TRS-80 DOS – TRSDOS v1.3 &#8211; BOOT/SYS Disassembled

Call RST 28H run a specific routine in a specific DOS OVERLAY, all as held in Register A.

4033

JP 44B0H

JUMP to 44B0H.

NOTE: 44B0H is the INPUT/OUTPUT DISPATCHER.

4036-
403C

Keyboard Bit Image

403D

JUMP to 35FAH.

NOTE: This is INTERRUPT VECTOR 3. It isn’t used.

4040

JUMP to 35FAH.

NOTE: This is INTERRUPT VECTOR 6. It isn’t used.

4043

JUMP to 35FAH.

NOTE: This is INTERRUPT VECTOR 7. It isn’t used.

4046

JP 3529H

JUMP to 3529H.

NOTE: This is INTERRUPT VECTOR 2 and JUMPs to 3529H in the Model III ROM. It sets and flashes the cursor and updates the clock/calendar.

4049

JP 42BDH

JUMP to 42BDH.

NOTE: This is NON-MASKABLE INTERRUPT VECTOR.

4243

JP 022EH

JUMP to 022EH.

NOTE: This is VECTOR for a BREAK when hit during RS-232 or CASSETTE I/O.

4246

JUMP to 35FAH.

NOTE: This is INTERRUPT VECTOR 4.

4249

JUMP to 35FAH.

NOTE: This is INTERRUPT VECTOR 5.

427BH – Start of the Overlay in Earnest.

427B

LD A,9AH

LET Register A = 9AH (1001 1010).
In the context of setting up Register A for a RST 28H call, x001 is the routine number within the overlay to run (i.e, routine 1) and 1010 is the overlay number being called (i.e., overlay 10). This would be the BASIC Error Routine

427D

Call RST 28H run a specific routine in a specific DOS OVERLAY, all as held in Register A.

427E

LD A,8CH

LET Register A = 8CH (1000 1100).
In the context of setting up Register A for a RST 28H call, x000 is the routine number within the overlay to run (i.e, routine 0) and 1100 is the overlay number being called (i.e., overlay 12). This would be the BASIC Renumber Routine

4280

Call RST 28H run a specific routine in a specific DOS OVERLAY, all as held in Register A.

4281

LD A,9CH

LET Register A = 9CH (1001 1100).
In the context of setting up Register A for a RST 28H call, x001 is the routine number within the overlay to run (i.e, routine 1) and 1100 is the overlay number being called (i.e., overlay 12).

4283

Call RST 28H run a specific routine in a specific DOS OVERLAY, all as held in Register A.

4284

LD A,8DH

LET Register A = 8DH (1000 1101).
In the context of setting up Register A for a RST 28H call, x000 is the routine number within the overlay to run (i.e, routine 0) and 1101 is the overlay number being called (i.e., overlay 13).

4286

Call RST 28H run a specific routine in a specific DOS OVERLAY, all as held in Register A.

4287

LD A,9DH

LET Register A = 9DH (1001 1101).
In the context of setting up Register A for a RST 28H call, x001 is the routine number within the overlay to run (i.e, routine 1) and 1101 is the overlay number being called (i.e., overlay 13).

4289

Call RST 28H run a specific routine in a specific DOS OVERLAY, all as held in Register A.

428A

LD A,ADH

LET Register A = ADH (1010 1101).
In the context of setting up Register A for a RST 28H call, x010 is the routine number within the overlay to run (i.e, routine 2) and 1101 is the overlay number being called (i.e., overlay 13).

428C

Call RST 28H run a specific routine in a specific DOS OVERLAY, all as held in Register A.

428D

LD A,8EH

LET Register A = 8EH (1000 1110).
In the context of setting up Register A for a RST 28H call, x000 is the routine number within the overlay to run (i.e, routine 0) and 1110 is the overlay number being called (i.e., overlay 14). This will return the file pointers to the user.

428F

Call RST 28H run a specific routine in a specific DOS OVERLAY, all as held in Register A.

4290

LD A,9EH

LET Register A = 9EH (1001 1110).
In the context of setting up Register A for a RST 28H call, x001 is the routine number within the overlay to run (i.e, routine 1) and 1110 is the overlay number being called (i.e., overlay 14). This will return the directory of files in memory.

4292

Call RST 28H run a specific routine in a specific DOS OVERLAY, all as held in Register A.

4293

LD A,8FH

LET Register A = 8FH (1000 1111).
In the context of setting up Register A for a RST 28H call, x000 is the routine number within the overlay to run (i.e, routine 0) and 1111 is the overlay number being called (i.e., overlay 15). There is no overlay 15.

4295

Call RST 28H run a specific routine in a specific DOS OVERLAY, all as held in Register A.

4296

LD A,91H

LET Register A = 91H (1001 0001).
In the context of setting up Register A for a RST 28H call, x001 is the routine number within the overlay to run (i.e, routine 1) and 0001 is the overlay number being called (i.e., overlay 1). This is the normal DOS command processor (DOS READY).

4298

Call RST 28H run a specific routine in a specific DOS OVERLAY, all as held in Register A.

4299

LD A,B1H

LET Register A = B1H (1011 0001).
In the context of setting up Register A for a RST 28H call, x011 is the routine number within the overlay to run (i.e, routine 3) and 0001 is the overlay number being called (i.e., overlay 1). This will execute a DOS COMMAND and RETURN to DOS READY.

429B

Call RST 28H run a specific routine in a specific DOS OVERLAY, all as held in Register A.

429C

LD A,F1H

LET Register A = F1H (1111 0001).
In the context of setting up Register A for a RST 28H call, x111 is the routine number within the overlay to run (i.e, routine 7) and 0001 is the overlay number being called (i.e., overlay 1). This will execute a DOS COMMAND and RETURN to the USER PROGRAM.

429E

Call RST 28H run a specific routine in a specific DOS OVERLAY, all as held in Register A.

429F

LD A,A7H

LET Register A = A7H (1010 0111).
In the context of setting up Register A for a RST 28H call, x010 is the routine number within the overlay to run (i.e, routine 2) and 0111 is the overlay number being called (i.e., overlay 7). This will execute a FORMAT COMMAND.

42A1

Call RST 28H run a specific routine in a specific DOS OVERLAY, all as held in Register A.

42A2

LD A,97H

LET Register A = 97H (1001 0111).
In the context of setting up Register A for a RST 28H call, x001 is the routine number within the overlay to run (i.e, routine 1) and 0111 is the overlay number being called (i.e., overlay 7). This will execute a BACKUP COMMAND.

42A4

Call RST 28H run a specific routine in a specific DOS OVERLAY, all as held in Register A.

42A5

LD A,98H

LET Register A = 98H (1001 1000).
In the context of setting up Register A for a RST 28H call, x001 is the routine number within the overlay to run (i.e, routine 1) and 1000 is the overlay number being called (i.e., overlay 8). This will execute a HELP COMMAND.

42A7

Call RST 28H run a specific routine in a specific DOS OVERLAY, all as held in Register A.

42A8

LD A,99H

LET Register A = 99H (1001 1001).
In the context of setting up Register A for a RST 28H call, x001 is the routine number within the overlay to run (i.e, routine 1) and 1001 is the overlay number being called (i.e., overlay 9). This will process LIBRARY commands.

42AA

Call RST 28H run a specific routine in a specific DOS OVERLAY, all as held in Register A.

42AB

LD A,9BH

LET Register A = 9BH (1001 1011).
In the context of setting up Register A for a RST 28H call, x001 is the routine number within the overlay to run (i.e, routine 1) and 1011 is the overlay number being called (i.e., overlay 11). This will process MORE LIBRARY commands.

42AD

Call RST 28H run a specific routine in a specific DOS OVERLAY, all as held in Register A.

42AE

JP 4B00H

If we are here then there was a bad overlay request, so JUMP to 4B00H. Note: This memory location may be changed to 4A85H (or other values from other overlays) based on whether OVERLAY 1 has been loaded.

42B1

DEFB …

Byte Storage.

42B2

DEFB …

Byte Storage.

42B3

DEFB …

Byte Storage.

42B4

DEFB …

Byte Storage.

42B5

DEFB …

Byte Storage.

42B6

DEFB …

Byte Storage.

42B7

DEFB …

Byte Storage.

42B8

DEFB …

Byte Storage.

42B9

DEFB …

Byte Storage.

42BA

LD A,86H

LET Register A = 86H (1000 0110).
In the context of setting up Register A for a RST 28H call, x000 is the routine number within the overlay to run (i.e, routine 0) and 0110 is the overlay number being called (i.e., overlay 6). This will process EVEN MORE LIBRARY commands.

42BC

Call RST 28H run a specific routine in a specific DOS OVERLAY, all as held in Register A.

42BD

RETN

RETN is used at the end of a NMI service routine. It POPs the value at the top of STACK into PC and copies the value of IFF2 into IFF1 (so that maskable interrupts are allowed to continue).

42BFH – Set the clock heartbeat counter to 25 and JUMP to ROM.

42BF

LD A,(4216H)

Fetch the value stored at memory location 4216H and put it into Register A.

NOTE: 4216H is the storage location for the Clock Data: Heartbeat Counter.

42C2

CP 1EH

Compare the value held in Register A against 1EH. Results: If Register A equals 1EH, the Z FLAG is set; otherwise the NZ FLAG is set.

42C4

JR NZ,42CBH

If the NZ FLAG (Not Zero) is set, JUMP to 42CBH.

42C6

LD A,19H

LET Register A = 19H (Decimal: 25).

42C8

LD (4216H),A

Store the value held in Register A into the memory location 4216H.

NOTE: 4216H is the storage location for the Clock Data: Heartbeat Counter.

42CB

JP 3529H

JUMP to 3529H.

NOTE: 3529H is in the Model III ROM. It sets and flashes the cursor and updates the clock/calendar.

42CEH – Execute the KILL command. On entry, DE needs to point to the memory location which states whether a file is currently open or not

42CE

LD A,(DE)

Fetch the value stored at memory location pointed to by Register Pair DE and put it into Register A. This should indicate if the file is currently open or not.

42CF

OR A

Since a LD command does not set any FLAGS, Set FLAGS based on the contents of Register A.

42D0

JP M,42D9H

If the SIGN FLAG is set then the file is currently open, JUMP away to 42D9H.

42D3

PUSH DE

Preseve Register DE to the top of the STACK.

42D4

CALL 4424H

GOSUB to 4424H to open the file.

42D7

POP DE

Restore Register Pair DE from the top of the STACK.

42D8

RET NZ

If the NZ FLAG (Not Zero) is set then we have an error, so RETurn to the caller.

42D9

LD A,A3H

If we are here then there was no error, so LET Register A = A3H (1010 0011).
In the context of setting up Register A for a RST 28H call, x010 is the routine number within the overlay to run (i.e, routine 2) and 0011 is the overlay number being called (i.e., overlay 3).

42DB

Call RST 28H run a specific routine in a specific DOS OVERLAY, all as held in Register A.

42DCH – Reset the FDC (including restore the drive to track 0).

42DC

GOSUB to 4566H to reset the FDC.

42DF

AND 10H

MASK the value of Register A against 10H (0001 0000). This has the effect of turning off bits 7, 6, 5, 3, 2, 1, 0, leaving only bit 4 active. Bit 4 from Port F0H is the flag for RECORD NOT FOUND.

42E1

RET Z

If the Z FLAG (Zero) is set then we found the record and have no error so RETurn to the caller.

42E2

PUSH BC

PUSH Register BC to the top of the STACK.

42E3

PUSH DE

PUSH Register DE to the top of the STACK.

42E4

PUSH HL

PUSH Register HL to the top of the STACK.

42E5

LD B,00H

42E7

LD HL,445BH

LET Register Pair HL = 445BH, which is the track table in RAM.

42EA

ADD HL,BC

ADD the value held in Register Pair BC to Register Pair HL so that HL now points to the track table as offset by the approriate offset in BC.

42EB

LD (HL),00H

Store 00H into the memory location pointed to by Register Pair HL, which marks that particular track as track 0.

42ED

LD A,0CH

LET Register A = 0CH (0000 1100).
When this bit pattern in sent to Port F0H (the Floppy Disk Control Port) and bits 7-4 are 0000, this is a RESTORE command and is a Type I command. Bits 0-1 are the stepping rate (00=6ms, 01=12ms, 10=20ms, 11=30ms), Bit 2 is the Track Number Verify Flag (0=No Verify), and Bit 3 is the Head Load Flag (1=Load Head).

When activated, the Track Register is checked, and if its 0 (meaning at Track 0) an interrupt is generated. If it is not 0, stepping pulses are issued until the TR00 goes to 0, and then an interrupt is generated.

42EF

OUTput the value held in Register A to port F0H.

NOTE: Port F0H is the Floppy Disk Command/Status Register. OUTputting to Port F0H cannot be simply summarized as there are more than 11 variables held in the bit pattern.

42F1

POP HL

Restore Register Pair HL from the top of the STACK

42F2

POP DE

Restore Register Pair DE from the top of the STACK

42F3

POP BC

Restore Register Pair BC from the top of the STACK

42F4

RET

RETurn to the caller.

42FFH-43FF – Data Storage Area.

42FF

00H

Storage Byte for whether a password check is skipped or undertaken in an OPEN command. If Bit 0 is set, the check is skipped

4300-
43FF

00H

256 Byte Storage Area for Disk I/O

4400H – Short Delay Routine to Accomodate the FDC.

4400

PUSH AF

PUSH Register AF to the top of the STACK.

4401

POP AF

MOVE the value held at the stop of the STACK into Register Pair AF.

4402

PUSH AF

PUSH Register AF to the top of the STACK.

4403

POP AF

MOVE the value held at the stop of the STACK into Register Pair AF.

4404

PUSH AF

PUSH Register AF to the top of the STACK.

4405

POP AF

MOVE the value held at the stop of the STACK into Register Pair AF.

4406

PUSH AF

PUSH Register AF to the top of the STACK.

4407

POP AF

MOVE the value held at the stop of the STACK into Register Pair AF.

4408

RET

RETurn to the caller.

4409H – Process DOS Errors.

4409

PUSH AF

Since an overlay call uses Register A, which is otherwise holding the DOS ERROR CODE, PUSH Register AF to the top of the STACK.

440A

LD A,84H

LET Register A = 84H (1000 0100).
In the context of setting up Register A for a RST 28H call, x000 is the routine number within the overlay to run (i.e, routine 0) and 0100 is the overlay number being called (i.e., overlay 4).

440C

Call RST 28H run a specific routine in a specific DOS OVERLAY, all as held in Register A.

440DH – Process DEBUG.

440D

PUSH AF

Since an overlay call uses Register A, PUSH Register AF to the top of the STACK to save Register A.

440E

LD A,85H

LET Register A = 85H (1000 0101).
In the context of setting up Register A for a RST 28H call, x000 is the routine number within the overlay to run (i.e, routine 0) and 0101 is the overlay number being called (i.e., overlay 5).

4410

Call RST 28H run a specific routine in a specific DOS OVERLAY, all as held in Register A.

4411H – Data Storage Area

4411

Storage for the MEMORY PROTECT address.

4413

Storage for the NUMBER OF DISK DRIVES in the system.

4414

Storage for the NUMBER OF THE OVERLAY CURRENTLY IN MEMORY.

4415

Storage for the ADDRESS OF THE END OF PHYSICAL MEMORY.

4419H – Routine to display a SHORT DIRECTORY to the screen.

4419

LD A,8AH

LET Register A = 8AH (1000 1010).
In the context of setting up Register A for a RST 28H call, x000 is the routine number within the overlay to run (i.e, routine 0) and 1010 is the overlay number being called (i.e., overlay 10).

441B

Call RST 28H run a specific routine in a specific DOS OVERLAY, all as held in Register A.

441CH – Routine to check the syntax of a filespec before it is used.

441C

LD A,C1H

LET Register A = C1H (1100 0001).
In the context of setting up Register A for a RST 28H call, x100 is the routine number within the overlay to run (i.e, routine 4) and 0001 is the overlay number being called (i.e., overlay 1).

441E

Call RST 28H run a specific routine in a specific DOS OVERLAY, all as held in Register A.

4420H – Routine to INIT a disk file (i.e., create or open).

4420

LD A,A2H

LET Register A = A2H (1010 0010).
In the context of setting up Register A for a RST 28H call, x010 is the routine number within the overlay to run (i.e, routine 2) and 0010 is the overlay number being called (i.e., overlay 2).

4422

Call RST 28H run a specific routine in a specific DOS OVERLAY, all as held in Register A.

4423H – Data Storage Area

4423

00H

Storage for the SAVED DISK DRIVE SELECT CODE.

4424H – Routine to OPEN A DISK FILE for use

LD A,92H

LET Register A = 92H (1001 0010).
In the context of setting up Register A for a RST 28H call, x001 is the routine number within the overlay to run (i.e, routine 1) and 0010 is the overlay number being called (i.e., overlay 2).

4426

Call RST 28H run a specific routine in a specific DOS OVERLAY, all as held in Register A.

4427H – Data Storage Area

4427

00H

Storage for the CURRENTLY SELECTED DISK DRIVE NUMBR.

4428H – Routine to CLOSE A DISK FILE

4428

LD A,93H

LET Register A = 93H (1001 0011).
In the context of setting up Register A for a RST 28H call, x001 is the routine number within the overlay to run (i.e, routine 1) and 0011 is the overlay number being called (i.e., overlay 3).

442A

Call RST 28H run a specific routine in a specific DOS OVERLAY, all as held in Register A.

442BH – Data Storage Area

442B

00H

Storage for the LOAD/EXECUTE flag when the disk loader is called. Bit 7 – Z=EXECUTE, NZ=LOAD

442CH – Vector for the KILL command

442C

JP 42CEH

JUMP to 42CEH.

442FH – Data Storage Area

442F

00H

Storage for the ROUTE/DUAL flag. 80H=ROUTE, FFH=DUAL, 00H=Neither

4430H – Various Jump Vectors

4430

JP 4BE6H

JUMP to 4BE6H to LOAD PROGRAM INTO RAM.

4433

JP 4BD9H

JUMP to 4BD9H to LOAD AND EXECUTE PROGRAM.

4436

JP 4737H

JUMP to 4737H which is the READ VECTOR.

4439

JP 47B3H

JUMP to 47B3H which is the WRITE VECTOR.

443C

JP 47B3H

JUMP to 47B3H which is the VERIFY VECTOR when VERIFY is OFF. When VERIFY is ON, this JUMP is changed in RAM to 47CCH.

443F

JP 4720H

JUMP to 4720H which is the REWIND VECTOR.

4442

JP 46CFH

JUMP to 46CFH which is the POSN VECTOR.

4445

JP 4701H

JUMP to 4701H which is the BACKSPACE VECTOR.

4448

JP 4729H

JUMP to 4729H which is the POSEOF VECTOR.

444BH – This routine will add a default extension to a filespec

444B

LD A,D1H

LET Register A = D1H (1101 0001).
In the context of setting up Register A for a RST 28H call, x101 is the routine number within the overlay to run (i.e, routine 5) and 0001 is the overlay number being called (i.e., overlay 1).

444D

Call RST 28H run a specific routine in a specific DOS OVERLAY, all as held in Register A.

444EH – Various Jump Vectors

444E

JP 4B55H

JUMP to 4B55H which is the DMULT VECTOR – Multiply Register Pair HL by Register A.

4451

JP 4B6FH

JUMP to 4B6FH which is the DIVIDE VECTOR.

4454H – This routine will parse a parameter string.

4454

LD A,E1H

LET Register A = E1H (1110 0001).
In the context of setting up Register A for a RST 28H call, x110 is the routine number within the overlay to run (i.e, routine 6) and 0001 is the overlay number being called (i.e., overlay 1).

4456

Call RST 28H run a specific routine in a specific DOS OVERLAY, all as held in Register A.

4457H – Data Storage Area

4457

Table of Directory Tracks (4 Bytes)

445B

Table of Current Head Positions (4 Bytes)

4463

Storage for the old system disk side number

4465

Storage for FILENAME/EXT.PASSWORD:D

4497H – Data Storage Area for the System Overlay File Control Block

4497

FCB + 00 – This holds the FILE TYPE byte

4498

FCB + 01 – This holds the OPTIONS byte. When paired with IX+01H, this can be the LSB of the device driver OR the new DCB if not working off disk.

Bit 0, 1, 2 = Access Level Permitted, 0=Yes
Bit 4 = Unwritten data in the buffer, 1=yes
Bit 5 = Is sector loaded, 0=Yes
Bit 6 = RANDOM I/O
Bit 7 = BIT 7 = FILE TYPE. 1=BYTE LEVEL I/O, 0=SECTOR LEVEL I/O.

4499

FCB + 02 – This is currently unused. When paired with IX+01H, this can be the MSB of the device driver OR the new DCB if not working off disk.

449A

FCB + 03 – This holds the BUFFER ADDRESS (2 Bytes)

449C

FCB + 05 – This holds the BYTE POSITION FOR NEXT

449D

FCB + 06 – This holds the DRIVE NUMBER OF THE FILE

449E

FCB + 07 – This holds the LOGICAL FILE NUMBER IN THE DIRECTORY

449F

FCB + 08 – This holds the BYTE POSITION OF THE EOF

44A0

FCB + 09 – This holds the LOGICAL RECORD LENGTH

44A1

FCB + 10 – This holds the SECTOR NUMBER FOR NEXT FILE (2 Bytes)

44A3

FCB + 12 – This holds the SECTOR NUMBER FOR THE EOF (2 Bytes)

44A4

FCB + 13 – This holds the SIDE NUMBER byte

44A5

FCB + 14 – This is currently unused

44A6

FCB + 15 – is currently unused

44A7

FCB + 16 – This holds the FIRST EXTENT ELEMENT

44A9H – Data Storage Area

44A9

Storage for the Saved Caller Address for DOS functions (2 Bytes)

44AB

Storage for the Saved FFCB Address for DOS functions (2 Bytes)

44ADH – Set up Special Index Register Pair IX with the address of the DCB

44AD

PUSH HL

PUSH Register HL (which is holding the NEW DCB) to the top of the STACK.

44AE

POP IX

Restore the NEW DCB from the top of the STACK into Register Pair IX.

44B0H – Continue the I/O Dispatcher Routine which started in ROM

44B0

LD L,(IX+01H)

Fetch the value stored at memory location IX+01H and put it into Register L. Set HL to be the device driver OR the new DCB if not diskette.

44B3

LD H,(IX+02H)

Fetch the value stored at memory location IX+02H and put it into Register H.

44B6

LD A,(IX+00H)

Fetch the value stored at memory location IX+00H and put it into Register A. Fetch the FILE TYPE byte into Register A.

44B9

CP 10H

Compare the value held in Register A against 10H. Results: If Register A equals 10H, the Z FLAG is set; otherwise the NZ FLAG is set.

44BB

JR Z,44ADH

If the Z FLAG (Zero) is set then we are needing to ROUTE to a NEW DCB so JUMP BACK to 44ADH to restore HL into IX and start this routine again.

44BD

JR NC,44C3H

If the NC FLAG (No Carry) is set then we are needing to ROUTE to a disk file so JUMP to 44C3H.

44BF

LD A,B

LET Register A = Register B, which is holding the KIND of I/O needed.

44C0

CP 02H

Compare the value held in Register A against 02H. This is done with an eye to setting the CARRY FLAG if read. Results:

If Register A equals 02H, the Z FLAG is set.
If A < 02H, the CARRY FLAG will be set.
if A >= 02H, the NO CARRY FLAG will be set.

44C2

JP (HL)

JUMP to (HL).

44C3H – Routine to ROUTE to a Disk File. Register B holds the KIND of I/O needed and Register C holds the byte to be sent.

44C3

SET 7,(IX+01H)

SET (i.e., set as 1) BIT 7 of Register (IX+01H) which is the OPTIONS byte. This sets the FILE TYPE for BYTE LEVEL I/O.

44C7

GOSUB to 4881H to set up for DISK I/O. This routine requires that Register Pair DE be pointing to the location in the buffer.

44CA

LD A,B

LET Register A = Register B, which is holding the KIND of I/O needed.

