TRS-80 DOS &#8211; TRSDOS v1.3 &#8211; SYS07/SYS Disassembled

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.

5811

CALL SDELAY
[4404H]

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

5814

LD B,05H

LET Register B = 05H for a DJNZ loop for FIVE revolutions.

5816

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

5818

BIT 1,A

Test Bit Number 1 of Register A. Z FLAG will be set if that bit is 0, and NZ FLAG will be set if that bit is 1.

581A

JR Z,5816H
[5816H]

If the Z FLAG (Zero) is set then we haven’t found the index mark yet for this iteration so, JUMP to 5816H.

581C

LD A,(FDRIVE)

Fetch the drive select code (stored at memory location 4E7DH) and put it into Register A.

581F

CALL DSEL0
[44E0H]

GOSUB to 44E0H to select the drive based on the DRIVE SELECT CODE.

5822

DJNZ 5816H
[5816H]

LOOP back to 5816H, reducing Register B each time, and continue to LOOP until Register B has been reduced to ZERO, in which case, continue with the next instruction.

“L1OOP”
5824

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

5826

BIT 2,A

Test Bit Number 2 of Register A to check for arrival at track 0. Z FLAG will be set if that bit is 0, and NZ FLAG will be set if that bit is 1.

5828

JR NZ,L2OOP
[5832H]

If the NZ FLAG (Not Zero) is set then we have arrived at TRACK 0 so we have no need to position there; JUMP to 5832H to continue with the formatting.

582A

LD A,(FDRIVE)

Fetch the drive select code (stored at memory location 4E7DH) and put it into Register A.

582D

CALL DSEL0
[44E0H]

GOSUB to 44E0H to select the drive based on the DRIVE SELECT CODE.

5830

JR L1OOP
[5824H]

JUMP to 5824H until we have hit track 0.

5832H – Continuation of FORMAT routine above; jumped here once we hit track 0.

“L2OOP”
5832

CALL FORM
[5CBDH]

GOSUB to 5CBDH to format the diskette.

5835

LD A,(BCKFLG)

LET Register A = the value placed into 5836H elsewhere in this overlay. This value will be 0FH if we called a BACKUP routine.

5837

OR A

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

5838

JR NZ,BCKFL1
[5840H]

If the NZ FLAG (Not Zero) is set, meaning that we had called for a BACKUP, skip the next few instructions which display messages and JUMP to 5840H.

583A

LD HL,WRTDIR

LET Register Pair HL = 5B6DH to point to the message “0AH + Writing System Information + 0DH.

583D

CALL PRINT
[021BH]

GOSUB to 021BH.
NOTE: 021BH is the Model III ROM routine to display the character at (HL) until a 03H is found.

“BCKFL1”
5840

LD A,0CH

LET Register A = 0CH (0000 1100), which is the RESTORE TO TRACK 0 command when sent to Port 0F0H.

5842

5844

CALL SDELAY
[4404H]

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

“L3OOP”
5847

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

5849

BIT 2,A

Set Bit Number 2 of Register A to check for arrival at track 0. Z FLAG will be set if that bit is 0, and NZ FLAG will be set if that bit is 1.

584B

JP Z,L3OOP
[5847H]

If the Z FLAG (Zero) is set then we have not yet arrived at track 0; JUMP to 5847H.

584E

CALL BOTWRI
[5A53H]

GOSUB to 5A53H to write the Boot / System Information onto the Diskette.

5851

Enable Interrupts.

5852

RET

RETurn to the caller.

4300H – EXACT COPY OF BOOT/SYS … This is what gets dumped onto a disk. The JUMP values do not match the instruction addresses because it will be Relocated

“ENT”
5853

CP 11H

Compare the value held in Register A against 11H (=17). This is the byte which instructs DOS where the directory track is.

“TRYAGN”
5855

LD A,D0H

LET Register A = D0H (1101 0000). When applied to Port F0H, 1101 0000 is the command to Reset, which puts FDC in mode 1 (INTRQ; “000” terminate command without interrupt).

5857

OUTput the value held in Register A to port F0H (the Floppy Disk Command/Status Register).

5859

LD HL,0002H

LET Register Pair HL = 0002H, which will mean TRACK 00H, SECTOR 02H.

585C

LD (TRKSAV),HL

Store the value held in Register HL into the memory location 43EAH.
NOTE: 43EAH is where the DISK SECTOR NUMBER and 43EBH is where the DISK TRACK NUMBER is stored.

585F

XOR A

Set Register A to ZERO and clear all Flags.

5860

LD (EOFBYT),A

Store the value held in Register A (which is now a 0) into the memory location 43ECH.
NOTE: 43ECH is where the EOF BYTE is stored.