44CB

CP 02H

Compare the value held in Register A against 02H. This is done with an eye to setting the CARRY FLAG if read. Results:

If Register A equals 02H, the Z FLAG is set.
If A < 02H, the CARRY FLAG will be set.
if A >= 02H, the NO CARRY FLAG will be set.

44CD

LD A,C

LET Register A = Register C so that the byte to send is put into Register A.

44CE

JP NC,4837H

If the NC FLAG (No Carry) is set then we want to WRITE, so JUMP to 4837H.

44D1

JP 4791H

Otherwise, JUMP to 4791H to do a READ.

44D4H – Handle DRIVE SELECT. Register C holds the drive number to select.

44D4

LD A,C

LET Register A = Register C (which is the drive number).

44D5

GOSUB to 45E9H to calculate the DRIVE SELECT CODE.

44D8

OR 80H

OR Register A against 80H (1000 0000). This has the effect of turning on bit 7, thus selecting DOUBLE DENSITY when sent to Port F4H.

44DA

LD (4423H),A

Store the value held in Register A into the memory location 4423H.

44DD

LD A,(4423H)

Fetch the value stored at memory location 4423H (which holds the Disk Drive Select Code) and put it into Register A.

44E0

OUT (F4H),A

OUTput the value held in Register A to port F4H.

NOTE: Port F4H is the Floppy drive select and options register. For OUTPUT:

Bit 0: Drive 0 Select
Bit 1: Drive 1 Select
Bit 2: Drive 2 Select
Bit 3: Drive 3 Select
Bit 4: Side Select (0 = Select Side 0, 1 = Select Side 1)
Bit 5: Write Precompensation (0 = Disable WP, 1 = Enable WP)
Bit 6: Wait State Generation (0 = Disable WSG, 1 = Enable WSG)
Bit 7: Density Select (0 = Single/FM, 1 = Double/MFM)

44E2

RET

RETurn to the caller.

44E3H – Position the Disk Read/Write Head to the Proper Track. On entry, Register C holds the drive number, and Register D holds the track number.

44E3

PUSH BC

PUSH Register BC to the top of the STACK.

44E4

PUSH DE

PUSH Register DE to the top of the STACK.

44E5

PUSH HL

PUSH Register HL to the top of the STACK.

44E6

LD A,C

LET Register A = Register C (which holds the drive number).

44E7

LD (4427H),A

Store the value held in Register A (the drive number) into the memory location 4427H (which stores the currently selected disk drive number).

44EA

PUSH BC

Sve Register BC to the top of the STACK.

44EB

GOSUB to 45E9H to calculate the DRIVE SELECT CODE.

44EE

OR 80H

OR Register A against 80H (1000 0000). This has the effect of turning on bit 7, thus selecting DOUBLE DENSITY when sent to Port F4H.

44F0

LD C,A

LET Register C = Register A (which is the drive select code, modified to double density).

44F1

LD A,D

LET Register A = Register D (which is the track number we want).

44F2

CP 16H

Compare the value held in Register A against 16H. Results:

If Register A equals 16H, the Z FLAG is set.
If A < 16H, the CARRY FLAG will be set.
if A >= 16H, the NO CARRY FLAG will be set.

44F4

JR C,44F8H

If the C FLAG (No Carry) is set, then the track number want is less than 16H (which is track 25), skip the next couple of instructions and JUMP to 44F8H.

44F6

SET 5,C

SET (i.e., set as 1) BIT 5 of Register C to enable WRITE PRECOMPENSATION.

44F8

LD A,C

LET Register A = Register C (which is the DRIVE SELECT CODE).

44F9

LD (4423H),A

Store the value held in Register A into the memory location 4423H (which holds the Disk Drive Select Code).

44FC

POP BC

Restore Register Pair from the top of the STACK into Register Pair BC.

44FD

CALL 4547H

GOSUB to 4547H to wait for the controller.

4500

JR NZ,4536H

If the NZ FLAG (Not Zero) is set then that call returned an error, so JUMP to 4536H.

4502

PUSH BC

Save Register BC to the STACK.

4503

LD B,00H

LET Register B = 00H sso that BC will be the OFFSET on the TRACK TABLE.

4505

LD HL,445BH

LET Register Pair HL = 445BH (which is the TRACK TABLE).

4508

ADD HL,BC

ADD the value held in Register Pair BC to Register Pair HL. The results (he entry in the TRACK TABLE for the track we are on) are held in Register Pair HL.

4509

POP BC

Restore Register Pair BC from the STACK.

450A

LD A,(HL)

Fetch the value stored at memory location pointed to by Register Pair HL (which is the entry in the TRACK TABLE for the track we are on) and put it into Register A.

450B

CP D

Compare the value held in Register A (the current track number) against the value held in Register D (the track number we want). Results:

If Register A equals D, the Z FLAG is set.
If A < D, the CARRY FLAG will be set.
if A >= D, the NO CARRY FLAG will be set.

450C

CALL 453AH

GOSUB to 453AH to seek to the new track.

450F

JR NZ,4536H

If the NZ FLAG (Not Zero) is set, then there was an errior, so JUMP to 4536H to handle the error.

4511

BIT 4,A

Test Bit 4 of Register A, which would be a SEEK ERROR.

4513

JR Z,4527H

If the Z FLAG (Zero) is set, meaning that we did not get a SEEK ERROR, JUMP to 4527H.

4515

LD A,0CH

4517

4519

CALL 4400H

GOSUB to 4400H to wait for the FDC to be ready.

451C

INput a byte from Port F0H and put it into Register A.

NOTE: Port F0H is the FDC Status port on input. Input Results:

Bit 0: Busy
Bit 1: Index/DRQ
Bit 2: Track 0/Data Lost
Bit 3: CRC error
Bit 4: Seek error/Record not found
Bit 5: If Reading, Record Type, if Writing, Write Fault/Head loaded
Bit 6: Write Protect
Bit 7: Not ready

451E

BIT 2,A

Test Bit 2 of Register A to see if we are at TRACK 0 yet.

4520

JR Z,451CH

If the Z FLAG (Zero) is set then we are not yet at TRACK 0, so LOOP BACK 2 instructions to 451CH.

4522

CALL 453CH

GOSUB to 453CH to seek the track.

4525

JR NZ,4536H

If the NZ FLAG (Not Zero) is set then we had an error in seeking so JUMP FORWARD to 4536H to deal with the error.

4527

PUSH AF

PUSH Register AF to the top of the STACK to save Register A and the FLAGs.

4528

LD A,(4427H)

Fetch the value stored at memory location 4427H (the currently selected disk drive number) and put it into Register A.

452B

LD C,A

LET Register C = Register A.

452C

LD B,00H

LET Register B = 00H. This leaves Register Pair BC holding the currently selected disk drive number.

452E

LD HL,445BH

LET Register Pair HL = 445BH, which is the track table in RAM.

4531

ADD HL,BC

ADD the value held in Register Pair BC to Register Pair HL. The results (the table entry for the desired track) are held in Register Pair HL.

4532

IN A,(F1H)

Read the actual diskette drive track number from the FDC by doing an INput from Port F1H.

4534

LD (HL),A

Store the value held in Register A into the memory location pointed to by Register Pair HL (which is the table entry for the desired track).

4535

POP AF

Restore Register A and the FLAGs from the STACK.

4536

POP HL

Restore Register Register Pair HL from the STACK.

4537

POP DE

Restore Register Register Pair DE from the STACK.

4538

POP BC

Restore Register Register Pair BC from the STACK.

4539

RET

RETurn to the caller.

453AH – Prepare to do a TRACK SEEK. On entry, Register A should hold the current track number and Register D should hold the desired track number.

453A

OUT (F1H),A

Put the track we want to find (held in Register A) into the TRACK SELECT port for the FDC (Port F1H).

453C

LD A,D

LET Register A = Register D (the desired track number).

453D

OUT (F3H),A

OUTput the value held in Register A to the FDC Data Register (Port F3H).
The reason this is going to the data register is that the FDC seek command requires that the Track Register contains the track number of the current position of the head and the Data Register contains the desired track number.

453F

JR NZ,4543H

If the NZ FLAG (Not Zero) is set, then the actual track number is not the same as the desired track number so JUMP to 4543H to seek the track.

4541

XOR A

Set Register A to ZERO and clear all Flags.

4542

RET

RETurn to the caller.

4543H – Routine to SEEK a TRACK on the diskette.

4543

LD A,1CH

LET Register A = 1CH (0001 1100).
When this bit pattern in sent to Port F0H (the Floppy Disk Control Port) and bits 7-4 are 0001, this is a SEEK command and is a Type I command. Bits 0-1 are the stepping rate, Bit 2 is the Track Number Verify Flag (0=No Verify), and Bit 3 is the Head Load Flag (1=Load Head).
So long as the Track Register holds the current position of the head and the Data Register contains the desired track number, the FDC will update the Track Register and issue stepping pulses in the appropriate direction until the Track Register and Data Register match. An interrupt is generated when done.

4545

Now we need to wait for the disk drive to actually finish seeking to the track.

4547

GOSUB to 44DDH to re-select the drive.

454A

CALL 4400H

GOSUB to 4400H to delay a little bit.

454D

PUSH BC

PUSH Register BC to the top of the STACK.

454E

LD BC,0000H

LET Register Pair BC = 0000H to set up for a counter.

4551

INput a byte from Port F0H and put it into Register A.

NOTE: Port F0H is the FDC Status port on input. Input Results:

Bit 0: Busy
Bit 1: Index/DRQ
Bit 2: Track 0/Data Lost
Bit 3: CRC error
Bit 4: Seek error/Record not found
Bit 5: If Reading, Record Type, if Writing, Write Fault/Head loaded
Bit 6: Write Protect
Bit 7: Not ready

4553

BIT 0,A

Test Bit 0 of Register A, which would be the FDC BUSY bit from Port F0H.

4555

JR Z,4564H

If the Z FLAG (Zero) is set, meaning that the FDC is NOT busy, JUMP DOWN to 4564H to continue.

4557

DEC BC

If we’re here, we got FDC BUSY, so DECrement Register Pair BC by 1.

4558

LD A,B

We need to check the counter, but we can’t test Register B, so LET Register A = Register B.

4559

OR A

Since a LD command does not set any FLAGS, Set FLAGS based on the contents of Register A.

455A

JR Z,4561H

If the Z FLAG (Zero) is set then we did this FFFFH times, so JUMP to 4561H to error out.

455C

Otherwise, GOSUB to 44DDH to re-select the drive.

455F

JR 4551H

LOOP BACK to 4551H until the drive times out.

4561H – Routine to ERROR OUT.

4561

LD A,0CH

LET Register A = 0CH (0000 1100).

4563

OR A

Since a LD command does not set any FLAGS, Set FLAGS based on the contents of Register A.

4564

POP BC

MOVE the value held at the stop of the STACK into Register Pair BC.

4565

RET

RETurn to the caller.

4566H – Reset the FDC

4566

PUSH AF

SAVE Register AF to the STACK.

4567

LD A,D0H

LET Register A = D0H (1101 0000).
When this bit pattern in sent to Port F0H (the Floppy Disk Control Port) and bits 7-4 are 1101, this is a FORCE INTERRUPT command and is a Type IV command. Bit 3 designates IMMEDIATE INTERRUPT. Bit 2 designates INDEX PULSE. Bit 1 designates READY to NOT READY TRANSITION. Bit 0 designates NOT READY to READY TRANSITION.

4569

456B

POP AF

Restore Register Pair AF from the STACK.

456C

RET

RETurn to the caller.

456DH – Routine to WRITE SECTOR TO DISK. Assumes the head is already properly positioned.

456D

LD BC,00F3H

LET Register B = 00H for a counter and LET Register C = the FDC data port F3H.

4570

GOSUB to 44DDH to select the drive.

4573

OR 40H

OR Register A against 40H (0100 0000). This has the effect of turning on bit 6 which enables the WAIT BIT.

4575

LD D,A

LET Register D = Register A (which is holding the modified select code).

4576

CALL 459CH

GOSUB to 459CH to set the appropriate NMI Vector and clear the FDC status.

4579

LD A,C0H

LET Register A = C0H (1100 0000). When sent to the NMI port of E4H this actives NMI for FDC time-out and Disk Drive time-out

457B

OUTput the value held in Register A to port F4H.

NOTE: Port E4H is the Non-Maskable Interrupt Latch.

457D

LD E,02H

LET Register E = 02H to set up a mask of 0000 0010, which, when applied, would test for CONTROLLER READY.

457F

LD HL,0000H

LET Register Pair HL = 0000H.
NOTE: This value is set elsewhere and is likely to be 4580H.

4582

LD A,A5H

LET Register A = A5H (1010 0101).
When this bit pattern in sent to Port F0H (the Floppy Disk Control Port) and bits 7-5 are 101, this is a WRITE SECTOR command and is a Type II command. Bit 4 is the Multiple Record Flag (0=Single Record), Bit 3 is the Side Compare Flag (0=Side 0), Bit 2 is a 15ms Delay Flag (0=Off), Bit 1 is a Side Compare Flag (0=Disable). Bit 0 sets the Data Address Mark (0=FBH, 1=F8H).

4584

4586

CALL 4404H

GOSUB to 4404H for a slight delay.

4589

INput a byte from Port F0H and put it into Register A.

NOTE: Port F0H is the FDC Status port on input. Input Results:

Bit 0: Busy
Bit 1: Index/DRQ
Bit 2: Track 0/Data Lost
Bit 3: CRC error
Bit 4: Seek error/Record not found
Bit 5: If Reading, Record Type, if Writing, Write Fault/Head loaded
Bit 6: Write Protect
Bit 7: Not ready

458B

AND E

MASK the value of Register A against the value held in Register E to check for CONTROLLER READY.

458C

JR Z,4589H

If the Z FLAG (Zero) is set then the FDC is not ready so LOOP BACK 2 instructions to 4589H.

458E

OUTI

Write a byte from the memory location pointed to by HL to port C. Register B is DECremented and Register Pair HL is INCremented.

4590

LD B,64H

LET Register B = 64H (0110 0100).

4592

DJNZ 4592H

LOOP back to this very instruction 64H times for a short delay.

4594

LD A,D

LET Register A = Register D (which was holding the drive select code).

4595

OUT (F4H),A

OUTput the value held in Register A to port F4H.

NOTE: Port F4H is the Floppy drive select and options register. For OUTPUT:

Bit 0: Drive 0 Select
Bit 1: Drive 1 Select
Bit 2: Drive 2 Select
Bit 3: Drive 3 Select
Bit 4: Side Select (0 = Select Side 0, 1 = Select Side 1)
Bit 5: Write Precompensation (0 = Disable WP, 1 = Enable WP)
Bit 6: Wait State Generation (0 = Disable WSG, 1 = Enable WSG)
Bit 7: Density Select (0 = Single/FM, 1 = Double/MFM)

4597

OUTI

Write a byte from the memory location pointed to by HL to port C. Register B is DECremented and Register Pair HL is INCremented.

4599

JP 4595H

LOOP back to 4595H to keep writing until a NMI exits the loop.

459CH – Routine to set the NMI vector to 45C1H and clear the FDC status.

459C

PUSH HL

SAVE Register HL to the STACK.

459D

LD HL,45ADH

Part 1 of 4 instructions to set the NMI vector to 45C1H.

45A0

LD (404AH),HL

Store the value held in Register HL into the memory location 404AH.

NOTE: 404AH is part of the NMI VECTOR. I do not make believe I understand how that vector is set or how it works.

45A3

LD A,C3H

LET Register A = C3H (1100 0011).

45A5

LD (4049H),A

Store the value held in Register A into the memory location 4049H.

NOTE: 4049H is part of the NMI VECTOR. I do not make believe I understand how that vector is set or how it works.

45A8

INput a byte from Port F0H and put it into Register A.

NOTE: Port F0H is the FDC Status port on input. Input Results:

Bit 0: Busy
Bit 1: Index/DRQ
Bit 2: Track 0/Data Lost
Bit 3: CRC error
Bit 4: Seek error/Record not found
Bit 5: If Reading, Record Type, if Writing, Write Fault/Head loaded
Bit 6: Write Protect
Bit 7: Not ready

45AA

POP HL

Restore Register Pair BC from the STACK.

45AB

Disable Maskable Interrupts.

45AC

RET

RETurn to the caller.

45ADH – Routine to clear the NMI vector and disable NMIs.

45AD

XOR A

Set Register A to ZERO and clear all Flags.

45AE

OUTput the value held in Register A to port F4H.

NOTE: Port E4H is the Non-Maskable Interrupt Latch. Sending a 0 to E4 will turn off all Non-Maskable Interrupts.

45B0

LD HL,42BDH

LET Register Pair HL = 42BDH.

45B3

LD (404AH),HL

Store the value held in Register HL (i.e., 42BDH) into the memory location 404AH.

NOTE: 404AH is part of the NMI VECTOR. I do not make believe I understand how that vector is set or how it works.

45B6

JR 45A8H

JUMP to 45A8H in the prior routine to clear the FDC status and disable Maskable Interrupts, and RETurn.

45B8H – Routine to READ SECTOR FROM DISK. Assumes the head is already properly positioned and that Register A holds the drive select code.

45B8

LD BC,00F3H

LET Register B = 00H for a counter and LET Register C = the FDC data port F3H.

45BB

GOSUB to 44DDH (a short routine to re-select a selected drive).

45BE

LD D,A

Preserve the drive select code (held in Register A) into Register D.

45BF

CALL 459CH

GOSUB to 459CH to set the appropriate NMI Vector and clear the FDC status.

45C2

LD A,C0H

LET Register A = C0H (1100 0000) so that the NMI is programmed for controller and drive time-out when sent to Port E4H.

45C4

OUTput the value held in Register A to port F4H.

NOTE: Port E4H is the Non-Maskable Interrupt Latch. Sending a 0 to E4 will turn off all Non-Maskable Interrupts.

45C6

LD E,02H

LET Register E = 02H (0000 0010) to mask for CONTROLLER READY.

45C8

LD HL,0000H

LET Register Pair HL = 0000H.

45CB

LD A,84H

LET Register A = 84H (1000 0100).
When this bit pattern in sent to Port F0H (the Floppy Disk Control Port) and bits 7-5 are 100, this is a READ SECTOR command and is a Type II command. Bit 4 is the Multiple Record Flag (0=Single Record), Bit 3 is the Side Compare Flag (0=Side 0), Bit 2 is a 15ms Delay Flag (0=Off), Bit 1 is a Side Compare Flag (0=Disable). Bit 0 is supposed to be 0.

45CD

OUTput the value held in Register A to port F0H.

NOTE: Port F0H is the Floppy Disk Command/Status Register.

45CF

CALL 4404H

GOSUB to 4404H.

NOTE: 4404H is the routine which has a slight delay.

45D2

INput a byte from Port F0H and put it into Register A.

NOTE: Port F0H is the FDC Status port on input. Input Results:

Bit 0: Busy
Bit 1: Index/DRQ
Bit 2: Track 0/Data Lost
Bit 3: CRC error
Bit 4: Seek error/Record not found
Bit 5: If Reading, Record Type, if Writing, Write Fault/Head loaded
Bit 6: Write Protect
Bit 7: Not ready

45D4

AND E

MASK the value of Register A against the value held in Register E.

45D5

JR Z,45D2H

If the Z FLAG (Zero) is set, meaning CONTROLLER NOT READY, LOOP back 2 entries to poll the FDC again.

45D7

INI

Read a byte from Port C into the memory location pointed to by HL, then DECrement HL and INCrement B. Z is set if B = 0; otherwise it is reset.

45D9

LD A,D

Restore the drive select code (saved to Register D) back into Register A.

45DA

OR 40H

OR Register A against 40H (0100 0000). This has the effect of turning on bit 6 which is the WAIT STATE ON bit when sent to Port F4H.

45DC

OUT (F4H),A

OUTput the value held in Register A to port F4H.

NOTE: Port F4H is the Floppy drive select and options register. For OUTPUT:

Bit 0: Drive 0 Select
Bit 1: Drive 1 Select
Bit 2: Drive 2 Select
Bit 3: Drive 3 Select
Bit 4: Side Select (0 = Select Side 0, 1 = Select Side 1)
Bit 5: Write Precompensation (0 = Disable WP, 1 = Enable WP)
Bit 6: Wait State Generation (0 = Disable WSG, 1 = Enable WSG)
Bit 7: Density Select (0 = Single/FM, 1 = Double/MFM)

45DE

INI

Read a byte from Port C into the memory location pointed to by HL, then DECrement HL and INCrement B. Z is set if B = 0; otherwise it is reset.

45E0

JP NZ,45DCH

If the NZ FLAG (Not Zero) is set then the buffer is not yet full so LOOP BACK to 45DCH to keep reading bytes.

45E3

LD HL,0000H

If we are here then buffer IS full, so toss out HL by setting Register Pair HL = 0000H.

45E6

JP 45DCH

LOOP Back 4 instructions (45DCH). This routine will only exit via an interrupt.

45E9H – Routine to make a disk drive select code.

45E9

AND 07H

MASK the value of Register A against 07H (0000 0111). This has the effect of turning off bits 7, 6, 5, 4, 3, leaving only bits 2, 1, 0 active.

45EB
45EC
45ED

RLCA
RLCA
RLCA

Rotate the bits in A left (with Bit 7 going into both the CARRY and Bit 0) three times. This rorates the irrelevant bits and moves the bits we need to bits 3-5.

45EE

OR C7H

OR Register A against C7H to create an opcode using bits 3-5 from the mask.

45F0

LD (45F5H),A

Store the value held in Register A into the memory location 45F5H.

45F3

XOR A

Set Register A to ZERO and clear all Flags.

45F4

SET 0,A

SET (i.e., set as 1) BIT 0 of Register A.

45F6

RET

RETurn to the caller.

45F7H – WRITE SECTOR TO DISK. Positions the head first. Register C must be set to … dunno.

First we need to check for write protect.

45F7

LD A,FFH

LET Register A = FFH to check to see if the disk is write protected. Note that other routines will put a different value in here (i.e., 45F8H).

45F9

CP C

Compare the value held in Register A against the value held in Register C. Results: If Register A equals C, the Z FLAG is set; otherwise the NZ FLAG is set.

45FA

JR NZ,4600H

If the NZ FLAG (Not Zero) is set then the disks is NOT write protected, so continue by JUMPing to 4600H.

45FC

LD A,0FH

LET Register A = 0FH (0000 1111).

45FE

OR A

Since a LD command does not set any FLAGS, Set FLAGS based on the contents of Register A.

45FF

RET

RETurn to the caller.

4600H – WRITE SECTOR TO DISK. Positions the head first (continuation after verification that the disk isn’t write protected).

4600

LD (4580H),HL

Store the value held in Register HL (which is the buffer address) into the memory location 4580H.

4603

CALL 4C6BH

GOSUB to 4C6BH to start up and the check the disk drive.

4606

RET NZ

If the NZ FLAG (Not Zero) is set then there was an error from that routine so, RETurn to the caller.

4607

JR C,45FCH

If the C FLAG (No Carry) is set then the disk is write protected so, JUMP back a few instructions to the WRITE PROTECT ERROR routine (45FCH).

Start of a BIG loop for 9 iterations. The bottom of the loop is at 464EH.

4609

LD B,09H

LET Register B = 09H (0000 1001).

460B

CALL 44E3H

GOSUB to 44E3H.

NOTE: 44E3H is the routine which handles positions the READ/WRITE HEAD to the proper track (Register C holds the drive number, and Register D holds the track).

460E

CP 0CH

Compare the value held in Register A against 0CH to see if we got a TIME OUT. Results: If Register A equals 0CH, the Z FLAG is set; otherwise the NZ FLAG is set.

4610

JR NZ,4614H

If the NZ FLAG (Not Zero) is set then we are good, so JUMP to 4614H.

4612

OR A

Prepare to return with error by setting the FLAGS based on the contents of Register A.

4613

RET

RETurn to the caller.

4614H – WRITE SECTOR TO DISK. Positions the head first (continuation after positioning the head successfully.

4614

BIT 6,A

Test Bit 6 of Register A to see if the disk is write protected.

4616

JR NZ,45FCH

If the NZ FLAG (Not Zero) is set, then it is, so JUMP back a few instructions to the WRITE PROTECT ERROR routine (45FCH).

4618

AND 19H

MASK the value of Register A against 19H (0001 1001). This has the effect of turning off bits 7, 6, 5, 2, 1, leaving only bits 4 (FDC Seek Error), 3 (FDC CRC Error), 0 (FDC Busy) active.

461A

JR NZ,465CH

If the NZ FLAG (Not Zero) is set then one of those things happened, so JUMP to 465CH to reset the FDC and try again.

At this point, we know the disk isnt write protected, that we are on the right track, and that the FDC wasn’t busy and didn’t have an error … so we are good to start writing.

461C

LD A,D

LET Register A = Register D (which is holding the track number).

461D

CP 11H

Compare the value held in Register A against 11H (which is the directory track). Results: If Register A equals 11H, the Z FLAG is set; otherwise the NZ FLAG is set.

461F

JR NZ,4629H

If the NZ FLAG (Not Zero) is set (meaning we are NOT on the directory track), skip the next instructions (which set the READ PROTECT flag) and JUMP to 4629H.

4621

LD A,(4583H)

Fetch the value stored at memory location 4583H (which would be the write command) and put it into Register A.

4624

AND FEH

MASK the value of Register A (which is the write command) against FEH (1111 1110). This has the effect of turning off bit 0, leaving only bits 7, 6, 5, 4, 3, 2, 1 active. This change to bit 0 sets to READ PROTECT.

4626

LD (4583H),A

Store masked/changed write commend (held in Register A) back into the memory location 4583H.

4629

LD A,E

LET Register A = Register E.

462A

OUT (F2H),A

OUTput the value held in Register A to Port F2H.

NOTE: Port F2H is the Floppy Disk Controller SECTOR Register.

462C

PUSH BC

PUSH Register BC to the top of the STACK.

462D

PUSH DE

PUSH Register DE to the top of the STACK.

462E

PUSH HL

PUSH Register HL to the top of the STACK.

462F

LD (47D8H),BC

Store the value held in Register BC (the FILE NUMBER and the DISK DRIVE) into the memory location 47D8H.

4633

LD (47DBH),DE

Store the value held in Register DE (the TRACK NUMBER and SECTOR NUMBER) into the memory location 47DBH.

4637

CALL 456DH

GOSUB to 456DH.

NOTE: 456DH is the routine which to write a sector to diskette after the head is already properly positioned.

463A

POP HL

MOVE the value held at the stop of the STACK into Register Pair HL.

463B

POP DE

MOVE the value held at the stop of the STACK into Register Pair DE.

463C

POP BC

MOVE the value held at the stop of the STACK into Register Pair BC.

463D

INput a byte from Port F0H and put it into Register A.

NOTE: Port F0H is the FDC Status port on input. Input Results:

Bit 0: Busy
Bit 1: Index/DRQ
Bit 2: Track 0/Data Lost
Bit 3: CRC error
Bit 4: Seek error/Record not found
Bit 5: If Reading, Record Type, if Writing, Write Fault/Head loaded
Bit 6: Write Protect
Bit 7: Not ready

463F

BIT 7,A

Test Bit 7 of Register A (which is the NOT READY flag).

4641

JR NZ,464BH

If the NZ FLAG (Not Zero) is set then the DRIVE IS NOT READY, so JUMP to 464BH to reset the FDC and try again.

4643

BIT 6,A

If we’re here, then the drive is ready so test Bit 6 (WRITE PROTECT Flag) of Register A (the FDC status from 463DH).

4645

JR NZ,45FCH

If the NZ FLAG (Not Zero) is set then the WRITE PROTECT FLAG is set, so we have an error and need to JUMP to the WRITE PROTECT ERROR routine (45FCH).

4647

AND 3DH

MASK the value of Register A against 3DH (0011 1101). This has the effect of turning off bits 7, 6, 1, leaving only bits 5 (WRITE FAULT), 4 (RECORD NOT FOUND), 3 (CRC ERROR), 2 (LOST DATA), 0 (BUSY) active.

4649

JR Z,4667H

If the Z FLAG (Zero) is set then we are error free so, JUMP to 4667H to DISABLE the READ-PROTECT.

464B

CALL 42DCH

GOSUB to 42DCH to reset the FDC.

464E

DJNZ 460BH

LOOP back to 460BH until Register B is ZERO.

End of loop of 9 tries.

4650

CALL 4667H

GOSUB to 4667H to DISABLE the READ-PROTECT.

4653

LD B,0EH

LET Register B = 0EH in case there is a WRITE FAULT.

4655

BIT 5,A

Test Bit 5 of Register A for a WRITE FAULT.

4657

JP NZ,46CCH

If the NZ FLAG (Not Zero) is set, meaning there was a WRITE FAULT, JUMP to 46CCH.

465A

JR 46A9H

JUMP to 46A9H to process the error.

465CH – Reset the FDC, Try Again, Disable Read-Protect, and JUMP to the error routine

465C

GOSUB to 4566H.

NOTE: 4566H is the routine which resets the FDC.

465F

DJNZ 460BH

LOOP back to 460BH until Register B is ZERO.

4661

CALL 4667H

GOSUB to 4667H to DISABLE the READ-PROTECT.

4664

JP 46C4H

JUMP to 46C4H to process the error.

4667H – Disable the READ-PROTECT bit.

4667

PUSH AF

Save Register Pair AF to STACK.

4668

LD A,(4583H)

Fetch the value stored at memory location 4583H and put it into Register A.

NOTE: 4583H is the storage location for the WRITE COMMAND to be used.

466B

OR 01H

OR Register A against 01H (0000 0001). This has the effect of turning on bit 0 (to disable the READ-PROTECT bit).

466D

LD (4583H),A

Store the value held in Register A into the memory location 4583H.

NOTE: 4583H is the storage location for the WRITE COMMAND to be used.

4670

POP AF

Restore Register Pair AF from the top of the STACK.

4671

RET

RETurn to the caller.

4672H/4675H – Position the Head and Read Requested Sector. This has 2 entry points. The first empties HL (to toss away any data held there) one one leaves it alone.

4672

LD HL,0000H

LET Register Pair HL = 0000H. This discards whatever was in HL.

4675

LD (45C9H),HL

Store the value held in Register HL into the memory location 45C9H.

4678

CALL 4C6BH

GOSUB to 4C6BH to start up the disk drive.

467B

RET NZ

If the NZ FLAG (Not Zero) is set, meaning we received an error from that call, RETurn to the caller.

Start of a BIG loop for 9 iterations. The bottom of the loop is at 46A7H.

467C

LD B,09H

LET Register B = 09H (0000 1001) for a loop of 9 tries.

467E

CALL 44E3H

GOSUB to 44E3H.

NOTE: 44E3H is the routine which handles positions the READ/WRITE HEAD to the proper track (Register C holds the drive number, and Register D holds the track).

4681

CP 0CH

Test for a time-out by comparing the value held in Register A against 0CH. Results: If Register A equals 0CH, the Z FLAG is set; otherwise the NZ FLAG is set.

4683

JR NZ,4687H

If the NZ FLAG (Not Zero) is set then we did not get a time-out so continue the routine by a JUMP to 4687H.

4685

OR A

Set FLAGS based on the contents of Register A in preparation to error out.

4686

RET

RETurn to the caller.

4687

AND 19H

MASK the value of Register A against 19H (0001 1001). This has the effect of turning off bits 7, 6, 5, 2, 1, leaving only bits 4 (SEEK ERROR), 3 (CRC ERROR), 0 (BUSY) active.

4689

JR NZ,46BFH

If the NZ FLAG (Not Zero) is set then we have one of those so JUMP to 46BFH to deal with the error.

468B

LD A,E

LET Register A = Register E (which is holding the SECTOR TO READ).

468C

OUT (F2H),A

OUTput the value held in Register A to Port F2H.

NOTE: Port F2H is the Floppy Disk Controller SECTOR Register.

468E
468F
4690

PUSH BC
PUSH DE
PUSH HL

Preserve Register Pairs BC, DE, and HL to the STACK.

4691

CALL 45B8H

GOSUB to 45B8H to READ SECTOR FROM DISK (routine assumes the head is already properly positioned and that Register A holds the drive select code).

4694
4695
4696

POP HL
POP DE
POP BC

Restore Register Pairs BC, DE, and HL from the STACK.

4697

INput a byte from Port F0H and put it into Register A.

NOTE: Port F0H is the FDC Status port on input. Input Results:

Bit 0: Busy
Bit 1: Index/DRQ
Bit 2: Track 0/Data Lost
Bit 3: CRC error
Bit 4: Seek error/Record not found
Bit 5: If Reading, Record Type, if Writing, Write Fault/Head loaded
Bit 6: Write Protect
Bit 7: Not ready

4699

Enable Interrupts.

469A

LD (4B12H),A

Store the byte read from the FDC status (held in Register A) into the memory location 4B12H – which is in the middle of a LD A,nnH instruction inside the READ DIRECTORY SECTOR routine at 4B0DH.

469D

BIT 7,A

Test Bit 7 (NOT READY) of the FDC status (held in Register A).

469F

JR NZ,46A4H

If the NZ FLAG (Not Zero) is set, then the FDC returned NOT READY, so JUMP to 46A4H.

46A1

AND 1DH

MASK the value of Register A against 1DH (0001 1101). This has the effect of turning off bits 7, 6, 5, 1, leaving only bits 4 (RECORD NOT FOUND), 3 (CRC ERROR), 2 (LOST DATA), 0 (BUSY) active.

46A3

RET Z

If the Z FLAG (Zero) is set then we have no error so RETurn to the caller.

If we are here, then we need to deal with a NOT READY FDC error from 469FH.

46A4

CALL 42DCH

GOSUB to 42DCH to reset the FDC (including restore the drive to track 0).

46A7

DJNZ 467EH

LOOP back to 467EH to try again until Register B is ZERO.

46A9H – Error Processing Routine.

46A9

LD B,06H

LET Register B = 06H to set up for a HARDWARE FAULT error

46AB

BIT 7,A

Test Bit 7 of Register A (NOT READY).

46AD

If the NZ FLAG (Not Zero) is set, JUMP to 46CCH to display the error held in Register B.

46AF

LD B,03H

LET Register B = 03H.

46B1

BIT 2,A

Test Bit 2 (LOST DATA) of Register A.

46B3

If the NZ FLAG (Not Zero) is set, JUMP to 46CCH to display the error held in Register B.

46B5

LD B,01H

LET Register B = 01H.

46B7

BIT 3,A

Test Bit 3 (CRC ERROR) of Register A.

46B9

If the NZ FLAG (Not Zero) is set, JUMP to 46CCH to display the error held in Register B.

46BB

LD B,05H

LET Register B = 05H (SECTOR NOT FOUND).

46BD

JR 46CCH

JUMP to 46CCH to display the error held in Register B.

46BFH – Jumped here from 4689H to deal with a SEEK ERROR, CRC ERROR or BUSY FDC code.

46BF

GOSUB to 4566H.

NOTE: 4566H is the routine which resets the FDC.

46C2

DJNZ 467EH

LOOP back to 467EH until Register B is ZERO.

46C4

LD B,04H

LET Register B = 04H (0000 0100).

46C6

BIT 4,A

Test Bit 4 (span class=”code”>SEEK ERROR) of Register A.

46C8

If the NZ FLAG (Not Zero) is set, JUMP to 46CCH to display the error held in Register B.

46CA

LD B,01H

LET Register B = 01H (0000 0001).

46CCH – Set up to return with an error code.

46CC

LD A,B

LET Register A = Register B (holding the error code).

46CD

OR A

Since a LD command does not set any FLAGS, Set FLAGS based on the contents of Register A.

46CE

RET

RETurn to the caller.

46CFH – This is the VECTOR call for POSN – Position a File for Access.

46CF

GOSUB to 4881H to set up for I/O. This routine requires that Register Pair DE be pointing to the location in the buffer.

46D2

BIT 7,(IX+01H)

Test Bit 7 of Register (IX+01H), which is the OPTIONS byte, to see if we were set up for SECTOR I/O.

46D6

JR Z,46E2H

If the Z FLAG (Zero) is set then we are looking for SECTOR I/O so JUMP to 46E2H.

46D8
46D9

LD H,B
LD L,C

LET Register Pair HL = Register BC (the record number).

46DA

LD A,(IX+09H)

Fetch the value stored at memory location IX+09H (Record Length) and put it into Register A.

46DD

CALL 4B55H

GOSUB to 4B55H to calculate the NEXT record number, calculated as Record Number * Record Length. On exit, Register A = Overflow and Register Pair HL = the result of the multiplication.

46E0
46E0

LD B,H
LD C,L

LET Register Pair BC = Register Pair HL (the new record number).

46E2

LD (46F2H),A

Store the value held in Register A (the overflow, if any, from the multiplication) into the memory location 46F2H (the byte offset in the sector).

46E5

LD (46F7H),BC

Store the value held in Register BC (the new record number) into the memory location 46F7H (record number).

46E9

CALL 4868H

GOSUB to 4868H to write the buffer (if necessary).

46EC

RET NZ

If the NZ FLAG (Not Zero) is set then there was an error, so RETurn to the caller.

46ED

SET 5,(IX+01H)

SET (i.e., set as 1) BIT 5 of Register (IX+01H), which is the OPTIONS byte, to indicate that the sector is not loaded.

46F1

LD A,00H

LET Register A = 00H. I suspect this byte is modified by code.

46F3

LD (IX+05H),A

Store the value held in Register A into the memory location IX+05H (which is the BYTE POSITION FOR NEXT).

46F6

LD BC,0000H

LET Register Pair BC = 0000H. I suspect this byte is modified by code.

46F9

LD (IX+0AH),C

Store the value held in Register C (LSB of the new “NEXT” sector number) into the memory location IX+0AH (which is the LSB of the sector number of the next file).

46FC

LD (IX+0BH),B

Store the value held in Register B (MSB of the new “NEXT” sector number) into the memory location IX+0BH (which is the MSB of the sector number of the next file).

46FF

XOR A

Set Register A to ZERO and clear all Flags to indicate NO ERROR.

4700

RET

RETurn to the caller.

4701H – This is the VECTOR call for BACKSPACE – Back Up To the Previous Record in a File.

4701

GOSUB to 4881H to set up for I/O. This routine requires that Register Pair DE be pointing to the location in the buffer.

4704

LD C,(IX+0AH)

Fetch the value stored at memory location IX+0AH (which is the LSB of the sector number of the “next” file) and put it into Register C.

4707

LD B,(IX+0BH)

Fetch the value stored at memory location IX+0BH (which is the LSB of the sector number of the “next” file) and put it into Register B.

470A

BIT 7,(IX+01H)

Test Bit 7 of Register (IX+01H), which is the OPTIONS byte, to see if we were set up for SECTOR I/O.

470E

JR Z,471DH

If the Z FLAG (Zero) is set then we were set up for SECTOR I/O so JUMP to 471DH.

4710

LD A,(IX+09H)

Fetch the value stored at memory location IX+09H (Logical Record Length) and put it into Register A.

4713

OR A

Since a LD command does not set any FLAGS, Set FLAGS based on the contents of Register A.

4714

JR Z,471DH

If the Z FLAG (Zero) is set then we were set for FULL sector records so JUMP to 471DH.

4716

NEG

NEGate Register A so that it is -LRL

4718

ADD A,(IX+05H)

Apply the -LRL to (IX+05H) (which is the BYTE POSITION FOR NEXT) to adjust the “NEXT” value

471B

JR C,46E2H

If the C FLAG (No Carry) is set then we are still within the same sector so JUMP to 46E2H to finish up and RETurn.

471D

DEC BC

DECrement Register Pair BC by 1 to go to the prior sector.

471E

JR 46E2H

JUMP to 46E2H to finish up and RETurn.

4720H – This is the VECTOR call for REWIND – Position to the Start of a File.

4720

GOSUB to 4881H to set up for I/O. This routine requires that Register Pair DE be pointing to the location in the buffer.

4723

LD BC,0000H

LET Register Pair BC = 0000H so that the NEXT sector will be 0.

4726

XOR A

Set Register A to ZERO and clear all Flags so that the NEXT byte will be 0.

4727

JR 46E2H

JUMP to 46E2H to finish up and RETurn.

4729H – This is the VECTOR call for POSEOF – Position to the End of a File.

4729

GOSUB to 4881H to set up for I/O. This routine requires that Register Pair DE be pointing to the location in the buffer.

472C

LD C,(IX+0CH)

Fetch the value stored at memory location IX+0CH (LSB of the EOF sector number) and put it into Register C.

472F

LD B,(IX+0DH)

Fetch the value stored at memory location IX+0DH (MSB of the EOF sector number) and put it into Register B.

4732

LD A,(IX+08H)

Fetch the value stored at memory location IX+08H (the Byte Position of the EOF) and put it into Register A.

4735

JR 46E2H

JUMP to 46E2H to finish up and RETurn.

4737H – This is the VECTOR call for READ – Read the Logical Record at the “NEXT” Position.

4737

GOSUB to 4881H to set up for I/O. This routine requires that Register Pair DE be pointing to the location in the buffer.

473A

CALL 4868H

GOSUB to 4868H to write the buffer (if necessary).

473D

RET NZ

If the NZ FLAG (Not Zero) is set then we have an error so RETurn to the caller.

473E

RES 6,(IX+01H)

RESet (i.e., set as 0) BIT 6 of Register (IX+01H), which is the OPTIONS byte, to designate that that we need to update the EOF after all writes.

4742

BIT 7,(IX+01H)

Test Bit 7 of Register (IX+01H), which is the OPTIONS byte, to see if we were set up for SECTOR I/O.

4746

JR Z,4772H

If the Z FLAG (Zero) is set then we were set up for SECTOR I/O so JUMP to 4772H to check the file security settings.

4748

LD B,(IX+09H)

Fetch the value stored at memory location IX+09H (Logical Record Length) and put it into Register B. This will serve as a DJNZ loop to read that number of bytes.

474B
474C

PUSH HL
PUSH BC

Preserve Register Pairs HL and BC to the STACK because the next routine uses them both.

474D

CALL 4791H

GOSUB to 4791H to read a byte.

4750
4751

POP BC
POP BC

RESTORE Register Pairs BC and HL from the STACK.

4752

RET NZ

If the NZ FLAG (Not Zero) is set then we have an error so RETurn to the caller.

4753

LD (HL),A

Store the byte read from disk (held in Register A) into the memory buffer (pointed to by Register Pair HL).

4754

INC HL

Bump Register Pair HL by 1 to point to the next byte in the memory buffer.

4755

DJNZ 474BH

LOOP back to 474BH until Register B is ZERO.

4757

XOR A

Set Register A to ZERO and clear all Flags so as to indicate NO ERROR.

4758

RET

RETurn to the caller.

4759H – SUBROUTINE to first verify that READ ACCESS or higher is permitted, and then continue at 476CH. Called from 4772 and 4797.

4759

LD A,(IX+01H)

Fetch the value stored at memory location IX+01H, which is the OPTIONS byte, and put it into Register A.

475C

AND 07H

MASK the value of Register A against 07H (0000 0111). This has the effect of turning off bits 7, 6, 5, 4, 3, leaving only bits 2, 1, 0 (the ACCESS LEVEL) active.

475E

CP 06H

Compare the value held in Register A against 06H to check for READ LEVEL ACCESS or higher. Results:

If Register A equals 06H, the Z FLAG is set.
If A < 06H, the CARRY FLAG will be set.
if A >= 06H, the NO CARRY FLAG will be set.

4760

JR C,476CH

If the C FLAG (No Carry) is set then we have READ LEVEL ACCESS or higher so, JUMP to 476CH.

4762

LD A,19H

LET Register A = 19H to set up fror a FILE ACCESS DENIED error.

4764

OR A

Since a LD command does not set any FLAGS, Set FLAGS based on the contents of Register A.

4765

RET

RETurn to the caller.

4766H – Read the Sector / Return With Error or Continue. Called from 4C58.

4766

CALL 476CH

GOSUB to 476CH to read a sector from the file.

4769

RET NZ

If the NZ FLAG (Not Zero) is set then there is an error, so RETurn to the caller.

476A

JR 4776H

JUMP to 4776H to update “NEXT” and RETURN.

476CH – Routine to check If Past EOF / Return With Error or Continue at 4780H.

476C

CALL 4A2EH

GOSUB to 4A2EH to Check to see if we are past the EOF.

476F

RET NZ

If the NZ FLAG (Not Zero) is set then there is an error, so RETurn to the caller.

4770

JR 4780H

JUMP to 4780H.

4772H – Routine to check the file security settings and increase the IX next sector memory locations.

4772

CALL 4759H

GOSUB to 4759H to check the file security settings.

4775

RET NZ

If the NZ FLAG (Not Zero) is set then we have an error, so RETurn to the caller.

4776

INC (IX+0AH)

Bump the “NEXT” sector LSB. IX+0AH holds the LSB of the sector number of the “next” file.

4779

JR NZ,477EH

If the NZ FLAG (Not Zero) is set then don’t have an overflow (i.e., no need to update the MSB), so skip the next instruction.

477B

INC (IX+0BH)

Bump the “NEXT” sector MSB because the bump of the LSB cycled back to zero. IX+0BH holds the MSB of the sector number of the “next” file.

477E

XOR A

Set Register A to ZERO and clear all Flags.

477F

RET

RETurn to the caller.

4780H – Continues from routine at 476CH (which makes sure we are not past EOF). Sets up HL for re-entry into the position head and read sector routine.

4780

CALL 49D4H

GOSUB to 49D4H to find the physical track and sector number of the “NEXT” sector.

4783

RET NZ

If the NZ FLAG (Not Zero) is set then we have an error, so RETurn to the caller.

4784

RES 5,(IX+01H)

RESet (i.e., set as 0) BIT 5 of Register (IX+01H), which is the OPTIONS byte, to indicate sector IS loaded.

4788

LD L,(IX+03H)

Fetch the value stored at memory location IX+03H (the LSB of the 2 byte BUFFER ADDRESS) and put it into Register L.

478B

LD H,(IX+04H)

Fetch the value stored at memory location IX+04H (the MSB of the 2 byte BUFFER ADDRESS) and put it into Register H.

478E

JP 4675H

JUMP to 4675H (the Position the Head and Read Requested Sector routine; although THIS time witH HL set).

4791H – Continues the READ LOGICAL RECORD routine

4791

BIT 5,(IX+01H)

Test Bit 5 of Register (IX+01H), which is the OPTIONS byte, to see if the current sector is loaded.

4795

JR Z,479BH

If the Z FLAG (Zero) is set then the current sector is already loaded so JUMP to 479BH.

4797

CALL 4759H

GOSUB to 4759H to check the file’s security.

479A

RET NZ

If the NZ FLAG (Not Zero) is set then we have an error, so RETurn to the caller.

479B

CALL 4A2EH

GOSUB to 4A2EH to check to see if we are past the EOF.

479E

RET NZ

If the NZ FLAG (Not Zero) is set then we have an error, so RETurn to the caller.

479F

CALL 4872H

GOSUB to 4872H to determine the applicable byte position in the buffer (by adding the offset to the base buffer address, and then loading Register Pair DE to point to the buffer address of that specific byte).

47A2

XOR A

Set Register A to ZERO and clear all Flags.

47A3

LD A,(DE)

Fetch the value stored at memory location pointed to by Register Pair DE (which is the byte from the buffer) and put it into Register A.

47A4

PUSH AF

Preserve Register AF (which includes the byte from the buffer) to the top of the STACK.

47A5

INC (IX+05H)

IX+05H is the BYTE POSITION FOR NEXT, so we need to bump that to the next byte

47A8

JR NZ,47B1H

If the NZ FLAG (Not Zero) is set then we don’t have a new sector, so skip the next 2 bytes and JUMP to 47B1H.

47AA

SET 5,(IX+01H)

SET (i.e., set as 1) BIT 5 of Register (IX+01H), which is the OPTIONS byte, to signify that the current sector is NOT loaded.

47AE

CALL 4776H

GOSUB to 4776H to update the “NEXT” sector.

47B1

POP AF

Restore Register Pair AF (including the byte read, which is held in Register A) from the STACK.

47B2

RET

RETurn to the caller.

47B3H – This is the WRITE VECTOR. This routine writes the file sector WITHOUT verify.