“BOTLOP”
5863

GOSUB to 433EH (the read A BYTE FROM DISKETTE routine).

5866

CP 01H

At this point, Register A holds the byte which was read from Diskette; but we don’t know what kind of byte it is. First, we will compare that byte (held in Register A) against 01H (which is a DATA BLOCK designator). If Register A equals 01H, the Z FLAG is set; otherwise the NZ FLAG is set.

5868

JR Z,BOTLO1

If the Z FLAG (Zero) is set then we have a DATA BLOCK so JUMP to 4323H to deal with a data block.

586A

CP 02H

All we know now is that it isn’t a DATA BLOCK, so next we compare that byte (held in Register A) against 02H (which is a JUMP ADDRESS designator). If Register A equals 02H, the Z FLAG is set; otherwise the NZ FLAG is set.

586C

JR NZ,TRYAGN

If the NZ FLAG (Not Zero) is set meaning we did not get a JUMP ADDRESS then we need to restart the boot routine by JUMPing BACK to 5855H.

586E

If we’re here then we got a JUMP ADDRESS designator, so GOSUB to 433EH (the read A BYTE FROM DISKETTE routine) to get the byte.

5871

CALL GETADD

GOSUB to 4335H (the GET AN ADDRESS FROM DISKETTE routine). Note that the routine exits with the address in Register Pair HL.

5874

JP (HL)

The memory location pointed to by Register Pair HL now holds the jump point, so JUMP to it!

4322H – Data Byte. Number of Backups Permitted.

“CNT1”
5875

FFH

Storage Byte for NUMBER OF BACKUPS FOR RESTRICTED FILES ALLOWED. FFH means unlimited. Any directory entry which has Bit 5 of the first relative byte set will be blocked from being backed up once this counter hits 0. Fun Fact: PATCH will not permit you to change this value.

4323H – Read a Data Block from Diskette into a Memory Buffer Pointed To By HL. If we are here, the byte we read from disk was a DATA BLOCK designator.

“BOTLO1”
5876

First, a little primer. Common data block structure is an 01H followed by a data block length byte (either 00H, 01H, or 02H to indicate whether the block is 254, 255, or 256 bytes), followed by the loading address for that block.

So, at this point we have the 01H, so we need to pull the rest of the header bytes from disk. To do this we GOSUB to 433EH (the READ A BYTE FROM DISKETTE routine).

5879

SUB 02H

Here we are going to covert 00H / 01H / 02H into 254 / 255 / 256. To do this, we SUBtract the value 02H from Register A, so that a 00H becomes FEH (254), a 01H becomes FFH (255), and a 02H becomes 00H (256). Those numbers will be the LOOP to read bytes.

587B

LD B,A

Since Register B is pretty much universally used as a loop counter (and definitely by DJNZ), we must put the modified Register A (which is now set as a loop based on the byte read from diskette for the number of bytes to expect) into Register B.

587C

CALL GETADD

Now that we got the block length indicator, we need to get the 2 byte loading address. To do this we GOSUB to 4335H (the GET AN ADDRESS FROM DISKETTE routine). That routine exits with the address in Register Pair HL.

587FH – Start of LOOP to read “B” bytes from the diskette and put them into a buffer starting at HL.

“BOTLO2”
587F

Get a byte from diskette by GOSUBing to 433EH (the read A BYTE FROM DISKETTE routine).

5882

LD (HL),A

Store the byte which was read from diskette into the memory location pointed to by Register Pair HL.

5883

INC HL

Bump Register Pair HL by 1.

5884

DJNZ BOTLO2

LOOP back to 432CH, reducing Register B each time, and continue to LOOP until Register B has been reduced to ZERO, in which case, continue with the next instruction.

5886

JR BOTLOP

JUMP to 5863H.

5888H – Get an address from diskette. Basically, this just reads 2 bytes into Register Pair HL; LSB into L and MSB into H.

“GETADD”
5888

GOSUB to 433EH (the read A BYTE FROM DISKETTE routine).

588B

LD L,A

LET the LSB of Register Pair HL = Register A.

588C

GOSUB to 433EH (the read A BYTE FROM DISKETTE routine).

588F

LD H,A

LET the MSB of Register Pair HL = Register A.

5890

RET

RETurn to the caller.

5891H – Read a Byte from Diskette

“GETBYT”
5891

PUSH BC

PUSH Register BC to the top of the stack.

5892

PUSH HL

PUSH Register HL to the top of the stack.

5893

LD A,(EOFBYT)

Fetch the value stored at memory location 43ECH and put it into Register A.
NOTE: 43ECH is where the BYTE NUMBER is stored.