47B3

GOSUB to 4881H to prepare for the write. This routine requires that Register Pair DE be pointing to the location in the buffer.

47B6

BIT 7,(IX+01H)

Test Bit 7 of Register (IX+01H), which is the OPTIONS byte, to see if we are looking for SECTOR I/O.

47BA

JR Z,47E9H

If the Z FLAG (Zero) is set then we are looking for SECTOR I/O so JUMP to 47E9H.

47BC

LD B,(IX+09H)

Fetch the value stored at memory location IX+09H, which is the LRL, and put it into Register B since Register B will be the counter for the DJNZ write loop.

Start of a loop for the entire LRL of the sector.

47BF

LD A,(HL)

Fetch the byte to write (which is the value stored at memory location pointed to by Register Pair HL) and put it into Register A.

47C0

INC HL

Bump the pointer (held in Register Pair HL) by 1.

47C1

PUSH HL

PUSH Register HL to the top of the STACK.

47C2

PUSH BC

PUSH Register BC to the top of the STACK.

47C3

CALL 4837H

GOSUB to 4837H to write the byte to diskette.

47C6

POP BC

MOVE the value held at the stop of the STACK into Register Pair BC.

47C7

POP HL

MOVE the value held at the stop of the STACK into Register Pair HL.

47C8

RET NZ

If the NZ FLAG (Not Zero) is set then we have an error, so RETurn to the caller.

47C9

DJNZ 47BFH

LOOP back to 47BFH until Register B (which was the LRL) is ZERO.

End of the loop.

47CB

RET

RETurn to the caller.

47CCH – This is the WRITE VECTOR. This routine writes the file sector WITH verify.

47CC

GOSUB to 4881H to prepare for the write. This routine requires that Register Pair DE be pointing to the location in the buffer.

47CF

LD B,03H

LET Register B = 03H for a write of up to three tries.

47D1

PUSH BC

PUSH Register BC (i.e., save the counter held in Register B) to the top of the STACK because the B is used for the LRL counter in 47B6H.

47D2

CALL 47B6H

GOSUB to 47B6H in the prior routine to loop the write of the sector.

47D5

JR NZ,47E7H

If the NZ FLAG (Not Zero) is set then we have an error, so JUMP to 47E7H (to restore BC from the STACK and RETurn).

47D7

LD BC,0000H

LET Register Pair BC = 0000H. Note: This value will be changed dynamically as part of DOS operations. B is the file number, C is the drive number.

47DA

LD DE,0000H

LET Register Pair DE = 0000H. Note: This value will be changed dynamically as part of DOS operations. D is the track number, E is the sector number.

47DD

CALL 4672H

GOSUB to 4672H to perform a dummy read.

47E0

JR Z,47E7H

If the Z FLAG (Zero) is set then we got a good read, so JUMP to 47E7H (to restore BC from the STACK and RETurn).

47E2

POP BC

Restore Register Pair BC from the top of the STACK to get the “3 try” counter back into Register B.

47E3

DJNZ 47D1H

LOOP back to 47D1H until Register B is ZERO (for a total of 3 tries).

47E5

OR A

Set FLAGS based on the contents of Register A.

47E6

RET

RETurn to the caller.

47E7H – This little routine is called both when there is an error, or when there isn’t, in the above routine.

47E7

POP BC

MOVE the value held at the stop of the STACK into Register Pair BC.

47E8

RET

RETurn to the caller.

47E9H – Called from 47B3H (the WRITE VECTOR) if it was determined that SECTOR I/O was required.

47E9

SET 6,(IX+01H)

SET (i.e., set as 1) BIT 6 of Register (IX+01H), which is the OPTIONS byte, to signify RANDOM I/O.

47ED

LD A,(IX+01H)

Fetch the value stored at memory location IX+01H (the OPTIONS byte) and put it into Register A.

47F0

AND 07H

MASK the value of Register A against 07H (0000 0111). This has the effect of turning off bits 7, 6, 5, 4, 3, leaving only bits 2, 1, 0 active. Those bits are the ACCESS CODE.

47F2

CP 05H

Compare the value held in Register A against 05H. Results:

If Register A equals 05H, the Z FLAG is set.
If A < 05H, the CARRY FLAG will be set.
if A >= 05H, the NO CARRY FLAG will be set.

47F4

JP NC,4762H

If the NC FLAG (No Carry) is set then we do not have WRITE access so JUMP to 4762H.

47F7

CALL 49D4H

GOSUB to 49D4H to find the physical location of the track and sector for the I/O.

47FA

JR Z,4803H

If the Z FLAG (Zero) is set then we had no errors in that CALL so JUMP to 4803H.

If we are here then we have an error, so we need to figure out which error and waht to do about it.

47FC

CP 1DH

Compare the value held in Register A against 1DH to see if we are past the EOF. Results:

If Register A equals 1DH, the Z FLAG is set.
If A < 1DH, the CARRY FLAG will be set.
if A >= 1DH, the NO CARRY FLAG will be set.

47FE

RET NZ

If the NZ FLAG (Not Zero) is set, meaning we are NOT past the EOF, RETurn to the caller.

47FF

CALL 48ACH

GOSUB to 48ACH to allocate filespace and expand the file.

4802

RET NZ

If the NZ FLAG (Not Zero) is set then we have an error from that CALL so RETurn to the caller.

4803

LD L,(IX+03H)

Fetch the value stored at memory location IX+03H (the LSB of the 2 byte BUFFER ADDRESS) and put it into Register L.

4806

LD H,(IX+04H)

Fetch the value stored at memory location IX+04H (the MSB of the 2 byte BUFFER ADDRESS) and put it into Register H.

4809

GOSUB to 45F7H to write the sectors.

480C

RET NZ

If the NZ FLAG (Not Zero) is set then we have an error from that CALL so RETurn to the caller.

480D

SET 3,(IX+01H)

SET (i.e., set as 1) BIT 3 of Register (IX+01H), which is the OPTIONS byte.

4811

RES 4,(IX+01H)

RESet (i.e., set as 0) BIT 4 of Register (IX+01H), which is the OPTIONS byte.

4815

CALL 4A2EH

GOSUB to 4A2EH to see if we have past the EOF.

4818

JR Z,482AH

If the Z FLAG (Zero) is set then we have NOT gone past the EOF so JUMP to 482AH.

481A

BIT 6,(IX+01H)

Test Bit 6 of Register (IX+01H), which is the OPTIONS byte for EOF.

481E

JR Z,4821H

If the Z FLAG (Zero) is set then we do not need to update the EOF so JUMP to 4821H.

4820

INC HL

Bump Register Pair HL (which holds the EOF) by 1.

4821

LD (IX+0CH),L

Store the value held in Register L (LSB of the EOF) into the memory location IX+0CH (LSB of the EOF sector number).

4824

LD (IX+0DH),H

Store the value held in Register H (MSB of the EOF) into the memory location IX+0DH (MSB of the EOF sector number).

4827

LD (IX+08H),C

Store the value held in Register C into the memory location IX+08H (the Byte Position of the EOF).

482A

BIT 6,(IX+01H)

Test Bit 6 of Register (IX+01H), which is the OPTIONS byte, to see if we need to update the EOF.

482E

CALL NZ,4776H

If the NZ FLAG (Not Zero) is set, then we don’t have to update the EOF, but we need to update the “NEXT” so GOSUB to 4776H.

4831

RES 6,(IX+01H)

RESet (i.e., set as 0) BIT 6 of Register (IX+01H), which is the OPTIONS byte, to flag that we need to udpate the EOF.

4835

XOR A

Set Register A to ZERO and clear all Flags.

4836

RET

RETurn to the caller.

4837H – Subroutine to Write a Byte at the “Next” Position.

4837

PUSH AF

PUSH Register Pair AF (which contains the byte to write) to the top of the STACK.

4838

SET 3,(IX+01H)

SET (i.e., set as 1) BIT 3 of Register (IX+01H), which is the OPTIONS byte.

483C

BIT 5,(IX+01H)

Test Bit 5 of Register (IX+01H), which is the OPTIONS byte to test to see if the current sector is loaded.

4840

JR Z,4850H

If the Z FLAG (Zero) is set then the current sector is loaded so JUMP to 4850H.

4842

CALL 4780H

GOSUB to 4780H to load the current sector.

4845

JR Z,4850H

If the Z FLAG (Zero) is set then we had no errors so continue at 4850H.

4847

AND FEH

If we’re here then we had an error, so we need to find out which one. First, MASK the value of Register A against FEH (1111 1110). This has the effect of turning off bit 0 (the BUSY bit), leaving only bits 7, 6, 5, 4, 3, 2, 1 active.

4849

CP 1CH

Compare the value held in Register A against 1CH (END OF FILE error). Results: If Register A equals 1CH, the Z FLAG is set; otherwise the NZ FLAG is set.

484B

JR Z,4850H

If the Z FLAG (Zero) is set then we hit the END OF FILE so JUMP to 4850H.

484D

EX (SP),HL

Clear the STACK.

484E

POP HL

Clear the STACK.

484F

RET

RETurn to the caller.

4850H – Called from the prior WRITE A BYTE AT NEXT POSITION routine if the current sector is already loaded.

4850

CALL 4872H

GOSUB to 4872H to compute the applicable position in the buffer (by adding the offset to the base buffer address, and then loading Register Pair DE to point to the buffer address of that specific byte).

4853

POP AF

Restore the byte to be written from the top into Register A.

4854

LD (DE),A

Store the byte to be written (held in Register A) into the buffer (the memory location pointed to by Register Pair DE).

4855

INC DE

Move to the next buffer location by bumping Register Pair DE (which is the pointer) by 1.

4856

SET 4,(IX+01H)

SET (i.e., set as 1) BIT 4 of Register (IX+01H), which is the OPTIONS byte, to indicate that there is unwritten data in the buffer.

485A

INC (IX+05H)

IX+05H is the BYTE POSITION FOR NEXT in the FCB, so bump that as well.

485D

JR Z,4861H

If the Z FLAG (Zero) is set then we have pushed from 256 bytes to 0, so it is time to actually write the buffer. To do so JUMP to 4861H.

485F

XOR A

Set Register A to ZERO and clear all Flags to indicate no error.

4860

RET

RETurn to the caller.

4861H – Jumped to from the prior routine when it is time to write the buffer out to diskette because the buffer is full.

4861

SET 5,(IX+01H)

SET (i.e., set as 1) BIT 5 of Register (IX+01H), which is the OPTIONS byte, to indicate that the sector is NOT loaded.

4865

JP 47E9H

JUMP to 47E9H (the WRITE VECTOR) to write the sector.

4868H – Write the Buffer to Disk (if necessary).

4868

LD A,(IX+01H)

Fetch the value stored at memory location IX+01H (which is the OPTIONS byte) and put it into Register A.

486B

AND 50H

MASK the value of Register A against 50H (0101 0000). This has the effect of turning off bits 7, 5, 3, 2, 1, 0, leaving only bits 6 (random I/O selected, 1=random), 4 (Unwritten data in the buffer, 1=yes) active.

486D

JP NZ,47EDH

If the NZ FLAG (Not Zero) is set then either RANDOM I/O was selected or there is unwritten data in the buffer. If either is the case JUMP to 47EDH to write the sector.

4870

XOR A

Set Register A to ZERO and clear all Flags to indicate no error.

4871

RET

RETurn to the caller.

4872H – Compute the Applicable Position in the Buffer.

4872

LD A,(IX+05H)

Fetch the value stored at memory location IX+05H (which is the BYTE POSITION FOR NEXT) and put it into Register A. This will act as an offset value.

4875

LD C,A

Save the offset value in Register C.

4876

ADD A,(IX+03H)

IX+03H holds the LSB of the 2 byte BUFFER ADDRESS, so this adds the OFFSET to the BASE buffer address

4879

LD E,A

Put the Offset + Base address into Register E (so that Register Pair DE will point to the actual offset byte in the buffer).

487A

LD A,(IX+04H)

Fetch the value stored at memory location IX+04H (MSB of the 2 byte BUFFER ADDRESS) and put it into Register A.

487D

ADC 00H

This is cute. If the offset is such that it would have caused an overflow (i.e., CARRY) when added to the buffer, the CARRY FLAG will be set to 1. By doing an ADD WITH CARRY for a value of 00H, this will increase the MSB by 1 if there was a carry, and increase the MSB by 0 if there wasn’t!

487F

LD D,A

To round out having the MSB and LSB of the BUFFER which points to the OFFSET byte held in Register Pair DE, let Register D = Register A.

4880

RET

RETurn to the caller.

4881H – This routine is called a LOT and prepares for an actual diskette write.

4881

LD A,(DE)

Fetch the value stored in the buffer (the memory location pointed to by Register Pair DE) and put it into Register A.

4882

RLCA

Rotate the bits in A left (with Bit 7 going into both the CARRY and Bit 0). This will have the effect of checking to see if the file is open by putting that indicator bit into the CARRY FLAG.

4883

JR NC,48A7H

If the NC FLAG (No Carry) is set, then the file isn’t open, and we cannot do a write, so EXIT by JUMPing to 48A7H.

4885

POP AF

Get the return address by moving the value held at the stop of the STACK into Register Pair AF.

4886

EX (SP),HL

Save the calling address (which is at the top of the STACK) by putting it into Register Pair HL.

4887

LD (44A9H),HL

Store the value held in Register HL into the memory location 44A9H.

NOTE: 44A9H is the storage location for the Saved Caller Address for DOS functions (2 Bytes).

488A

LD (44ABH),DE

Store the value held in Register DE into the memory location 44ABH.

NOTE: 44ABH is the storage location for the Saved FCB Address for DOS functions (2 Bytes).

488E

EX (SP),HL

Take the value that was held in 44A9H and overwrite the value at the top of the STACK.

488F

PUSH DE

PUSH Register DE (the buffer memory location) to the top of the STACK.

4890

EX (SP),IX

Swap the value that was held in 44A9H into Register Pair IX, and put the base DCB value (which is held in IX) into the top of the STACK.

4892

PUSH HL

PUSH Register HL to the top of the STACK.

4893

PUSH DE

PUSH Register DE to the top of the STACK.

4894

PUSH BC

PUSH Register BC to the top of the STACK.

4895

PUSH IY

PUSH Register IY to the top of the STACK.

4897

PUSH HL

PUSH Register HL to the top of the STACK. Why push it again? This is to set up a for a routine return via 489FH.

4898

LD HL,489FH

LET Register Pair HL = 489FH.

489B

EX (SP),HL

Put 489FH at the top of the STACK

489C

PUSH AF

PUSH Register AF to the top of the STACK so as to save t he old return.

489D

XOR A

Set Register A to ZERO and clear all Flags to signify no error.

489E

RET

RETurn to the caller.

489FH – This routine will finish up the diskette write and RETURN. It was set in 489BH.

489F

POP IY

MOVE the value held at the stop of the STACK into Register Pair IY.

48A1

POP BC

MOVE the value held at the stop of the STACK into Register Pair BC.

48A2

POP DE

MOVE the value held at the stop of the STACK into Register Pair DE.

48A3

POP HL

MOVE the value held at the stop of the STACK into Register Pair HL.

48A4

POP IX

MOVE the value held at the stop of the STACK into Register Pair IX.

48A6

RET

RETurn to the caller.

48A7H – This routine is JUMPed to from 4883H if the file we wanted to do I/O upon was not open.

48A7

POP AF

Since we are aborting; clear the STACK.

48A8

LD A,26H

LET Register A = 26H, which is the FILE NOT OPEN error.

48AA

OR A

Since a LD command does not set any FLAGS, set FLAGS based on the contents of Register A, which will be NZ for sure to signify an error.

48AB

RET

RETurn to the caller.

48ACH – This subroutine is called from 47FF to allocate filespace (and/or expand if we hit the EOF).

48AC

LD A,(IX+07H)

Fetch the value stored at memory location IX+07H (which is the LOGICAL FILE NUMBER IN THE DIRECTORY) and put it into Register A.

48AF

LD B,A

Preserve that file number into Register B.

48B0

LD C,(IX+06H)

Fetch the value stored at memory location IX+06H (which is the DRIVE NUMBER OF THE FILE) and put it into Register C.

48B3

CALL 4A93H

GOSUB to 4A93H to read the GAT into Buffer # 2.

48B6

RET NZ

If the NZ FLAG (Not Zero) is set, then we have an error, so RETurn to the caller.

48B7

CALL 4A67H

GOSUB to 4A67H to read the directory entry directly to Buffer # 1.

48BA

RET NZ

If the NZ FLAG (Not Zero) is set, then we have an error, so RETurn to the caller.

48BB

LD (4955H),HL

Store the directory entry pointer (held in Register HL) into the memory location 4955H (which is actually the middle of an OPCODE for LD HL,xxxx).

48BE

LD L,(IX+0AH)

Fetch the value stored at memory location IX+0AH (which is the LSB of the sector number of the “next” file) and put it into Register L.

48C1

LD H,(IX+0BH)

Fetch the value stored at memory location IX+0BH (which is the MSB of the sector number of the “next” file) and put it into Register H.

48C4

LD E,(IX+0CH)

Fetch the value stored at memory location IX+0CH (which is the LSB of the EOF sector number) and put it into Register E.

48C7

LD D,(IX+0DH)

Fetch the value stored at memory location IX+0DH (which is the MSB of the EOF sector number) and put it into Register D.

48CA

OR A

Set FLAGS based on the contents of Register A which is really just trying to clear the CARRY FLAG as we are about to use it.

48CB

SBC HL,DE

HL = HL – DE (with Carry), so this way HL = the NEXT – the EOF.

48CD

JR NC,48D2H

If the NC FLAG (No Carry) is set, then NEXT=EOF so we can skip the next code and JUMP to 48D2H.

48CF

LD HL,0001H

LET Register Pair HL = 0001H to set up for an allocation of ONE sector.

48D2

LD A,03H

To prepare for a division routine, set Register A = 03H.

48D4

CALL 4B6FH

GOSUB to 4B6FH to divide HL by 3, so as to determine the number of grans.

48D7

INC HL

No matter what, HL needs to be higher than ZERO, so bump Register Pair HL by 1 so we have at least 1 gran. This leaves Register Pair HL holding the number of grans to allocate.

48D8

LD (4942H),HL

Store the value held in Register HL (the number of grans to allocate) into the memory location 4942H (which is actually the middle of an OPCODE for LD HL,xxxx).

The next 4 instructions are designed to set Register Pair HL to be the extent elements by setting HL as a base of 10, putting the DCB into Register Pair DE, and then adding DE to HL.

48DB

LD HL,0010H

LET Register Pair HL = 0010H as a base value

48DE

PUSH IX

PUSH Register IX to the top of the STACK.

48E0

POP DE

Restore Register IX from the top of the STACK into Register Pair DE.

48E1

ADD HL,DE

ADD the value held in Register Pair DE to Register Pair HL. The results are held in Register Pair HL.

48E2

LD A,(HL)

Fetch the value stored at memory location pointed to by Register Pair HL (which is the first element) and put it into Register A.

48E3

CP FFH

Compare the value held in Register A against FFH to see if it is free. If Register A equals FFH, the Z FLAG is set; otherwise the NZ FLAG is set.

48E5

DEC HL

DECrement Register Pair HL by 1 to back one one byte because the JP in the next instruction starts with an INC HL.

48E6

JP Z,4985H

If the Z FLAG (Zero) is set, meaning that the element is free, JUMP to 4985H (to search for a free gran and if none found, return DISK FULL).

48E9

LD B,1AH

LET Register B = 1AH as a counter to go through 26 bytes.

48EB

INC HL

Bump Register Pair HL by 1 to get to the next byte.

48EC

LD A,(HL)

Fetch the value stored at memory location pointed to by Register Pair HL (which is the next element) and put it into Register A.

48ED

CP FFH

Compare the value held in Register A against FFH to see if it is free. If Register A equals FFH, the Z FLAG is set; otherwise the NZ FLAG is set.

48EF

JR Z,48F7H

If the Z FLAG (Zero) is set then the element is free so stop looking and JUMP to 48F7H.

48F1

DJNZ 48EBH

LOOP back to 48EBH until Register B is ZERO.

48F3

LD A,1EH

LET Register A = 1EH to signify NO MORE EXTENTS AVAILABLE

48F5

OR A

Since a LD command does not set any FLAGS, set FLAGS based on the contents of Register A, which will be NZ for sure to signify an error.

48F6

RET

RETurn to the caller.

48F7H – Check to see if there are free extents.

48F7
48F8

DEC HL
DEC HL

DECrement Register Pair HL by 2 so as to back up to the start of the last extent.

48F9

LD A,(HL)

Fetch the value stored at memory location pointed to by Register Pair HL (which would be the track number) and put it into Register A.

48FA

LD DE,4D00H

LET Register Pair DE = 4D00H so as to point to the GAT table.

48FD

ADD A,E

ADD the value held in Register E to Register A (Results held in Register A) to point to the entry for the given track.

48FE

LD E,A

Preserve the entry for the given track into Register E.

48FF

INC HL

Bump Register Pair HL by 1 to point to the storage location for the number of grans in the selected extent.

4900

LD A,(HL)

Fetch the value stored at memory location pointed to by Register Pair HL (i.e., the number of grants in the selected extent) and put it into Register A.

4901

AND 1FH

MASK the value of Register A against 1FH (0001 1111). This has the effect of turning off bits 7, 6, 5, leaving only bits 4, 3, 2, 1, 0 active.

4903

CP 1FH

Compare the value held in Register A against 1FH to see if the extent is full. Results: If Register A equals 1FH (= the extent is full), the Z FLAG is set; otherwise the NZ FLAG (= not full) is set.

4905

JR NZ,490AH

If the NZ FLAG (Not Zero) is set, then the extent is not full, so JUMP to 490AH.

4907

INC HL

If we are here, then Z was set and the extent is full, so we need to bump Register Pair HL by 1 to check for the next extent.

4908

JR 48E2H

LOOP BACK to 48E2H.

490AH – This routine is JUMPed to from 4905H if an open extent is found.

490A

LD A,(HL)

Fetch the value stored at memory location pointed to by Register Pair HL (the extent data) and put it into Register A.

490B

PUSH AF

PUSH Register AF to the top of the STACK.

The next few instructions basically set Register C to be the number of grans offset from the start of the track.

490C

AND E0H

MASK the value of Register A against E0H (1110 0000). This has the effect of turning off bits 4, 3, 2, 1, 0, leaving only bits 7, 6, 5 active.

490E

RLCA

Move bits 7,6,5 to 0,7,6 by rotating the bits in A left (with Bit 7 going into both the CARRY and Bit 0).

490F

RLCA

Move bits 0,7,6 to 1,0,7 by rotating the bits in A left (with Bit 7 going into both the CARRY and Bit 0).

4910

RLCA

Move bits 1,0,7 to 2,1,0 by rotating the bits in A left (with Bit 7 going into both the CARRY and Bit 0).

4911

LD C,A

LET Register C = Register A.

The next few instructions basically set Register A to be the number of grans in the extent.

4912

POP AF

MOVE the value held at the stop of the STACK into Register Pair AF.

4913

AND 1FH

MASK the value of Register A against 1FH (0001 1111). This has the effect of turning off bits 7, 6, 5, leaving only bits 4, 3, 2, 1, 0 active.

So now C holds the number of grans offset from the start of the track, and A holds the grans in the extent.

4915

ADD A,C

ADD the value held in Register C to Register A (Results held in Register A) so that Register A now equals the number of grans in total from the start.

4916

PUSH HL

Preserve Register Pair HL by pushing it to the top of the STACK.

4917
4918

LD L,A
LD H,00H

Set Register Pair HL to be the number of grans total from the start of the track.

491A

LD A,06H

Since TRSDOS v1.3 has 6 grans per track, set up Register A with a 6.

491C

CALL 4B6FH

GOSUB to 4B6FH to compute the track offset.

491F

PUSH AF

PUSH Register AF (the number of grans to go) to the top of the STACK.

There aren’t a lot of variables in the Z-80 so the next 3 instructions simply set DE to point to the GAT Table, by setting A=L, A=A+E, E=A.

4920

LD A,L

LET Register A = Register L.

4921

ADD A,E

ADD the value held in Register E to Register A (Results held in Register A).

4922

LD E,A

LET Register E = Register A.

4923

POP BC

MOVE the value held at the stop of the STACK (the number of grans into the track) into Register Pair BC.

4924

POP HL

Restore Register Pair HL from the top of the STACK.

4925

CP 28H

Test Register A against 40 (which is the highest number of tracks). Results:

If Register A equals 28H, the Z FLAG is set.
If A < 28H, the CARRY FLAG will be set.
if A >= 28H, the NO CARRY FLAG will be set.

4927

JP NC,4985H

If the NC FLAG (No Carry) is set then we have exceeded track 40, which is a problem, so JUMP to 4985H (to search for a free gran and if none found, return DISK FULL).

492A

LD A,(DE)

Fetch the value stored at memory location pointed to by Register Pair DE (the GAT entry) and put it into Register A.

492B

CP 3FH

Compare the value held in Register A against 3FH to see if the track is completely allocated or not. Results: If Register A equals 3FH then the track is completely allocated and the Z FLAG is set; otherwise the NZ FLAG is set.

492D

JR Z,4985H

If the Z FLAG (Zero) is set then the track is completely allocated so JUMP to 4985H (to search for a free gran and if none found, return DISK FULL).

492F

LD C,B

If we are here, then the track has room so we continue by setting Register C = the gran offset (held in Register B).

4930

LD B,A

LET Register B = the GAT entry (held in Register A).

4931

LD A,C

LET Register A = Register C (the gran offset).

4932

CALL 4B48H

GOSUB to 4B48H to test to see if the gran is allocated.

4935

JP NZ,4985H

If the NZ FLAG (Not Zero) is set then the gran IS allocated, so create a new extent by JUMPing to 4985H.

4938

LD A,C

If we are here, then the gran is not allocated. LET Register A = Register C (the gran offset).

4939

GOSUB to 45E9H (the routine to calculate the DRIVE SELECT CODE to be used). This will set the bit for the gran.

493C

EX DE,HL

Swap Register Pair DE (the GAT table) and Register Pair HL (the extent element).

493D

OR (HL)

OR the value stored at (HL) against Register A to allocate the gran. The results are stored in Register A.

493E

LD (HL),A

Store the value held in Register A into the GAT table (i.e., the memory location pointed to by Register Pair HL).

493F

EX DE,HL

Swap Register Pair De and Register Pair HL back. Now HL will contain the extent element.

4940

INC (HL)

Since HL points to the extent element, bump it by 1 to allocate another gran in the extent

The next bunch of instructions set that there is now one less gran on the disk to allocate.

4941

LD HL,0000H

LET Register Pair HL = 0000H. Note: This 0000H is replaced at 48D8H and at 4945H with other values, and is supposed to be the number of grans to allocate.

4944

DEC HL

Reduce the number of grans remaining available by (held in Register Pair HL) by 1.

4945

LD (4942H),HL

Store the value held in Register HL (the newly reduced number of available grans) into the memory location 4942H (which is actually the middle of an OPCODE for LD HL,xxxx).

4948
4949

LD A,H
OR L

Since the Z-80 cannot test Register Pair HL against zero, the common trick is to set Register A to one, and Or the other. Only if every single bit was off in both will the Z FLAG be set.

494A

JP NZ,48DBH

If the NZ FLAG (Not Zero) is set, meaning that HL was not zero, LOOP BACK to 48DBH.

494D

CALL 4954H

GOSUB to 4954H.

4950

RET NZ

If the NZ FLAG (Not Zero) is set then there was an error from that routine, so RETurn to the caller.

4951

JP 49D4H

JUMP to 49D4H to continue the routine.

4954H – This subroutine is called from 494DH to continue with the file space allocation.

The first part of this routine figures out what the EOF data is in the buffer and then saves the new EOF in the directory.

4954

LD HL,0000H

LET Register Pair HL = 0000H. This value gets repopulated at 48BBH to be the applicable directory entry pointer.

4957

LD DE,0014H

LET Register Pair DE = 0014H, which is the EOF data in the ENTRY.

495A

ADD HL,DE

ADD the value held in Register Pair DE (the EOF data in the entry) to Register Pair HL (the directory entry). The results, which should be the EOF data in the buffer) are held in Register Pair HL.

495B

LD A,(IX+0AH)

Fetch the value stored at memory location IX+0AH (which is the LSB of the sector number of the next file) and put it into Register A.

495E

LD (HL),A

Store the value held in Register A into the memory location pointed to by Register Pair HL.

495F

INC HL

Bump Register Pair HL by 1 to point to the next location.

4960

LD A,(IX+0BH)

Fetch the value stored at memory location IX+0BH (which is the MSB of the sector number of the next file) and put it into Register A.

4963

LD (HL),A

Store the value held in Register A into the memory location pointed to by Register Pair HL.

4964

INC HL

Bump Register Pair HL by 1.

Now that the new EOF has ben saved, need to deal with the extents.

4965

PUSH IX

PUSH Register IX (which is the point to the DCB in RAM) to the top of the STACK.

4967

LD DE,0010H

LET Register Pair DE = 0010H, which is the offset for the extent information.

496A

ADD IX,DE

ADD the value held in Register Pair DE (the offset) to Register Pair IX (the DCB). The results are held in Register Pair IX.

496C
496E

PUSH IX
POP DE

Copy IX to DE via a PUSH and a POP.

496F

POP IX

Restore the DCB to Register Pair IX.

4971

EX DE,HL

Swap Register Pair DE and Register Pair HL (so now HL will hold the extent information)

4972

LD BC,001AH

LET Register Pair BC = 001AH to set up a LDIR to move 26 bytes from the DCB to the directory.

4975

LDIR

Do that loop

4977

LD C,(IX+06H)

Fetch the value stored at memory location IX+06H (which is the DRIVE NUMBER OF THE FILE) and put it into Register C.

497A

LD B,(IX+07H)

Fetch the value stored at memory location IX+07H (which is the LOGICAL FILE NUMBER IN THE DIRECTORY) and put it into Register B.

497D

CALL 4A9BH

GOSUB to 4A9BH to write the new GAT sector.

4980

RET NZ

If the NZ FLAG (Not Zero) is set then there was an error from that routine, so RETurn to the caller.

4981

CALL 4A7BH

GOSUB to 4A7BH to write the directory entry.

4984

RET

RETurn to the caller.

4985H – Routine to search for an available gran and if none, error out.

4985

INC HL

Bump Register Pair HL by 1 so that it points to the next extent element.

4986

PUSH HL

Preserve Register Pair HL by PUSHing Register HL to the top of the STACK.

4987

LD (49C9H),HL

Preserve Register Pair HL AGAIN by storing it into the memory location 49C9H (whcih is actuall the opcode LD DE,nnnn).

498A

CALL 4997H

Search for a free gran by GOSUBing to 4997H. That routine should return a Z FLAG if a free gran has been found.

498D

POP HL

Restore HL from the top of the STACK.

498E

JR Z,4941H

If the Z FLAG (Zero) is set then a free gran was found so JUMP to 4941H.

4990

CALL 4954H

GOSUB to 4954H to preserve the new allocation information.

4993

LD A,1BH

LET Register A = 1BH, which is the error code for DISK FULL

4995

OR A

Since a LD command does not set any FLAGS, Set FLAGS based on the contents of Register A and, based on A, it will definitely be NZ FLAG.

4996

RET

RETurn to the caller.

4997H – SUBROUTINE called from 4997H to find a free gran on a diskette and either JUMP to 49A7H if there is one or error out if there isn’t.

4997

LD HL,4D00H

LET Register Pair HL = 4D00H which is the buffer address of the GAT.

499A

LD B,28H

LET Register B = 28H (Decimal: 40) to set a loop for 40 tracks to search.

499C

LD A,(HL)

Fetch the status of the current gran (which is the value stored at memory location pointed to by Register Pair HL) and put it into Register A.

499D

CP 3FH

Compare the value held in Register A against 3FH to test if that gran is full. Results: If Register A equals 3FH (the gran is full), the Z FLAG is set; otherwise the NZ FLAG is set.

499F

JR NZ,49A7H

If the NZ FLAG (Not Zero) is set then the gran is NOT full, so JUMP to 49A7H.

49A1

INC L

Bump Register L by 1 so that HL will point to the next entry.

49A2

DJNZ 499CH

LOOP back to 499CH until Register B is ZERO (i.e., all 40 tracks have been checked).

49A4

OR FFH

OR Register A against FFH to set the flags and otherwise prepare for a GRAN NOT FOUND.

49A6

RET

RETurn to the caller.

49A7H – Search for a free gran.

49A7

PUSH HL

PUSH Register HL (the element pointer) to the top of the STACK.

49A8

LD DE,0060H

LET Register Pair DE = 0060H as a base address to the granule existance table.

49AB

ADD HL,DE

ADD the value held in Register Pair DE to Register Pair HL so that HL now points to the appropriate element in the table.

49AC

LD A,(HL)

Fetch the value stored at memory location pointed to by Register Pair HL and put it into Register A.

49AD

POP HL

Restore the element pointer back into HL from the top of the STACK.

49AE

CP FFH

Compare the value held in Register A against FFH to see if the track is bad. Results: If Register A equals FFH then the track is bad and the Z FLAG is set; otherwise the NZ FLAG is set.

49B0

JR Z,49A1H

If the Z FLAG (Zero) is set then the track was marked bad so JUMP BACK to 49A1H to keep searching.

49B2

LD C,00H

If we are here, then the table isn’t bad, so to prepare, LET Register C = 00H as the first gran.

49B4

LD B,(HL)

Fetch the value stored at memory location pointed to by Register Pair HL (the GAT entry)and put it into Register B.

49B5

LD A,C

LET Register A = Register C.

49B6

CALL 4B48H

Test to see if the gran is allocated by GOSUBing to 4B48H.

49B9

JR Z,49BEH

If the Z FLAG (Zero) is set then the gran is NOT allocated so JUMP to 49BEH.

49BB

INC C

If we’re here then the gran was allocated so to move to the next one Bump Register C by 1.

49BC

JR 49B5H

LOOP BACK to 49B5H until a free gran is found.

49BEH – Routine to update the GAT entry and number of free grans.

49BE

LD A,C

LET Register A = Register C (the gran number in the track).

49BF

GOSUB to 45E9H.

NOTE: 45E9H is the routine to calculate the DRIVE SELECT CODE to be used.

49C2

OR (HL)

OR the value stored at (HL) against Register A to combine the gran number in track with the GAT entry. The results are stored in Register A.

49C3

LD (HL),A

Store the new GAT entry (held in Register A) into the memory location pointed to by Register Pair HL.

49C4

LD A,C

LET Register A = Register C (the gran number in the track).

49C5
49C6
49C7

RRCA
RRCA
RRCA

Rotate the gran offset into bits 5-7 by doing three RRCA’s (which rotates the contents of A right one bit position; so that, for example, the contents of bit 0 are copied bit 7).

49C8

LD DE,0000H

LET Register Pair DE = 0000H. This 0000H is filled in dynamically from 4987H to be the next extent element pointer.

49CB

PUSH AF

PUSH Register AF (the gran offset) to the top of the STACK.

49CC

LD A,L

LET Register A = Register L (which is the track number).

49CD

LD (DE),A

Store the track number (held in Register A) into the DCB (the memory location pointed to by Register Pair DE).

49CE

INC DE

Bump Register Pair DE by 1 to point to the next byte.

49CF

POP AF

Restore the gran offset from the top of the STACK into Register Pair AF.

49D0

INC A

Bump Register A by 1 to indicate that one gran has been allocated.

49D1

LD (DE),A

Save the updated extent byte by storing the value held in Register A into the memory location pointed to by Register Pair DE.

49D2

XOR A

Set Register A to ZERO and clear all Flags to indicate NO ERROR.

49D3

RET

RETurn to the caller.

49D4H – Routine to scan all the extents to see if there are any available.

49D4

PUSH HL

PUSH Register HL to the top of the STACK.

49D5

PUSH BC

PUSH Register BC to the top of the STACK.

49D6

PUSH IX

PUSH Register IX to the top of the STACK.

49D8

POP HL

MOVE the value held at the top of the STACK (which was Index Register IX) into Register Pair HL.

49D9

LD DE,0010H

LET Register Pair DE = 0010H.

49DC

ADD HL,DE

ADD the value held in Register Pair DE (10H) to Register Pair HL, so that HL points to the extent elements in the DCB.

49DD
49E0

LD E,(IX+0AH)
LD D,(IX+0BH)

Fetch the value for the next sector into Register Pair DE.

49E3

LD B,0DH

LET Register B = 0DH (Decimal: 13) to scan up to 13 elements.

49E5

LD A,(HL)

Fetch the extent element (which is thg value stored at memory location pointed to by Register Pair HL) and put it into Register A.

49E6

INC A

Bump Register A by 1 to see if it is unused.

49E7

JR Z,4A05H

If the Z FLAG (Zero) is set, meaning that the element is used, we have an error so JUMP to 4A05H.

49E9

INC HL

Bump Register Pair HL by 1 so that it will point to the number of grans in the extent in the DCB.

49EA

LD A,(HL)

Fetch the number of grants in the extent (the value stored at memory location pointed to by Register Pair HL) into Register A.

49EB

AND 1FH

MASK the value of Register A against 1FH (0001 1111). This has the effect of turning off bits 7, 6, 5, leaving only bits 4, 3, 2, 1, 0 active.

49ED

CALL 4A28H

GOSUB to 4A28H to convert that masked value into the number of sectors in the extent.

49F0

NEG

Negate Register A so that A = – sectors in the extent

49F2

PUSH DE

PUSH Register DE to the top of the STACK.

The next bunch of instructions only serves to set DE = DE – number of sectors in the extent and put that into Register A (with the CARRY FLAG set if the result is negative).

49F3

ADD A,E

ADD the value held in Register E to Register A (Results held in Register A).

49F4

LD E,A

LET Register E = Register A.

49F5

CCF

Invert the CARRY FLAG

49F6

LD A,D

LET Register A = Register D.

49F7

SBC 00H

Subtract 00H and the CARRY FLAG from Register A.

49F9

LD D,A

LET Register D = Register A.

DE now holds the number of sectors in the extent.

49FA

JR C,4A0AH

If the C FLAG (CARRY) is set, then the number of sectors in the extent are negative, so JUMP to 4A0AH.

49FC

POP AF

Clear the STACK.

49FD

INC HL

Bump Register Pair HL by 1 so HL points to the next element.

49FE

DJNZ 49E5H

LOOP back to 49E5H until Register B is ZERO and all the extents have been checked.

If we are here then every extent has been checked and all extents have been used. This is bad.

4A00

POP BC

Restore Register Pair BC from the STACK.

4A01

POP HL

Restore Register Pair HL from the STACK.

4A02

JP 48F3H

JUMP to 48F3H to exit with an error that ALL EXTENTS USED.

4A05H – Return a PAST END OF FILE error.

4A05

OR 1DH

OR Register A against 1DH (0001 1101). This has the effect of turning on bits 4, 3, 2, 0.

4A07

POP BC

MOVE the value held at the stop of the STACK into Register Pair BC.

4A08

POP HL

MOVE the value held at the stop of the STACK into Register Pair HL.

4A09

RET

RETurn to the caller.

4A0AH – This routine claculates the offset in grans for the track of the extent.

4A0A

POP DE

Restore Register Pair DE from the STACK.

4A0B

POP BC

MOVE the value held at the stop of the STACK into Register Pair BC.

The next few instructions get the offset (in grans) into the track of the extent.

4A0C

LD A,(HL)

Fetch the value stored at memory location pointed to by Register Pair HL and put it into Register A.

4A0D

AND E0H

MASK the value of Register A against E0H (1110 0000). This has the effect of turning off bits 4, 3, 2, 1, 0, leaving only bits 7, 6, 5 active.

4A0F
4A10
4A11

RLCA
RLCA
RLCA

Rotate the bits in A left 3 times (with Bit 7 going into both the CARRY and Bit 0), thus moving bits 7, 6, and 5 to bits 2, 1, and 0.

Now that the offset (in grans) is in Register A, prepare to calculate it and write it.

4A12

CALL 4A28H

GOSUB to 4A28H to convert that masked value (which is grans) into the sectors (by multipling them by 3).

4A15

ADD A,E

Calculate the ofset into the track by ADDing the value held in Register E to Register A (Results held in Register A).

4A16

DEC HL

DECrement Register Pair HL by 1 to point to where the track of the extent is stored.

4A17

LD D,(HL)

Fetch the value stored at memory location pointed to by Register Pair HL (whcih is the track of the extent) and put it into Register D.

4A18

LD E,A

LET Register E = Register A (the sector offset in the track).

4A19

SUB 12H

SUBtract the value 12H (Decimal: 18) from Register A to set up for a 1 track offset.

4A1B

JR C,4A20H

If the C FLAG (No Carry) is set, meaning that the offset is negative, JUMP to 4A20H.

4A1D

INC D

Bump Register D by 1 to point to the next track.

4A1E

JR 4A18H

LOOP BACK to 4A18H until we are at the proper track.

4A20H – Jumped here from 4A1BH if the offset is negative.

4A20

INC E

Bump Register E (the sector number) by 1.

4A21

LD A,(IX+06H)

Fetch the value stored at memory location IX+06H (which is the DRIVE NUMBER OF THE FILE) and put it into Register A.

4A24

LD C,A

LET Register C = Register A (the DRIVE NUMBER OF THE FILE).

4A25

XOR A

Set Register A to ZERO and clear all Flags to indicate no error.

4A26

POP HL

Restore Register Pair HL from the top of the STACK.

4A27

RET

RETurn to the caller.

4A28H – SUBROUTINE to convert the Grans (which was the masked value) into a sector.

4A28

PUSH BC

PUSH Register BC to the top of the STACK.

4A29

LD B,A

LET Register B = Register A (to preserve A).

4A2A

RLCA

Rotate the bits in A left, which effectively doubles Register A.

4A2B

ADD A,B

ADD the value held in Register B (which is Register A before it was doubled) to Register A. This results in A * 3.

4A2C

POP BC

Restore Register Pair BC from the top of the STACK.

4A2D

RET

RETurn to the caller.

4A2EH – Subroutine to check if the sought after sector is after the EOF. Called from 476CH, 479BH, and 4815H.

4A2E

PUSH DE

PUSH Register DE to the top of the STACK.

The combination of IX+08H, IX+0CH, and IX+0DH is the relative byte address of the EOF.

4A2F

LD D,(IX+0DH)

Fetch the value stored at memory location IX+0DH (MSB of the EOF sector number) and put it into Register D.

4A32

LD E,(IX+0CH)

Fetch the value stored at memory location IX+0CH (LSB of the EOF sector number) and put it into Register E.

4A35

LD A,(IX+08H)

Fetch the value stored at memory location IX+08H (the Byte Position of the EOF) and put it into Register A.

4A38

OR D

Check D vs A to see if they are empty by ORing Register D against Register A. The results are stored in Register A.

4A39

OR E

Check E vs A to see if they are empty by ORing Register E against Register A. The results are stored in Register A.

4A3A

JR Z,4A57H

If the Z FLAG (Zero) is set then D, E, and A are all zero meaning that the file is empty, so JUMP to 4A57H.

4A3C

LD H,(IX+0BH)

Fetch the value stored at memory location IX+0BH (MSB of the new “NEXT” sector number) and put it into Register H.

4A3F

LD L,(IX+0AH)

Fetch the value stored at memory location IX+0AH (LSB of the new “NEXT” sector number) and put it into Register L.

4A42

OR A

Since we are about to do a routine which checks the CARRY FLAG, we need to clear the CARRY FLAG.

4A43

SBC HL,DE

Subtract DE from HL to see if it the next sector (held in HL) is before or after the EOF sector (held in DE)

4A45

JR C,4A5AH

If the C FLAG (No Carry) is set then the next sector is BEFORE the EOF, so we are all good! JUMP to 4A5AH.

4A47

JR NZ,4A57H

If the NZ FLAG (Not Zero) is set AND the CARRY FLAG was set, the sector is AFTER the EOF so we need to exit with an EOF error by JUMPing to 4A57H.

4A49

BIT 6,(IX+01H)

If we are here, then we had NC (sector is before EOF) and Z, so we continue by testing Bit 6 of Register (IX+01H), which is the OPTIONS byte, to see if we need to update the EOF.

4A4D

JR NZ,4A57H

If the NZ FLAG (Not Zero) is set then we do not need to update the EOF, and we need to exit with an EOF error by JUMPing to 4A57H.

4A4F

LD A,(IX+05H)

Fetch the value stored at memory location IX+05H (which is the BYTE POSITION FOR NEXT) and put it into Register A. This will be the EOF byte number.

4A52

CP (IX+08H)

Compare the EOF Byte Number (held in Register A) against the the Byte Position of the EOF (held in the memory location (IX+08H)). Results:

If Register A equals (IX+08H), the Z FLAG is set.
If A < (IX+08H), the CARRY FLAG will be set.
if A >= (IX+08H), the NO CARRY FLAG will be set.

4A55

JR C,4A5AH

If the C FLAG (No Carry) is set then are good, so exit without an error by JUMPing to 4A5AH.

4A57

LD A,1CH

LET Register A = 1CH to set up an error for past EOF.

4A59

0EH

This byte is a mask to 4A5BH.

4A5A

XOR A

Clear all FLAGS.

4A5B

OR A

Set FLAGS based on the contents of Register A.

The combination of IX+05H, IX+0AH, and IX+0BH is the relative byte address of the “NEXT” sector.

4A5C

LD H,(IX+0BH)

Fetch the value stored at memory location IX+0BH (MSB of the new “NEXT” sector number) and put it into Register H.

4A5F

LD L,(IX+0AH)

Fetch the value stored at memory location IX+0AH (LSB of the new “NEXT” sector number) and put it into Register L.

4A62

LD C,(IX+05H)

Fetch the value stored at memory location IX+05H (which is the BYTE POSITION FOR NEXT) and put it into Register C.

4A65

POP DE

Restore Register Pair DE from the top of the STACK.

4A66

RET

RETurn to the caller.

4A67H – Subroutine to READ the Directory Entry into Buffer # 1.

4A67

PUSH BC

Preserve Register Pair BC by PUSHing it to the top of the STACK.

4A68

PUSH DE

Preserve Register Pair DE by PUSHing it to the top of the STACK.

4A69

CALL 4AD8H

GOSUB to 4AD8H to position for a read.

4A6C

JR NZ,4A76H

If the NZ FLAG (Not Zero) is set then the prior subroutine exited with an error, so JUMP to 4A76H.

4A6E

PUSH HL

Preserve Register Pair HL by PUSHing it to the top of the STACK.

4A6F

LD L,00H

LET Register L = 00H. This will form HL as pointing to Buffer # 1.

4A71

CALL 4B0DH

GOSUB to 4B0DH to read the sector.

4A74

POP HL

Restore Register Pair HL from the top of the STACK.

4A75

01H

Mask to 4A78H.

4A76

LD A, 11H

Set up for a DIRECTORY READ ERROR.

4A78

POP DE

Restore Register Pair DE from the top of the STACK.

4A79

POP BC

Restore Register Pair BC from the top of the STACK.

4A7A

RET

RETurn to the caller.

4A7BH – Subroutine to WRITE the Directory Entry from Buffer # 1. Called from 4981H.

4A7B

PUSH BC

Preserve Register Pair BC by PUSHing it to the top of the STACK.

4A7C

PUSH DE

Preserve Register Pair DE by PUSHing it to the top of the STACK.

4A7D

CALL 4AD8H

GOSUB to 4AD8H to position for a write.

4A80

JR NZ,4A89H

If the NZ FLAG (Not Zero) is set then the prior subroutine exited with an error, so 4A89H.

4A82

LD L,00H

LET Register L = 00H. This will form HL as pointing to Buffer # 1.

4A84

GOSUB to 45F7H to write the sector.

4A87

JR Z,4A90H

If the Z FLAG (Zero) is set then the prior routine had NO error, so JUMP to 4A90H.

4A89

CP 0FH

If we’re here then there was an error, so we need to deal with that. First, compare the value held in Register A against 0FH to check for a WRITE PROTECT ERROR. Results: If Register A equals 0FH, the Z FLAG is set; otherwise the NZ FLAG is set.

4A8B

JR Z,4A8FH

If the Z FLAG (Zero) is set then we had a WRITE PROTECT ERROR so skip the next instruction that changes the error to DIRECTORY WRITE ERROR and JUMP to 4A8FH.

4A8D

LD A,12H

LET Register A = 12H to for a DIRECTORY WRITE ERROR.

4A8F

OR A

Since a LD command does not set any FLAGS, Set FLAGS based on the contents of Register A.

4A90

POP DE

Restore Register Pair DE from the top of the STACK.

4A91

POP BC

Restore Register Pair BC from the top of the STACK.

4A92

RET

RETurn to the caller.

4A93H – Read the GAT TABLE into a Buffer. Jumped to from 48B3H.

4A93

CALL 4AADH

GOSUB to 4AADH to set up to read/write the GAT TABLE into the Buffer.

4A96

CALL 4B0DH

GOSUB to 4B0DH to read the GAT TABLE into the Buffer.

4A99

JR 4AAAH

JUMP to 4AAAH to continue.

4A9BH – Subroutine to WRITE the GAT TABLE from the Buffer. Called from 497DH.

4A9B

CALL 4AADH

GOSUB to 4AADH to set up to read/write the GAT TABLE into the Buffer.

4A9E

GOSUB to 45F7H to write the GAT TABLE from the Buffer.

4AA1

JR Z,4AAAH

If the Z FLAG (Zero) is set then the prior routine had NO error, so JUMP to 4AAAH.

4AA3

CP 0FH

4AA5

JR Z,4AA9H

If the Z FLAG (Zero) is set then we had a WRITE PROTECT ERROR so skip the next instruction that changes the error to GAT WRITE ERROR and JUMP to 4AA9H.

4AA7

LD A,15H

LET Register A = 15H for a GAT WRITE ERROR.

4AA9

OR A

Since a LD command does not set any FLAGS, Set FLAGS based on the contents of Register A.

4AAA

POP BC

Restore Register Pair BC from the top of the STACK.

4AAB

POP DE

Restore Register Pair DE from the top of the STACK.

4AAC

RET

RETurn to the caller.

4AADH – Subroutine to set up for a read or write of the GAT Table.

4AAD

POP HL

Restore the RETURN ADDRESS from the top of the STACK into Register Pair HL.

4AAE

PUSH DE

Preserve Register Pair DE by PUSHing it to the top of the STACK.

4AAF

PUSH BC

Preserve Register Pair BC by PUSHing it to the top of the STACK.

4AB0

PUSH HL

Preserve Register Pair HL by PUSHing it to the top of the STACK.

4AB1

CALL 4B3EH

GOSUB to 4B3EH to Get the Directory Track Number. Routine exits with the director track number in Register D.

4AB4

LD E,01H

LET Register E = 01H to point to the GAT sector in the directory track.

4AB6

LD HL,4D00H

LET Register Pair HL = 4D00H as the base address of BUFFER # 2

4AB9

RET

RETurn to the caller.

4ABAH – Routine to read the HASH INDEX TABLE and continue on.

4ABA

CALL 4ACBH

GOSUB to 4ACBH to set up to read/write the HASH INDEX TABLE.

4ABD

CALL 4B0DH

GOSUB to 4B0DH to read the HASH INDEX TABLE.

4AC0

JR 4AC8H

Continue on by POPing BC and DE and RETurning.

4AC2H – Routine to write the HASH INDEX TABLE and continue on.

4AC2

CALL 4ACBH

GOSUB to 4ACBH to set up to read/write the HASH INDEX TABLE.

4AC5

GOSUB to 45F7H to write the HASH INDEX TABLE to diskette.

4AC8

POP BC

Restore Register Pair BC from the top of the STACK.

4AC9

POP DE

Restore Register Pair DE from the top of the STACK.

4ACA

RET

RETurn to the caller.

4ACBH – Subroutine to set up to read/write the HASH INDEX TABLE.

4ACB

POP HL

Restore the RETURN ADDRESS from the top of the STACK into Register Pair HL.

4ACC

PUSH DE

Preserve Register Pair DE by PUSHing it to the top of the STACK.

4ACD

PUSH BC

Preserve Register Pair BC by PUSHing it to the top of the STACK.

4ACE

PUSH HL

Preserve Register Pair HL by PUSHing it to the top of the STACK.

4ACF

CALL 4B3EH

GOSUB to 4B3EH to Get the Directory Track Number. Routine exits with the director track number in Register D.

4AD2

LD E,02H

LET Register E = 02H to point to the HIT sector in the directory track.

4AD4

LD HL,4300H

LET Register Pair HL = 4300H as the base address of BUFFER # 1.

4AD7

RET

RETurn to the caller.

4AD8H – Compute where the directory entry for a file is located. This routine is called to position for a READ (called from 4A69H and 4A7DH) or for a wRITE (called from 4A7DH). Register L will hold the results

4AD8

CALL 4B3EH

GOSUB to 4B3EH to Get the Directory Track Number. Routine exits with the director track number in Register D.

4ADB

LD A,B

LET Register A = Register B (the logical file number).

4ADC

OR A

Since a LD command does not set any FLAGS, Set FLAGS based on the contents of Register A.

4ADD

JR Z,4B06H

If the Z FLAG (Zero) is set then the logical file number is zero, and we can skip a lot of hard work, so JUMP to 4B06H.

4ADF

CP 50H

Compare the value held in Register A against 50H (Decimal: 80). Results:

If Register A equals 50H, the Z FLAG is set.
If A < 50H, the CARRY FLAG will be set.
if A >= 50H, the NO CARRY FLAG will be set.

4AE1

JR NC,4B02H

If the NC FLAG (No Carry) is set then it is simply too large so JUMP to 4B02H to set up for an ILLEGAL LOGICAL FILE NUMBER error and RETurn.

4AE3

PUSH BC

Preserve Register Pair BC by PUSHing it to the top of the STACK.

4AE4

LD L,B

Set up for HL to point to the logical file number by first setting Register L to be Register B (the logical file number).

4AE5

LD H,00H

Continue that by setting Register H to 0.

4AE7

LD A,05H

LET Register A = 05H as the divisor.

4AE9

CALL 4B6FH

GOSUB to 4B6FH to perform HL = HL / A leaving the remainder in Register A.

4AEC

PUSH AF

Preserve the remainder (held in Register A) by PUSHing it to the top of the STACK.

The next 3 instructions set Register A to be the appropriate directory sector by setting Register E = Register L + 3.

4AED

LD A,L

LET Register A = Register L.

4AEE

ADD 03H

ADD the value 03H to Register A (Results held in Register A).

4AF0

LD E,A

LET Register E = Register A.

4AF1

POP BC

Restore the remainder (held in Register Pair AF) from the top of the STACK into Register Pair BC.

4AF2

LD HL,4300H

LET Register Pair HL = 4300H as the base address of BUFFER # 1.

We next want to test Register B (the remainder) to see if it is ZERO or not, but we don’t necessarily want to mess with other registers, so the simplest way to do that is to increase and decrease Register B.

4AF5

INC B

Bump Register B by 1.

4AF6

DEC B

DECrement Register B by 1.

4AF7

JR Z,4AFFH

If the Z FLAG (Zero) is set, meaning the remainder was 0, JUMP to 4AFFH.

If we are here, then the remainder was NOT zero, so we need to do some math, via a LOOP set to B interations.

4AF9

LD A,L

LET Register A = Register L. Register L was set to 00H at 4AF2H.

4AFA

ADD 30H

ADD the value 30H to Register A (Results held in Register A).

4AFC

DJNZ 4AFAH

LOOP back to the prior instruction until Register B is ZERO.

4AFE

LD L,A

Put that calculated Register A into Register L.

4AFF

POP BC

Restore Register BC from the top of the STACK.

4B00

XOR A

Set Register A to ZERO and clear all Flags to indicate NO ERROR.

4B01

RET

RETurn to the caller.

4B02H – Set up for an ILLEGAL LOGICAL FILE NUMBER error and RETurn.

4B02

LD A,10H

LET Register A = 10H to prepare for an ILLEGAL LOGICAL FILE NUMBER error.

4B04

OR A

Since a LD command does not set any FLAGS, Set FLAGS based on the contents of Register A. Since A was just loaded with 10H, it will be NZ – the signal for an error.

4B05

RET

RETurn to the caller.

4B06H – This is the jump point from the routine to compute where the directory entry for a file is located when the remainder is zero (meaning, no offset calculation was required).

4B06

LD E,03H

LET Register E = 03H to point to DIRECTORY SECTOR 0.

4B08

LD HL,4300H

LET Register Pair HL = 4300H as the base address of BUFFER # 1.

4B0B

XOR A

Set Register A to ZERO and clear all Flags to indicate NO ERROR.

4B0C

RET

RETurn to the caller.

4B0DH – Read a directory sector. Called from 4A71, 4A96, and 4ABD

4B0D

CALL 4675H

GOSUB to 4675H to read the sector.

4B10

RET NZ

If the NZ FLAG (Not Zero) is set then the prior subroutine exited with an error, so RETurn to the caller.

4B11

LD A,00H

LET Register A = 00H. Note: This may not be 00H. A value is put into the 00H at 469AH. That value was the FDC Status Port on input.

4B13

BIT 5,A

Test Bit 5 of Register A (which was the FDC Status Port). Bit 5 of the FDC Status Port response is READ PROTECTED.

4B15

RET Z

If the Z FLAG (Zero) is set then the sector is not READ PROTECTED so RETurn to the caller.

4B16

PUSH DE

Preserve Register Pair DE to the top of the STACK.

4B17

LD DE,0001H

LET Register Pair DE = 0001H, which set D as 00H and E as 01H to read Sector 1, Track 0.

4B1A

CALL 4675H

GOSUB to 4675H to read Sector 1, Track 0..

4B1D

POP DE

Restore Register Pair DE from the top of the STACK.

4B1E

RET NZ

If the NZ FLAG (Not Zero) is set then the prior subroutine exited with an error, so RETurn to the caller.

4B1F

PUSH HL

Presever Register HL (which holds the Buffer Pointer) to the top of the STACK.

4B20

LD A,(HL)

Fetch the value stored at memory location pointed to by Register Pair HL (i.e., the first byte of the BOOT SECTOR) and put it into Register A.

4B21

CP FEH

Compare the value held in Register A against FEH. Results: If Register A equals FEH, the Z FLAG is set; otherwise the NZ FLAG is set.

4B23

JR NZ,4B39H

If the NZ FLAG (Not Zero) is set then the first byte wasn’t 0FEH which means that it is not a TRSDOS v1.3 DOS disk, so JUMP to 4B39H to point that out.

4B25

INC HL

Bump Register Pair HL by 1 to point to the next byte of the BOOT SECTOR.

4B26

LD D,(HL)

Fetch the value stored at memory location pointed to by Register Pair HL (which, as the 2nd byte, is the directory track on the disk) and put it into Register D.

4B27

LD A,D

LET Register A = Register D (the location of the directory track on the disk).

4B28

CP 29H

Compare the value held in Register A against 29H (Decimal: 40H). Results:

If Register A equals 29H, the Z FLAG is set.
If A < 29H, the CARRY FLAG will be set.
if A >= 29H, the NO CARRY FLAG will be set.

4B2A

JR NC,4B39H

If the NC FLAG (No Carry) is set then the Directory Track exceeds Track 40., which is the maximum track number on TRSDOS v1.3 so JUMP to 4B39H.

4B2C

LD HL,4457H

LET Register Pair HL = 4457H, to point to the directory table.

4B2F

PUSH BC

Preserve Register BC to the top of the STACK.

4B30

LD B,00H

LET Register B = 00H, which will be the drive number.

4B32

ADD HL,BC

ADD the value held in Register Pair BC (offset) to Register Pair HL (the directory table). The results are held in Register Pair HL.

4B33

POP BC

Restore Register Pair BC from the top of the STACK.

4B34

LD (HL),D

Store the value held in Register D (the directory track number) into the memory location pointed to by Register Pair HL.

4B35

POP HL

Restore the buffer pointer held at the stop of the STACK into Register Pair HL.

4B36

JP 4675H

JUMP to 4675H to read the sector and return.

4B39H – This routine is JUMPed to from 4B23H if it didn’t find 0FEH as the first byte on the disk, meaning that there is no system!

4B39

POP HL

Clear the STACK.

4B3A

LD A,0DH

LET Register A = 0DH, which is the error for NON-SYSTEM DISK.

4B3C

OR A

Since A is not zero, ORing A sets the NZ flag to signify an error.

4B3D

RET

RETurn to the caller.

4B3EH – Get the Directory Track Number. Routine exits with the director track number in Register D.

4B3E

PUSH BC

Preserve Register BC to the top of the STACK.

4B3F

LD B,00H

LET Register B = 00H to set the drive number to :0

4B41

LD HL,4457H

LET Register Pair HL = 4457H to point to the directory track table

4B44

ADD HL,BC

ADD the value held in Register Pair BC (an offset) to Register Pair HL (the directory track table). The results are held in Register Pair HL.

4B45

POP BC

Restore Ragister Pair BC from the top of the STACK.

4B46

LD D,(HL)

Fetch the value stored at memory location pointed to by Register Pair HL (the directory track number) and put it into Register D.

4B47

RET

RETurn to the caller.

4B48H – Subroutine to test to see if a gran is allocated.

The next few instructions create an OPCODE along the lines of BIT nn of Register B.

4B48

AND 07H

MASK the value of Register A against 07H (0000 0111). This has the effect of turning off bits 7, 6, 5, 4, 3, leaving only bits 2, 1, 0 active.

4B4A
4B4B
4B4C

RLCA
RLCA
RLCA

Rotate the bits in A left 3 times so that Bits 2, 1, 0 move to bits 5, 4, 3.

4B4D

OR 40H

OR Register A against 40H (0100 0000). This has the effect of turning on bits 6.

Put the generated OPCODE into 4B53H. If tthe value is 40H then it tests Bit 0; 48H tests Bit 1, 50H tests Bit 2, 58H tests Bit 3, and so on.

4B4F

LD (4B53H),A

Store the value held in Register A into the memory location 4B53H.

4B52

BIT 0,B

Test Bit nn of Register B. The nn was set in the prior instruction.

4B54

RET

RETurn to the caller.

4B55H – Subroutine to multiply Register Pair HL by Register A. On exit, Register A = Overflow and Register Pair HL = the result of the multiplication.

4B55

PUSH BC

Preserve Register BC to the top of the STACK.

4B56

EX DE,HL

Swap Register Pair DE and HL, so that Register Pair DE holds the multiplicand.

4B57

LD C,A

LET Register C = Register A, so that Register C holds the multiplier.

4B58

LD HL,0000H

LET Register Pair HL = 0000H to clear the accumulator.

4B5B

LD A,L

LET Register A = Register L to clear the overflow.

4B5C

LD B,08H

LET Register B = 08H to set up for 8 bits of LOOP.

Start of an 8 bit loop.

4B5E

ADD HL,HL

ADD the value held in Register Pair HL to Register Pair HL. The results are held in Register Pair HL.

4B5F

RLA

Shift the OVERFLOW to the left by rotating the bits in Register A left one bit, with the contents of BIT 7 being put into the CARRY FLAG and the CARRY FLAG being put into BIT 0.

4B60

RLC C

Shift the multiplier into the CARRY FLAG

4B62

JR NC,4B67H

If the NC FLAG (No Carry) is set then the multiplier is ZERO so JUMP to 4B67H.

4B64

ADD HL,DE

ADD the value held in Register Pair DE (multiplicand) to Register Pair HL (the current result). The results are held in Register Pair HL.

4B65

ADC 00H

This ADD WITH CARRY of 00H functions to add the overflow bit to the overflow.

4B67

DJNZ 4B5EH

LOOP back to 4B5EH until Register B is ZERO (i.e., all 8 bits).

End of an 8 bit loop. The next bunch of varliable exchanges are to set Register A = Overflow and Register Pair HL = the result of the multiplication

4B69

LD C,A

LET Register C = Register A (the overflow).

4B6A

LD A,L

LET Register A = Register L.

4B6B

LD L,H

LET Register L = Register H.

4B6C

LD H,C

LET Register H = Register C.

4B6D

POP BC

Restore Register Pair BC from the stop of the STACK.

4B6E

RET

RETurn to the caller.

4B6FH – Subroutine to divide Register Pair HL by Register A.

4B6F

PUSH DE

Preserve Register BC to the top of the STACK.

4B70

LD D,A

LET Register D = Register A (the divisor).

4B71

LD E,10H

LET Register E = 10H to cover 16 bits of action.

4B73

XOR A

Set Register A to ZERO and clear all Flags.

Start of a loop.

4B74

ADD HL,HL

LET HL = HL * 2.

4B75

RLA

Put the remainder, if any, into the CARRY bit by rotating the bits in Register A left one bit, with the contents of BIT 7 being put into the CARRY FLAG and the CARRY FLAG being put into BIT 0.

4B76

JR C,4B7BH

If the C FLAG (No Carry) is set then there is a remainder overflow, so JUMP to 4B7BH.

4B78

CP D

Compare the value held in Register A against the value held in Register D. Results:

If Register A equals D, the Z FLAG is set.
If A < D, the CARRY FLAG will be set.
if A >= D, the NO CARRY FLAG will be set.

4B79

JR C,4B7DH

If the C FLAG (No Carry) is set then the remainder does not equal the divisor so JUMP to 4B7DH.

4B7B

SUB D

If we’re here, then the remainder = the divisor so set REMAINDER = REMAINDER – DIVISOR by SUBtracting the value held in Register D from Register A.

4B7C

INC L

Bump Register L (which is tracking the result) by 1.

4B7D

DEC E

DECrement Register E by 1 as we have 1 less subtraction to do.

4B7E

JR NZ,4B74H

If the NZ FLAG (Not Zero) is set then we have more bits to process so JUMP BACK to 4B74H.

End of the loop.

4B80

POP DE

Restore Register Pair DE from the top of the STACK.

4B81

RET

RETurn to the caller.

4B82H – RST 28H – DOS Overlay Loader. Register A to hold the requested overlay on entry.

4B82

EX (SP),HL

Clear the restart return from the stack.

4B83

POP HL

Clear the restart return from the stack.

4B84

OR A

Set FLAGS based on the contents of Register A.

4B85

JP P,42AEH

If the P FLAG is set, then the overlay request which was sent was not a proper request, so JUMP to 42AEH to exit.

4B88

PUSH HL

Preserve Register Pair HL to the top of the STACK.

4B89

LD H,A

LET Register H = Register A (the requested overlay).

4B8A

LD A,(4414H)

Fetch the value stored at memory location 4414H and put it into Register A.

NOTE: 4414H is the storage location for the NUMBER OF THE OVERLAY CURRENTLY IN MEMORY.

4B8D

XOR H

We want to see if the requested overlay is the one currently in memory, and we do that with the following 2 instructions. First, eXclusive OR Register H against Register A. The results are stored in Register A.

4B8E

AND 0FH

MASK the value of Register A against 0FH (0000 1111). This has the effect of turning off bits 7, 6, 5, 4, leaving only bits 3, 2, 1, 0 active.

4B90

LD A,H

LET Register A = Register H (the new overlay number).

4B91

LD (4414H),A

Store the value held in Register A into the memory location 4414H.

NOTE: 4414H is the storage location for the NUMBER OF THE OVERLAY CURRENTLY IN MEMORY.

4B94

JR Z,4BD2H

If the Z FLAG (Zero) is set then the requested overlay is the same one which is already in memory so there is no need to load it. Instead, JUMP to 4BD2H.

4B96

PUSH DE

Preserve Register DE to the top of the STACK.

4B97

PUSH BC

Preserve Register BC to the top of the STACK.

4B98

AND 0FH

MASK the value of Register A against 0FH (0000 1111) to isolate the overlay number. This has the effect of turning off bits 7, 6, 5, 4, leaving only bits 3, 2, 1, 0 active.

4B9A

LD B,A

LET Register B = masked Register A. As masked, it will also be the logical file number.

4B9B

LD (449EH),A

Store the logical file number held in Register A into the DCB at memory location 449EH.

4B9E

XOR A

Set Register A to ZERO and clear all Flags.

4B9F

LD (4498H),A

Store the value held in Register A into the memory location 4498H (default options byte).

4BA2

LD (449DH),A

Store the value held in Register A into the memory location 449DH (drive number byte).

4BA5

SBC HL,HL

Clear HL

4BA7

LD (44A1H),HL

Store the value held in Register HL into the memory location 44A1H (location in file).

4BAA

LD C,A

LET Register C = Register A (the drive number).

4BAB

CALL 4ABAH

GOSUB to 4ABAH to read the HIT SECTOR, POP BC and DE and RETurning here.

4BAE

If the NZ FLAG (Not Zero) is set then the prior subroutine exited with an error, so JUMP to 4409H.

The next 3 instructions set B = Logical File Number * 2 + 0E0H.

4BB1

LD A,B

LET Register A = Register B.

4BB2

RLCA

Rotate the bits in A left (with Bit 7 going into both the CARRY and Bit 0).

4BB3

ADD E0H

ADD the value E0H to Register A (Results held in Register A).

The next 2 instructions set HL = 4300H + B.

4BB5

LD L,A

LET Register L = Register A.

4BB6

LD H,43H

LET Register H = 43H (0100 0011).

The next 4 instructions set DE = the First Overlay Extent Element.

4BB8

LD D,(HL)

Fetch the value stored at memory location pointed to by Register Pair HL and put it into Register D.

4BB9

INC HL

Bump Register Pair HL by 1.

4BBA

LD E,(HL)

Fetch the value stored at memory location pointed to by Register Pair HL and put it into Register E.

4BBB

INC E

Bump Register E by 1 to test the value.

4BBC

JP Z,4CC0H

If INC of Register E caused the Z FLAG (Zero) to be set, then the overlay does not exist, so JUMP to 4CC0H.

4BBF

DEC E

DECrement Register E (the track number) by 1 to put it back where it was.

4BC0

LD (44A7H),DE

Store the value held in Register DE (the extenbt element) into the DCB at memory location 44A7H.

4BC4

LD DE,4497H

LET Register Pair DE = 4497H to point to the overlay DCB.

4BC7

CALL 4BFFH

GOSUB to 4BFFH to load the overlay pointed to by Register Pair DE.

4BCA

If the NZ FLAG (Not Zero) is set then the prior subroutine exited with an error, so JUMP to 4409H.

4BCD

LD (4BD7H),HL

Store the value held in Register HL (the execution address of the overlay) into the memory location 4BD7H, which is a JP nnnnH.

4BD0

POP BC

Restore Register Pair BC from the STACK

4BD1

POP DE

Restore Register Pair DE from the STACK

4BD2

POP HL

Restore Register Pair HL from the STACK

4BD3

LD A,(4414H)

Fetch the value stored at memory location 4414H (the requested overlay) and put it into Register A.

NOTE: 4414H is the storage location for the NUMBER OF THE OVERLAY CURRENTLY IN MEMORY.

4BD6

JP 0000H

JUMP to nnnnH. nnnnH was set at 4BCDH

4BD9H – Routine to load and execute a program.

4BD9

PUSH HL

Preserve Register HL to the top of the STACK.

4BDA

XOR A

Set Register A to ZERO and clear all Flags.

4BDB

LD (442BH),A

Store the value held in Register A into the memory location 442BH.

NOTE: 442BH is the storage location for the LOAD/EXECUTE flag when the disk loader is called. Bit 7 – Z=EXECUTE, NZ=LOAD.

4BDE

CALL 4430H

GOSUB to 4430H.

NOTE: 4430H is DOS PROGRAM LOADER routine and does the same as the DOS LIBRARY command of LOAD. Requires Register Pair DE to contain the applicable FCB. All Registers are modified.

4BE1

If the NZ FLAG (Not Zero) is set then the prior subroutine exited with an error, so JUMP to 4409H.

4BE4

EX (SP),HL

Get Register Pair HL from the top of the stack without removing it.

4BE5

RET

RETurn (which in this case, will execute the program).

4BE6H – Routine to load a program.

4BE6

LD B,00H

LET Register B = 00H, which is the same as 256. This is to be a LRL of a full sector.

4BE8

LD HL,4D00H

LET Register Pair HL = 4D00H as a buffer.

4BEB

CALL 4424H

GOSUB to 4424H to open the file (which just calls RST 28H with A loaded).

4BEE

RET NZ

If the NZ FLAG (Not Zero) is set then the prior subroutine exited with an error, so RETurn to the caller.

4BEF

INC DE

Bump Register Pair DE by 1. DE will now point to the ACCESS CODE.

4BF0

LD A,(DE)

Fetch the value stored at memory location pointed to by Register Pair DE (the ACCESS CODE) and put it into Register A.

4BF1

DEC DE

DECrement Register Pair DE by 1, so now DE points back to the DCB for the file.

4BF2

AND 07H

Isolate the ACCESS CODE by MASKing the value of Register A against 07H (0000 0111). This has the effect of turning off bits 7, 6, 5, 4, 3, leaving only bits 2, 1, 0 active.

4BF4

CP 06H

Compare the value held in Register A against 06H to check for READ or lower. Results:

If Register A equals 06H, the Z FLAG is set.
If A < 06H, the CARRY FLAG will be set.
if A >= 06H, the NO CARRY FLAG will be set.

4BF6

JR C,4BFFH

If the C FLAG (No Carry) is set then we had READ or lower access, so we won’t be executing the program. Instead, JUMP to 4BFFH.

4BF8

LD A,(442BH)

Fetch the value stored at memory location 442BH and put it into Register A.

NOTE: 442BH is the storage location for the LOAD/EXECUTE flag when the disk loader is called. Bit 7 – Z=EXECUTE, NZ=LOAD.

4BFB

RLCA

Rotate the bits in A left (with Bit 7 going into both the CARRY and Bit 0). This puts the EXECUTE vs LOAD bit into the CARRY FLAG

4BFC

LD A,19H

LET Register A = 19H to set up for a possible FILE ACCESS DENIED error.

4BFE

RET C

If the C FLAG (No Carry) is set then we can’t do the operation, RETurn to the caller.

4BFF

LD BC,4DFFH

LET Register Pair BC = 4DFFH (the last byte of the buffer) to set up the data pointer.

4C02

GOSUB to 4C4FH to get the byte.

4C05

CP 01H

Compare the value held in Register A against 01H (program code). Results: If Register A equals 01H, the Z FLAG is set; otherwise the NZ FLAG is set.

4C07

JR Z,4C32H

If the Z FLAG (Zero) is set, JUMP to 4C32H.

4C09

CP 02H

Compare the value held in Register A against 02H (execution address). Results: If Register A equals 02H, the Z FLAG is set; otherwise the NZ FLAG is set.

4C0B

JR Z,4C25H

If the Z FLAG (Zero) is set, JUMP to 4C25H to put the execution/transfer address into Register Pair HL and RETurn.

4C0D

CP 03H

Compare the value held in Register A against 03H (break key hit). Results: If Register A equals 03H, the Z FLAG is set; otherwise the NZ FLAG is set.

4C0F

JP Z,402DH

If the Z FLAG (Zero) is set, JUMP to 402DH.

4C12

CP 20H

Compare the value held in Register A against 20H (comment block). Results:

If Register A equals 20H, the Z FLAG is set.
If A < 20H, the CARRY FLAG will be set.
if A >= 20H, the NO CARRY FLAG will be set.

4C14

JR C,4C1AH

If the C FLAG (No Carry) is set, JUMP to 4C1AH to read the number of bytes in the length byte and jump back into this loop.

4C16

LD A,22H

LET Register A = 22H to set up for a NOT A PROGRAM FILE error

4C18

OR A

Since Register A is not ZERO, this will set the NZ flag to indicate that there is an error.

4C19

RET

RETurn to the caller.

4C1AH – Read the number of bytes set out in the LENGTH BYTE.

4C1A

GOSUB to 4C4FH to get the LENGTH BYTE into Register A.

4C1D

LD B,A

Save the LENGTH BYTE into Register B.

4C1E

GOSUB to 4C4FH to read more bytes and …

4C21

DJNZ 4C1EH

… keep reading bytes until Register B (which was the length byte) is ZERO.

4C23

JR 4C02H

JUMP to 4C02H to continue the loop.

4C25H – Read the execution/transfer address into Register Pair HL.

4C25

GOSUB to 4C4FH to get the LENGTH BYTE into Register A.

4C28

GOSUB to 4C4FH to read the next byte which the LSB and store it in Register A.

4C2B

LD L,A

LET Register L = Register A (the LSB).

4C2C

GOSUB to 4C4FH to read the next byte which the MSB and store it in Register A.

4C2F

LD H,A

LET Register H = Register A (the MSB).

4C30

XOR A

Set Register A to ZERO and clear all Flags to indicate no error.

4C31

RET

RETurn to the caller.

4C32H – Read data and store in the buffer.

4C32

GOSUB to 4C4FH to read the next byte (which the LENGTH OF DATA BLOCK + 2) and store it in Register A.

4C35

LD B,A

Put the LENGTH OF DATA BLOCK + 2 into Register B.

4C36

GOSUB to 4C4FH to read the next byte (which the LSB of the LOADING ADDRESS) and store it in Register A.

4C39

LD L,A

Put the LSB of the LOADING ADDRESS into Register L.

4C3A

DEC B

DECrement Register B by 1 so that it now holds LENGTH OF DATA BLOCK + 1.

4C3B

GOSUB to 4C4FH to read the next byte (which the MSB of the LOADING ADDRESS) and store it in Register A.

4C3E

LD H,A

Put the MSB of the LOADING ADDRESS into Register H.

4C3F

DEC B

DECrement Register B by 1 so that it now holds LENGTH OF DATA BLOCK.

4C40

GOSUB to 4C4FH to read the next byte (which data) and store it in Register A.

4C43

LD (HL),A

Store the data held in Register A into the memory location pointed to by Register Pair HL.

4C44

CP (HL)

Check to make sure it was really stored by comparing the value held in Register A against the value held in the memory location (HL). Results: If Register A equals (HL), the Z FLAG is set; otherwise the NZ FLAG is set.

4C45

JR NZ,4C4CH

If the NZ FLAG (Not Zero) is set then it didn’t store properly so JUMP to 4C4CH to set up for a MEMORY FAULT.

4C47

INC HL

Bump Register Pair HL by 1 to point to the next spot in the buffer.

4C48

DJNZ 4C40H

LOOP back to 4C40H until Register B is ZERO.

4C4A

JR 4C02H

JUMP to the main loop at 4C02H.

4C4CH – Process a MEMORY FAULT error.

4C4C

LD A,23H

LET Register A = 23H for the error of MEMORY FAULT

4C4E

RET

RETurn to the caller.

4C4FH – Routine to read a sector.

4C4F

INC C

Bump Register C by 1 to point to the next location.

4C50

JR NZ,4C65H

If the NZ FLAG (Not Zero) is set then this is not a new sector so JUMP to 4C65H.

4C52

PUSH DE

PUSH Register DE to the top of the STACK.

4C53

EX (SP),IX

Let Index Register IX = DE and put Register IX at the top of the STACK

4C55

PUSH HL

PUSH Register HL to the top of the STACK.

4C56

PUSH DE

PUSH Register DE to the top of the STACK.

4C57

PUSH BC

PUSH Register BC to the top of the STACK.

4C58

CALL 4766H

GOSUB to 4766H to read a Sector and Return With Error or Continue.

4C5B

POP BC

MOVE the value held at the stop of the STACK into Register Pair BC.

4C5C

POP DE

MOVE the value held at the stop of the STACK into Register Pair DE.

4C5D

POP HL

MOVE the value held at the stop of the STACK into Register Pair HL.

4C5E

POP IX

MOVE the value held at the stop of the STACK into Register Pair IX.

4C60

JR Z,4C65H

If the Z FLAG (Zero) is set then there was no error, so JUMP to 4C65H.

4C62
4C63

INC SP
INC SP

Bump Register Pair SP by 2 to clear the most recent RETURN address.

4C64

RET

RETurn to the caller.

4C65H – Continuation of the above routine when we are not at a new sector.

4C65

PUSH BC

PUSH Register BC to the top of the STACK.

4C66

LD B,4DH

LET Register B = 4DH. THis modified Register Pair BC to point to the data byte.

4C68

LD A,(BC)

Fetch the value stored at memory location pointed to by Register Pair BC (i.e., the data byte) and put it into Register A.

4C69

POP BC

Restore Register Pair BC from the top of the STACK.

4C6A

RET

RETurn to the caller.

4C6BH – Start up and check the drive. Register C holds the drive being sought.

4C6B

LD A,(4413H)

Fetch the value stored at memory location 4413H and put it into Register A.

NOTE: 4413H is the storage location for the NUMBER OF DISK DRIVES in the system.

4C6E

CP C

Compare the value held in Register A against the value held in Register C. Results:

If Register A equals C, the Z FLAG is set.
If A < C, the CARRY FLAG will be set.
if A >= C, the NO CARRY FLAG will be set.

4C6F

JR NC,4C75H

If the NC FLAG (No Carry) is set then the drive requested is on the system so continue the routine by JUMPing to 4C75H.

4C71

LD A,02H

LET Register A = 02H to set up for a DISK DRIVE NOT ON SYSTEM error.

4C73

OR A

Since Register A is not 00H, this sets the NZ FLAG to signify that an error occurred.

4C74

RET

RETurn to the caller.

4C75H – The prior routine jumps here if the requested drive id on the system.

4C75

INput a byte from Port F0H and put it into Register A.

NOTE: Port F0H is the FDC Status port on input. Input Results:

Bit 0: Busy
Bit 1: Index/DRQ
Bit 2: Track 0/Data Lost
Bit 3: CRC error
Bit 4: Seek error/Record not found
Bit 5: If Reading, Record Type, if Writing, Write Fault/Head loaded
Bit 6: Write Protect
Bit 7: Not ready

4C77

AND 80H

MASK the value of Register A against 80H (1000 0000). This has the effect of turning off bits 6, 5, 4, 3, 2, 1, 0, leaving only bit 7 (DRIVE NOT READY) active.

4C79

GOSUB to 4566H.

NOTE: 4566H is the routine which resets the FDC.

4C7C

JR Z,4C89H

If the Z FLAG (Zero) is set then the disk is good to go so JUMP to 4C89H.

4C7E

GOSUB to 44D4H.

NOTE: 44D4H is the routine which handles DRIVE SELECT (Register C holds the drive number to be used).

4C81

PUSH BC

Preserve Register Pair BC to the top of the STACK.

4C82

LD BC,9000H

LET Register Pair BC = 9000H to set up for a delay of 1/2 second.

4C85

CALL 0060H

GOSUB to 0060H to do the delay.

4C88

POP BC

Restore Register Pair BC from the top of the STACK.

4C89

GOSUB to 44D4H.

NOTE: 44D4H is the routine which handles DRIVE SELECT (Register C holds the drive number to be used).

4C8C

GOSUB to 4566H.

NOTE: 4566H is the routine which resets the FDC.

4C8F

LD A,(4427H)

Fetch the value stored at memory location 4427H (previously selected drive) and put it into Register A.

NOTE: 4427H is the storage location for the currently selected disk drive number.

4C92

CP C

Compare the value held in Register A against the value held in Register C. Results: If Register A equals C, the Z FLAG is set; otherwise the NZ FLAG is set.

4C93

JR NZ,4C97H

If the NZ FLAG (Not Zero) is set thenm the selected drive was NOT the same as the prior selected drive so JUMP to 4C97H.

4C95

XOR A

Set Register A to ZERO and clear all Flags to indicate no error.

4C96

RET

RETurn to the caller.

4C97H – Read until you get an INDEX LOW, INDEX HIGH, INDEX LOW, and then set CARRY FLAG if WRITE PROTECT is enabled.

4C97

PUSH BC

Preserve Register BC to the top of the STACK.

4C98

LD BC,62AAH

LET Register Pair BC = 62AAH to set up a counter.

4C9B

CALL 4CB0H

GOSUB to 4CB0H to check to see if the BC counter is zero and, if so, exit, and if not get the disk status and look for an index hole. Returns NZ if the index hole was found.

4C9E

JR NZ,4C9BH

If the NZ FLAG (Not Zero) is set then the prior subroutine exited with an error, so JUMP to 4C9BH.

4CA0

CALL 4CB0H

GOSUB to 4CB0H to check to see if the BC counter is zero and, if so, exit, and if not get the disk status and look for an index hole. Returns NZ if the index hole was found.

4CA3

JR Z,4CA0H

If the Z FLAG (Zero) is set, JUMP to 4CA0H.

4CA5

CALL 4CB0H

GOSUB to 4CB0H to check to see if the BC counter is zero and, if so, exit, and if not get the disk status and look for an index hole. Returns NZ if the index hole was found.

4CA8

JR NZ,4CA5H

If the NZ FLAG (Not Zero) is set then the prior subroutine exited with an error, so JUMP to 4CA5H.

4CAA

POP BC

Restore Register Pair BC from the top of the STACK into .

4CAB

RLCA

Rotate the bits in A left (with Bit 7 going into both the CARRY and Bit 0). This moves Bit 6 from the FDC (Write Protect) to Bit 7.

4CAC

AND 80H

MASK the value of Register A against 80H (1000 0000). This has the effect of turning off bits 6, 5, 4, 3, 2, 1, 0, leaving only bit 7 active.

4CAE

ADD A,A

ADD the value held in Register A to Register A (Results held in Register A) to set the CARRY FLAG if the WRITE PROTECT bit was on.

4CAF

RET

RETurn to the caller.

4CB0H – Subroutine to check to see if the BC counter is zero and, if so, exit, and if not get the disk status and look for an index hole. Returns NZ if the index hole was found.

4CB0

DEC BC

DECrement Register Pair BC by 1 to decrease the counter.

4CB1

LD A,B

Since there is no way to directly check BC for 0, a common way to handle this is to set Register A = Register B.

4CB2

OR C

and then OR Register C against Register A. The results are stored in Register A.

4CB3

JR Z,4CBAH

If the Z FLAG (Zero) is set, then both B and C were zero, so JUMP to 4CBAH.

4CB5

INput a byte from Port F0H and put it into Register A.

NOTE: Port F0H is the FDC Status port on input. Input Results:

Bit 0: Busy
Bit 1: Index/DRQ
Bit 2: Track 0/Data Lost
Bit 3: CRC error
Bit 4: Seek error/Record not found
Bit 5: If Reading, Record Type, if Writing, Write Fault/Head loaded
Bit 6: Write Protect
Bit 7: Not ready

4CB7

BIT 1,A

Check the INDEX FOUND by testing Bit 1 of Register A.

4CB9

RET

RETurn to the caller.

4CBAH – Clear the stack, set up for a DISK TIME OUT error, and return.

4CBA
4CBA

POP BC
POP BC

Clear the STACK.

4CBC

LD A,08H

LET Register A = 08H for a DISK TIME OUT error.

4CBE

OR A

Since Register A is not 00H, this sets the NZ FLAG to signify that an error occurred.

4CBF

RET

RETurn to the caller.

4CC0H – Display an error and return to DOS PROMPT.

4CC0

LD HL,4CCCH

LET Register Pair HL = 4CCCH to point to the error message.

4CC3

GOSUB to 021BH.

NOTE: 021BH will display the character at (HL) until a 03H is found.

4CC6

CALL 0049H

GOSUB to 0049H to wait for user action.

4CC9

JP 4030H

JUMP to 4030H.

NOTE: 4030H is the OPERATION ABORTED routine. Routine runs, displays an error, and returns to the DOS PROMPT.

4CCCH – NO SYSTEM ERROR storage location.

4CCC

1C 1F 17 “0 NS” 0D

NS Error

4CE1H – “Dual” Driver.

4CE1

PUSH BC

Preserve Register Pair BC to the top of the STACK. Register C has the character to display.

4CE2

LD IX,401DH

LET Register Pair IX = 401DH (the VIDEO DCB).

4CE6

CALL 0473H

GOSUB to 0473H to display a character.

4CE9

POP BC

Restore Register Pair BC from the top of the STACK.

4CEA

LD A,C

LET Register A = Register C (the character to display).

4CEB

CP 0DH

Compare the value held in Register A against 0DH. Results: If Register A equals 0DH, the Z FLAG is set; otherwise the NZ FLAG is set.

4CED

JR Z,4CF2H

If the Z FLAG (Zero) is set then we have a CARRIAGE RETURN, so JUMP to 4CF2H.

4CEF

CP 20H

Compare the value held in Register A against 20H looking for a CONTROL CHARACTER. Results:

If Register A equals 20H, the Z FLAG is set.
If A < 20H, the CARRY FLAG will be set.
if A >= 20H, the NO CARRY FLAG will be set.

4CF1

RET C

If the C FLAG (No Carry) is set then it is a CONTROL CHARACTER, and RETurn to the caller.

4CF2

PUSH BC

PUSH Register BC (the character to display) to the top of the STACK.

4CF3

LD IX,4025H

LET Register Pair IX = 4025H (the PRINTER DCB).

4CF7

CALL 03C2H

GOSUB to 03C2H to print the character held in Register C and return.

4CFA

POP BC

Restore Register Pair BC from the top of the STACK.

4CFB

LD A,C

LET Register A = Register C (the character).

4CFC

RET

RETurn to the caller.

4CFDH – DATA STORAGE LOCATION.

Is the “DO” flag active? 0=NO, 255=YES.

4E00H – OVERLAY 0 – DOS Initialization.

First we need to set up the interrupts

4E00

Disable Interrupts.

4E01

IM 1

Set Interrupt Mode to 1

4E03

LD SP,409FH

Set the System STACK to start at 409FH.

4E06

XOR A

Set Register A to ZERO and clear all Flags.

4E07

OUTput the value held in Register A to port F4H to disable the non-maskable interrupts.

NOTE: Port E4H is the Non-Maskable Interrupt Latch. Sending a 0 to E4 will turn off all Non-Maskable Interrupts.

4E09

LD A,20H

LET Register A = 20H (0010 0000).

4E0B

OUT (ECH),A

OUTput the value held in Register A (= Bit 5 On; everything else off) to port ECH to enable VIDEO WAITS and turn off the cassette motor, set to normal screen width, set to Kana character set, disable the I/O bus, and set the system to 2MHZ speed.

Port ECH is the various controls port.

4E0D

LD A,04H

LET Register A = 04H (0000 0100).

4E0F

OUT (E0H),A

OUTput the value held in Register A to port E0H (the Maskable Interrupt Port). Sending Bit 2 high sets this to 4046H to enable the CLOCK INTERRUPT

Decode the TRSDOS v1.3 disk serial number from Binary Coded Decimal to Text

4E11

LD HL,43F4H

LET Register Pair HL = 43F4H, which is the location of the encoded serial number of the TRSDOS v1.3 disk.

4E14

LD IX,5079H

LET Register Pair IX = 5079H, which is the location of the serial number text.

Start of a loop to convert 8 bytes of Binary Coded Decimal into Text

4E18

LD B,08H

LET Register B = 08H to prepare to count 8 Binary Coded Decimal numbers.

4E1A

LD D,00H

LET Register D = 00H.

4E1C

CALL 4FF8H

GOSUB to 4FF8H to convert the BCD number pointed to by Register Pair HL to ASCII.

4E1F

INC HL

Bump Register Pair HL by 1 to point to the next character.

4E20

DJNZ 4E1CH

LOOP back to 4E1CH until Register B is ZERO.

End of loop

4E22

LD (IX+00H),00H

Store the value of nn into the memory location pointed to by the Index Register IX + dd bytes. Note: nn and dd are very likely set elsewhere, and I wouldn’t be suprised if this was to add a CARRIAGE RETURN to the end of the text of the decoded serial number.

4E26H – Determine the system’s Memory Size

4E26

LD HL,FFFFH

LET Register Pair HL = FFFFH, which is the highest memory location for 64K memory when entering this routine..

4E29

LD A,(HL)

Fetch the value stored at memory location pointed to by Register Pair HL and put it into Register A.

4E2A

LD B,A

LET Register B = Register A.

4E2B

CPL

Invert Register A

4E2C

LD (HL),A

Store the value held in Register A (i.e., the inverted value) into the memory location pointed to by Register Pair HL.

4E2D

CP (HL)

Compare the value held in Register A against the value held in the memory location (HL). Results:

If Register A equals (HL), the Z FLAG is set.
If A < (HL), the CARRY FLAG will be set.
if A >= (HL), the NO CARRY FLAG will be set.

4E2E

LD (HL),B

Store the value held in Register B (i.e., the original value) into the memory location pointed to by Register Pair HL.

4E2F

JR Z,4E37H

If the Z FLAG (Zero) is set then we were able to write to that memory location, so JUMP to 4E37H. This is t he routine exit.

4E31

LD A,H

If we are here, then we were unable to write to that memory location, so we need to back up from 64K to 48K to 32K to 16K. To do this, we need to move the MSB into Register A for math.

4E32

SUB 40H

SUBtract the value 40H from Register A to drop down 16K.

4E34

LD H,A

LET Register H = Register A (which is the new MSB of the memory location to test).

4E35

JR 4E29H

JUMP to 4E29H.

4E37H – Continuing Routine to Determine Memory Size and Set Up to Display that Number.

4E37

LD (4411H),HL

Store the value held in Register HL into the memory location 4411H.

NOTE: 4411H is the storage location for the MEMORY PROTECT address.

4E3A

LD (40A0H),HL

Store the value held in Register HL into the memory location 40A0H.

NOTE: 40A0H is the storage location for the TOP OF FREE MEMORY

4E3D

LD (4415H),HL

Store the value held in Register HL into the memory location 4415H.

NOTE: 4415H is the storage location for the ADDRESS OF THE END OF PHYSICAL MEMORY.

4E40

CALL 01C9H

GOSUB to 01C9H to clear the screen.

4E43

LD A,(4412H)

Fetch the value stored at memory location 4412H (i.e., the MSB of the memory size) and put it into Register A.

4E46

LD HL,5010H

LET Register Pair HL = 5010H to point to the message storage area of 48.

4E49

AND F0H

MASK the value of Register A (the MSB of the memory size) against F0H (1111 0000) to test for 48K. This has the effect of turning off bits 3, 2, 1, 0, leaving only bits 7, 6, 5, 4 active.

4E4B

CP F0H

Compare the value held in Register A against F0H. Results: If Register A equals F0H, the Z FLAG is set; otherwise the NZ FLAG is set.

4E4D

JR Z,4E59H

If the Z FLAG (Zero) is set then the memory size is 48K so stop testing by JUMPing to 4E59H.

4E4F

LD HL,5012H

LET Register Pair HL = 5012H to point to the message storage area of 32.

4E52

CP B0H

Compare the value held in Register A against B0H to test for 32K. Results: If Register A equals B0H, the Z FLAG is set; otherwise the NZ FLAG is set.

4E54

JR Z,4E59H

If the Z FLAG (Zero) is set then the memory size is 32K so stop testing by JUMPing to 4E59H.

4E56

LD HL,5014H

No more testing; assume its 16K. LET Register Pair HL = 5014H to point to the message storage area of 16.

4E59

LD DE,504CH

LET Register Pair DE = 504CH to point to the message storage area of 00K

4E5C

LD BC,0002H

LET Register Pair BC = 0002H to set up for a 2 byte move under a LDIR instruction.

4E5F

LDIR

Transfer a byte of data from the memory location pointed to by HL to the memory location pointed to by DE. Then HL and DE are incremented and BC is decremented. If BC is not zero, this operation is repeated. Interrupts can trigger while this instruction is processing.

4E61H – Determine the Number of Disk Drives and Set Up to Display that Number.

4E61

LD C,00H

LET Register C = 00H for drive 0.

4E63

LD B,04H

LET Register B = 04H to signify a maximum of 4 drives.

4E65

GOSUB to 44D4H.

NOTE: 44D4H is the routine which handles DRIVE SELECT (Register C holds the drive number to be used).

4E68

PUSH BC

PUSH Register BC (the drive select code) to the top of the STACK.

4E69

CALL 4FDAH

GOSUB to 4FDAH to test to see if the disk drive (stored in Register C) is present on the system.

4E6C

POP BC

Restore Register BC (the drive select code) from the top of the STACK.

4E6D

JR Z,4E72H

If the Z FLAG (Zero) is set then the drive was NOT on the system, so JUMP to 4E72H to deal with that.

4E6F

INC C

Bump Register C by 1 to point to the next disk drive.

4E70

DJNZ 4E65H

LOOP BACK to 4E65H until Register B is ZERO and 4 drives were tested.

4E72

LD A,C

LET Register A = Register C (which is the last disk drive which was tested).

4E73

DEC A

DECrement Register A by 1 to go to the drive before that one.

4E74

LD (4413H),A

Store the value held in Register A into the memory location 4413H.

NOTE: 4413H is the storage location for the NUMBER OF DISK DRIVES in the system.

4E77

INC A

Bump Register A by 1 to prepare to turn it into an ASCII code for that drive number.

4E78

ADD 30H

ADD the value 30H to Register A (Results held in Register A) to turn Register A into an ASCII rendition of the number that was in Register A.

4E7A

LD (506BH),A

Store the value held in Register A (the ASCII rendition of the number of disk drives in the system) into the memory location 506BH.

4E7DH – Display the Model III Graphic Logo and the Copyright Notices.

4E7D

LD C,00H

LET Register C = 00H to point to drive 0

4E7F

GOSUB to 4566H.

NOTE: 4566H is the routine which resets the FDC.

4E82

GOSUB to 44D4H.

NOTE: 44D4H is the routine which handles DRIVE SELECT (Register C holds the drive number to be used).

4E85

LD HL,516CH

LET Register Pair HL = 516CH which is the message storage area for the Model III logo.

4E88

GOSUB to 021BH.

NOTE: 021BH will display the character at (HL) until a 03H is found.

4E8B

LD A,(38FFH)

Fetch the value stored at memory location 38FFH (which is the keyboard) and put it into Register A.

4E8E

LD (4F3AH),A

Store the value held in Register A (the key which was hit) into the memory location 4F3AH (which is the middle of an OPCODE).

4E91

LD HL,5016H

LET Register Pair HL = 5016H which is the message storage area for the DOS version

4E94

GOSUB to 021BH.

NOTE: 021BH will display the character at (HL) until a 03H is found.

4E97

LD HL,508AH

LET Register Pair HL = 508AH which is the messagr storage area for the copyright notice.

4E9A

GOSUB to 021BH.

NOTE: 021BH will display the character at (HL) until a 03H is found.

4E9D

Enable Interrupts.

4E9E

LD HL,(42B5H)

Fetch the value stored at memory location 42B5H (the DATE flag) and put it into Register HL.

4EA1

LD DE,4455H

LET Register Pair DE = 4455H to signify the valid date flag.

4EA4

PUSH HL

PUSH Register HL (the date flag) to the top of the STACK.

4EA5

OR A

Set FLAGS based on the contents of Register A; but really just to clear the CARRY FLAG (which is about to get used).

4EA6

SBC HL,DE

Subtract with Carry DE from HL to see if the date flag is valid.

4EA8

POP HL

Restore Register HL (the date flag) from the top of the STACK.

4EA9

JP Z,4F2EH

If the result of the SBC is zero, then the date flag is valid, so skip to 4F2EH

4EAC

LD (42B5H),DE

If we are here, then the date flag wasn’t valid, so store the value held in Register DE (the valid date flag) into the memory location 42B5H.

4EB0

JR 4EB8H

JUMP to 4EB8H to get the DATE and TIME from the user.

4EB2H – Get the Current Date from the System User.

4EB2

LD HL,512DH

LET Register Pair HL = 512DH to point to the message storage area of TRY AGAIN.

4EB5

GOSUB to 021BH.

NOTE: 021BH will display the character at (HL) until a 03H is found.

4EB8

LD HL,513BH

LET Register Pair HL = 513BH to point to the message storage area of the DATE PROMPT.

4EBB

CALL 4F84H

GOSUB to 4F84H to display the DATE PROMPT and get the date from the user. On exit, Register A holds the number of characters entered, and HL points to the buffer location where those characters are stored.

4EBE

CP 08H

Compare the value held in Register A against 08H (meaning, 8 characters were returned from the user). Results: If Register A equals 08H, the Z FLAG is set; otherwise the NZ FLAG is set.

4EC0

JR NZ,4EB2H

If the NZ FLAG (Not Zero) is set then we didn’t get 8 characters so JUMP BACK to 4EB2H to display the TRY AGAIN message and try to get a valid date from the user.

4EC2

LD C,2FH

LET Register C = 2FH (ASCII: / so we can further try to validate the 8 characters we got into a valid date.

4EC4

CALL 4FA6H

GOSUB to 4FA6H to decode the date from the input and check the delimiter in Register C. Return NZ FLAG for an error.

4EC7

JR NZ,4EB2H

If the NZ FLAG (Not Zero) is set then the prior subroutine exited with an error, so JUMP BACK to 4EB2H to display the TRY AGAIN message and try to get a valid date from the user.

4EC9

LD HL,4F81H

LET Register Pair HL = 4F81H (the TRSDOS v1.3 memory storage location for the decoded SECONDS or YEAR).

4ECC

LD DE,42B7H

LET Register Pair DE = 42B7H which is the memory location for the date which was held from boot up (i.e., the default date).

4ECF

LD BC,0003H

LET Register Pair BC = 0003H to signify a move of 3 bytes.

4ED2

LDIR

Overwrite the default (boot up) date with the one entered.

4ED4

LD A,(4F81H)

Fetch the value stored at memory location 4F81H (the TRSDOS v1.3 memory storage location for the decoded SECONDS or YEAR) and put it into Register A.

4ED7

CP 64H

Compare the value held in Register A against 64H (Decimal: 100). Results:

If Register A equals 64H, the Z FLAG is set.
If A < 64H, the CARRY FLAG will be set.
if A >= 64H, the NO CARRY FLAG will be set.

4ED9

JR NC,4EB2H

If the NC FLAG (No Carry) is set, then the YEAR is 100 or higher, which is too much, so so JUMP BACK to 4EB2H to display the TRY AGAIN message and try to get a valid date from the user.

4EDB

LD B,A

If we’re here the YEAR is GOOD, so LET Register B = Register A (the year).

4EDC

LD A,(4F83H)

Fetch the value stored at memory location 4F83H (the TRSDOS v1.3 memory storage location for the decoded HOURS or MONTH) and put it into Register A.

4EDF

CP 0DH

Compare the value held in Register A against 0DH (Decimal: 13). Results:

If Register A equals 0DH, the Z FLAG is set.
If A < 0DH, the CARRY FLAG will be set.
if A >= 0DH, the NO CARRY FLAG will be set.

4EE1

JR NC,4EB2H

If the NC FLAG (No Carry) is set, then the MONTH is 13 or higher, which is too much, so so JUMP BACK to 4EB2H to display the TRY AGAIN message and try to get a valid date from the user.

4EE3

OR A

Set FLAGS based on the contents of Register A to set up for a test against MONTH of ZERO.

4EE4

JR Z,4EB2H

If the Z FLAG (Zero) is set, then the MONTH is 0, which is too low, so so JUMP BACK to 4EB2H to display the TRY AGAIN message and try to get a valid date from the user.

4EE6

ADD A,B

If we’re here then we have a valid YEAR and a valid MONTH, so to prepare to test the DAY we must first ADD the value held in Register B (YEAR) to Register A (MONTH).

4EE7

LD B,A

Save that MONTH + YEAR (held in Register A) into Register B.

4EE8

LD A,(4F82H)

Fetch the value stored at memory location 4F82H (the TRSDOS v1.3 memory storage location for the decoded MINUTES or DAY) and put it into Register A.

4EEB

CP 20H

Compare the value held in Register A against 20H (Decimal: 32). Results:

If Register A equals 20H, the Z FLAG is set.
If A < 20H, the CARRY FLAG will be set.
if A >= 20H, the NO CARRY FLAG will be set.

4EED

JR NC,4EB2H

If the NC FLAG (No Carry) is set, then the DAY is 32 or higher, which is too much, so so JUMP BACK to 4EB2H to display the TRY AGAIN message and try to get a valid date from the user.

4EEF

OR A

Set FLAGS based on the contents of Register A to set up for a test against DAY of ZERO.

4EF0

JR Z,4EB2H

If the Z FLAG (Zero) is set, then the DAY is 0, which is too low, so so JUMP BACK to 4EB2H to display the TRY AGAIN message and try to get a valid date from the user.

4EF2

ADD A,B

ADD the value held in Register B (the MONTH + YEAR) to Register A (the DAY). Results held in Register A.

4EF3

OR A

Set FLAGS based on the contents of Register A.

4EF4

JR Z,4EB2H

If the Z FLAG (Zero) is set, then something is WAY off, so so JUMP BACK to 4EB2H to display the TRY AGAIN message and try to get a valid date from the user.

4EF6

JR 4EFEH

JUMP to 4EFEH to continue checking the date.

4EF8H – Get the Current TIME from the System User.

4EF8

LD HL,512DH

LET Register Pair HL = 512DH to point to the message storage area of TRY AGAIN.

4EFB

GOSUB to 021BH.

NOTE: 021BH will display the character at (HL) until a 03H is found.

4EFE

LD HL,5154H

LET Register Pair HL = 5154H to point to the message storage area of the TIME PROMPT.

4F01

CALL 4F84H

GOSUB to 4F84H to display the TIME PROMPT and get the time from the user. On exit, Register A holds the number of characters entered, and HL points to the buffer location where those characters are stored.

4F04

OR A

Set FLAGS based on the contents of Register A.

4F05

JR Z,4F2EH

If the Z FLAG (Zero) is set then we got a null response so JUMP to 4F2EH to continue.

4F07

LD C,3AH

LET Register C = 3AH (ASCII: :).

4F09

CALL 4FA6H

GOSUB to 4FA6H to decode the date from the input and check the delimiter in Register C. Return NZ FLAG for an error.

4F0C

JR NZ,4EF8H

If the NZ FLAG (Not Zero) is set then the prior subroutine exited with an error, so JUMP BACK to 4EF8H to display the TRY AGAIN message and try to get a valid time from the user.

4F0E

LD A,(4F81H)

Fetch the value stored at memory location 4F81H the TRSDOS v1.3 memory storage location for the decoded SECONDS or YEAR) and put it into Register A.

4F11

CP 3DH

Compare the value held in Register A against 3DH (Decimal: 61). Results:

If Register A equals 3DH, the Z FLAG is set.
If A < 3DH, the CARRY FLAG will be set.
if A >= 3DH, the NO CARRY FLAG will be set.

4F13

JR NC,4EF8H

If the NC FLAG (No Carry) is set, then the SECONDS are 61 or higher, which is too much, so so JUMP BACK to 4EF8H to display the TRY AGAIN message and try to get a valid time from the user.

4F15

LD A,(4F82H)

Fetch the value stored at memory location 4F82H (the TRSDOS v1.3 memory storage location for the decoded MINUTES or DAY) and put it into Register A.

4F18

CP 3DH

Compare the value held in Register A against 3DH (Decimal: 61). Results:

If Register A equals 3DH, the Z FLAG is set.
If A < 3DH, the CARRY FLAG will be set.
if A >= 3DH, the NO CARRY FLAG will be set.

4F1A

JR NC,4EF8H

If the NC FLAG (No Carry) is set, then the MINUTES are 61 or higher, which is too much, so so JUMP BACK to 4EF8H to display the TRY AGAIN message and try to get a valid time from the user.

4F1C

LD A,(4F83H)

Fetch the value stored at memory location 4F83H (the TRSDOS v1.3 memory storage location for the decoded HOURS or MONTH) and put it into Register A.

4F1F

CP 18H

Compare the value held in Register A against 18H (Decimal: 24). Results:

If Register A equals 18H, the Z FLAG is set.
If A < 18H, the CARRY FLAG will be set.
if A >= 18H, the NO CARRY FLAG will be set.

4F21

JR NC,4EF8H

If the NC FLAG (No Carry) is set, then the HOUR is 24 or higher, which is too much, so so JUMP BACK to 4EF8H to display the TRY AGAIN message and try to get a valid time from the user.

4F23

LD HL,4F81H

LET Register Pair HL = 4F81H (the TRSDOS v1.3 memory storage location for the decoded SECONDS or YEAR).

4F26

LD DE,4217H

LET Register Pair DE = 4217H (i.e., the time in the time storage area).

4F29

LD BC,0003H

LET Register Pair BC = 0003H (i.e., the number of bytes to move).

4F2C

LDIR

Move the user entered time into the system time storage area.

4F2E

LD HL,42B7H

LET Register Pair HL = 42B7H (i.e., the memory location for the boot-up date).

4F31

LD DE,421AH

LET Register Pair DE = 421AH (i.e., the memory location for the real time date).

4F34

LD BC,0003H

LET Register Pair BC = 0003H (i.e., the number of bytes to move).

4F37

LDIR

Move the boot up date memory location into the real time date memory location.

4F39

LD A,nn

LET Register A = nn. nn was set at 4E8EH to be the key which was pressed at bootup.

4F3B

OR A

Since a LD command does not set any FLAGS, Set FLAGS based on the contents of Register A.

4F3C

JP NZ,4296H

If the NZ FLAG (Not Zero) is set then a key was pressed so JUMP to 4296H.

NOTE: 4296H is the DOS READY jump routine.

4F3F

LD C,00H

LET Register C = 00H to signify DRIVE 0.

4F41

CALL 4A93H

GOSUB to 4A93H to read the GAT into Buffer # 2.

4F44

If the NZ FLAG (Not Zero) is set then the prior subroutine exited with an error, so JUMP to 4409H.

4F47

LD A,(4DE0H)

Fetch the value stored at memory location 4DE0H and put it into Register A.

NOTE: 4DE0H is the first byte of an AUTO command.

4F4A

CP 0DH

Compare the value held in Register A against 0DH. Results: If Register A equals 0DH, the Z FLAG is set; otherwise the NZ FLAG is set.

4F4C

JP Z,4296H

If the Z FLAG (Zero) is set then there was no AUTO command so JUMP to 4296H.

NOTE: 4296H is the DOS READY jump routine.

4F4F

LD A,C9H

LET Register A = C9H, which is the OPCODE for RET which, if placed in a vector, would basically ignore the jump and just return.

4F51

LD (42AEH),A

Store the value held in Register A into the memory location 42AEH which is currently a JUMP VECTOR (thus, nullifying that VECTOR).

4F54

LD HL,4DE0H

LET Register Pair HL = 4DE0H, which is a pointer to the AUTO command.

4F57

LD DE,4225H

LET Register Pair DE = 4225H, which is the start of the DOS command buffer.

4F5A

LD BC,0020H

LET Register Pair BC = 0020H (Decimal: 32) for the number of bytes to move.

4F5D

LDIR

Move the 32 potential bytes for an AUTO command into the DOS command buffer, so it is as if that command was just typed in.

4F5F

LD HL,4F6BH

LET Register Pair HL = 4F6BH, which is a pointed to the AUTO FUNCTION ENGAGED message.

4F62

GOSUB to 021BH.

NOTE: 021BH will display the character at (HL) until a 03H is found.

4F65

LD HL,4225H

LET Register Pair HL = 4225H, which is the start of the DOS command buffer.

4F68

JP 4299H

JUMP to 4299H to execute the command in the DOS command buffer and exit.

4F6BH – Message Storage Area.

4F6B

“AUTO FUNCTION ENGAGED” + 0DH

Message.

4F81

NOP

Storage location for SECONDS and YEAR

4F82

NOP

Storage location for MINUTES and DAY

4F83

NOP

Storage location for HOURS and MONTH

4F84H – Subroutine to display the DATE (or TIME) PROMPT and get the date (or time) from the user. On exit, Register A holds the number of characters entered, and HL points to the buffer location where those characters are stored.

4F84

GOSUB to 021BH.

NOTE: 021BH will display the character at (HL) until a 03H is found.

4F87

LD B,08H

LET Register B = 08H to get up to 8 characters from a ROM call to 0040H.

4F89

LD HL,4225H

LET Register Pair HL = 4225H (the keyboard buffer). HL must be set for a ROM call to 0040H.

4F8C

CALL 0040H

GOSUB to 0040H in the ROM.

NOTE: This routine gets a full line from the keyboard. The line is terminated by a carriage return or BREAK. Characters typed are echoed to the display. On entry, B must be the maximum length of line to be accepted and (HL) must be the storage buffer which should be set to B+1. On Exit, CARRY will be set if the BREAK key was hit, Register B will contain the number of characters entered, and (HL) will contain the line from the keyboard followed by the terminating character. Register paid DE is altered in this routine.

4F8F

LD A,B

Copy the number of characters received (which the ROM call of 0040H puts into Register B) into Register A.

4F90

CP 05H

Compare the value held in Register A against 05H. Results: If Register A equals 05H, the Z FLAG is set; otherwise the NZ FLAG is set.

4F92

RET NZ

If the NZ FLAG (Not Zero) is set, then we did not get exactly 5 characters, so RETurn to the caller.

This doesn’t make entire sense to me because this routine is shared by DATA and TIME, and the next instructions appear to say “Well, if we got 5 characters, then let’s just set the rest to :00”

4F93

PUSH HL

PUSH Register HL (the buffer location) to the top of the STACK.

4F94

LD HL,422BH

LET Register Pair HL = 422BH (the “:nn” [seconds] in the keyboard buffer collecting this information).

4F97

LD (HL),3AH

Store a : into the memory location pointed to by Register Pair HL.

4F99

INC HL

Bump Register Pair HL by 1.

4F9A

LD (HL),30H

Store a 0 into the memory location pointed to by Register Pair HL.

4F9C

INC HL

Bump Register Pair HL by 1.

4F9D

LD (HL),30H

Store a 0 into the memory location pointed to by Register Pair HL.

4F9F

INC HL

Bump Register Pair HL by 1.

4FA0

LD (HL),0DH

Store an ENTER into the memory location pointed to by Register Pair HL.

4FA2

POP HL

Restore the buffer location from the top of the STACK into Register Pair HL.

4FA3

LD A,08H

LET Register A = 08H to denote that now we have 8 characters (the original 5, plus “:00”).

4FA5

RET

RETurn to the caller.

4FA6H – Subroutine to read the value of HOURS/MONTHS, MINUTES/DAY, or SECONDS/YEARS from their memory location and convert that value from text.

4FA6

LD DE,4F83H

LET Register Pair DE = 4F83H (the TRSDOS v1.3 memory storage location for the decoded HOURS or MONTH).

4FA9

LD B,03H

LET Register B = 03H to set a loop for 3 numbers (i.e., memory storage locations 4F83 [HOURS/MONTH], 4F82 [MINUTES/DAY], and 4F81 [SECONDS/YEAR]).

4FAB

PUSH DE

Save the current pointer to the TIME or DATE storage area.

4FAC

CALL 4FBBH

GOSUB to 4FBBH to decode the number from text.

4FAF

POP DE

Restore the current pointer to the TIME or DATE storage agrea from the top of the STACK into Register Pair DE.

4FB0

RET NZ

If the NZ FLAG (Not Zero) is set then there was a conversion error reported by the routine at 4FBBH, so RETurn to the caller.

4FB1

LD (DE),A

Store the decided number (held in Register A) into the memory location pointed to by Register Pair DE.

4FB2

DEC DE

DECrement Register Pair DE by 1 (so 4F83H [H/M] changes to 4F82 [M/D] and again to 4F81 [S/Y]).

4FB3

DEC B

DECrement Register B by 1 to count down from 3 to 1.

4FB4

RET Z

If the Z FLAG (Zero) is set, meaning that we have processed all 3 memory locations, then RETurn to the caller.

4FB5

LD A,(HL)

Fetch the value stored at memory location pointed to by Register Pair HL and put it into Register A. This should be the applicable delimeter of either / if this routine involves processing the date, or : if this routine involves processing the time.

4FB6

INC HL

Bump Register Pair HL by 1 since we just filled the memory location pointed to by HL with a delimeter.

4FB7

CP C

Compare the value held in Register A (the delimeter in memory) against the value held in Register C (the appropriate delimeter fof the routine), to make sure they match. Results: If Register A equals C, the Z FLAG is set; otherwise the NZ FLAG is set.

4FB8

JR Z,4FABH

If the Z FLAG (Zero) is set then the appropriate delimiter was used, so continue on with the 3 numbers via a JUMP to 4FABH.

4FBA

RET

If we are here, NZ FLAG is set because the delimiter was not appropriate, so RETurn to the caller in an error state.

4FBBH – Subroutine to decode the two digit decimal number into BINARY.

4FBB

CALL 4FD2H

GOSUB to 4FD2H to decode a number from ASCII. The C FLAG will be set if there was an error.

4FBE

JR NC,4FD0H

If the NC FLAG (No Carry) is set then the digit provided was invalid, so JUMP to 4FD0H to set the error flag (via an OR A) and RETurn.

4FC0

LD E,A

LET Register E = Register A (the digit being processed).

4FC1

RLCA

Rotate the bits in A left, which is the same as A = A * 2.

4FC2

RLCA

Rotate the bits in A left, which is the same as A = A * 2 (so now the original A = A * 4).

4FC3

ADD A,E

ADD the value held in Register E to Register A (Results held in Register A, so now the original A = A * 5).

4FC4

RLCA

Rotate the bits in A left, which is the same as A = A * 2 (so now the original A = A * 10).

4FC5

LD E,A

LET Register E = Register A (which is the first digit * 10, forming the “tens place” number).

4FC6

CALL 4FD2H

GOSUB to 4FD2H to decode a number from ASCII. The C FLAG will be set if there was an error.

4FC9

JR NC,4FD0H

If the NC FLAG (No Carry) is set then the digit provided was invalid, so JUMP to 4FD0H to set the error flag (via an OR A) and RETurn.

4FCB

ADD A,E

ADD the value held in Register E (the “tens place”) to Register A (Results held in Register A).

4FCC

LD E,A

LET Register E = Register A (which is the completed number).

4FCD

XOR A

Set Register A to ZERO and clear all Flags.

4FCE

LD A,E

LET Register A = Register E.

4FCF

RET

RETurn to the caller.

4FD0H – Routine JUMPed to from the prior decoding routine if there was an error. This will set the NZ FLAG to indicate an error and RETurn.

4FD0

OR A

Set FLAGS based on the contents of Register A, which will trigger a NZ FLAG set – meaning an error condition.

4FD1

RET

RETurn to the caller.

4FD2H – Decode a number from ASCII. The C FLAG will be set if there was an error.

4FD2

LD A,(HL)

Fetch the value stored at memory location pointed to by Register Pair HL and put it into Register A.

4FD3

INC HL

Bump Register Pair HL by 1 to point to the next memory location.

4FD4

SUB 30H

SUBtract the value 30H from Register A which adjusts the number to BINARY.

4FD6

RET C

If the C FLAG (No Carry) is set then the value was too low to be a number, so RETurn to the caller.

4FD7

CP 0AH

Compare the value held in Register A against 0AH to see if the value was too high to be a number. Results:

If Register A equals 0AH, the Z FLAG is set.
If A < 0AH, the CARRY FLAG will be set.
if A >= 0AH, the NO CARRY FLAG will be set.

4FD9

RET

RETurn to the caller.

4FDAH – Subroutine called from 4E69H to est to see if the disk drive (stored in Register C) is present on the system.

4FDA

GOSUB to 4566H.

NOTE: 4566H is the routine which resets the FDC.

4FDD

CALL 4404H

GOSUB to 4404H.

NOTE: 4404H is the routine which has a slight delay.

4FE0

LD A,0CH

4FE2

OUTput the value held in Register A to port F0H.

NOTE: Port F0H is the Floppy Disk Command/Status Register.

4FE4

LD BC,6000H

LET Register Pair BC = 6000H as a delay.

4FE7

CALL 0060H

GOSUB to 0060H to delay for BC iterations.

4FEA

LD BC,0000H

LET Register Pair BC = 0000H to set up for a loop of 65,535.

Start of a loop of 65,535 tries.

4FED

DEC BC

DECrement Register Pair BC by 1.

4FEE
4FEF

LD A,B
OR C

Since the Z-80 cannot test Register Pair BC against zero, the common trick is to set Register A to one, and OR the other. Only if every single bit was off in both will the Z FLAG be set.

4FF0

RET Z

If the Z FLAG (Zero) is set, then BC=0 and we have run out of retries so RETurn to the caller.

4FF1