5896

OR A

Set FLAGS based on the contents of Register A.

5897

JR NZ,GETOK1

If the NZ FLAG (Not Zero) is set then we are at the sector we want, so JUMP to 4374H.

If we’re here, we are not at the sector we want.

5899

LD B,09H

LET Register B = 09H.

589BH is the start of a loop of 9 tries to get a good sector read.

“GETBY1”
589B

PUSH BC

Save the count by PUSHing Register BC to the top of the stack.

589C

CALL BOTRED

GOSUB to 437FH to read a sector from diskette.

589F

POP BC

RESTORE Register Pair BC from the top of the STACK.

58A0

AND 1DH

MASK the value of Register A against 1DH (0001 1101) to see if we have an error.

58A2

JR Z,GETOK

If the Z FLAG (Zero) is set then we have no error so JUMP to 4364H.

… uh oh, we have an error!

58A4

LD A,CTERM

LET Register A = D0H (1101 0000). When sent to the FLOPPY DISK COMMAND register, 1101 0000 is the command to Reset; puts FDC in mode 1 (INTRQ; “000” terminate command without interrupt).

58A6

OUTput the value held in Register A to port F0H (the Floppy Disk Command/Status Register).

58A8

DJNZ GETBY1

LOOP back to 4348H, reducing Register B each time, and continue to LOOP until Register B has been reduced to ZERO, in which case, continue with the next instruction.

At this point we failed 9 times, so its time to error out.

58AA

LD A,17H

LET Register A = 17H (ASCII: DOUBLE SIZED character code).

58AC

CALL DSP

GOSUB to 0033H to set the screen to double size characters.
NOTE: 0033H is the character print routine, to put the character held in Register A at the current cursor position.

58AF

LD HL,DSKERR

LET Register Pair HL = 43EDH to point to the ERROR message.

58B2

CALL PRINT

GOSUB to 021BH.
NOTE: 021BH will display the character at (HL) until a 03H or 0DH is found.

“GETBAD”
58B5

JR GETBAD

Keep executing this JUMP forever.

58B7H – Jumped to by 434FH if we weren’t originally at the sector we wanted, but we are now.

“GETOK”
58B7

LD HL,(TRKSAV)

Fetch the value stored at memory location 43EAH and put it into Register HL.
NOTE: 43EAH is where the DISK SECTOR NUMBER and 43EBH is where the DISK TRACK NUMBER is stored.

58BA

INC L

Bump Register L by 1 so as to point to the next sector.

58BB

LD A,L

LET Register A = Register L for testing.

58BC

CP TRACK+1

Compare the value held in Register A against 13H (Decimal: 19) which is 1 over the number of sectors in a track. Results:

If Register A equals 13H, the Z FLAG is set.
If A < 13H, the CARRY FLAG will be set.
if A >= 13H, the NO CARRY FLAG will be set.

58BE

JR C,GETOK0

If we are at less than 19 sectors the C FLAG (Not Zero) will be set, and in that case JUMP to 4370H to bypass the routine to BUMP the track number.

If we are here, the sector number is greater than 18 sectors, so we need to bump the track number and reset the sector number to 01H (which is the lowest sector in TRSDOS v1.3 land).

58C0

LD L,01H

LET Register L = 01H to set the sector at 1.

58C2

INC H

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

“GETOK0”
58C3

LD (TRKSAV),HL

Store the value held in Register HL into the memory location 43EAH.
NOTE: 43EAH is where the DISK SECTOR NUMBER and 43EBH is where the DISK TRACK NUMBER is stored.

58C6

XOR A

Set Register A to ZERO and clear all Flags.

“GETOK1”
58C7

LD L,A

We need to set HL to point to 4D00H, so first LET Register L = Register A (which is 00H).

58C8

LD H,BUFF2.HIGH.

LET Register H = 4DH.

58CA

INC A

Bump Register A by 1 to point to the next sector.

58CB

LD (EOFBYT),A

Store the value held in Register A into the memory location 43ECH.
NOTE: 43ECH is where the BYTE NUMBER is stored.

58CE

LD A,(HL)

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

58CF

POP HL

RESTORE Register Pair HL from the value held at the stop of the STACK.

58D0

POP BC

RESTORE Register Pair BC from the value held at the stop of the STACK.

58D1

RET

RETurn to the caller.

58D2H – Read a Sector from Diskette

“BOTRED”
58D2

CALL XSEEK

GOSUB to 43C5H (the POSITION THE READ HEAD routine) which polls the FDC, and sends the TRACK and SECTOR number (held in H and L, respectively) to the FDC.

58D5

LD BC,00F3H

LET Register Pair BC = 00F3H. In preparation for an INI command at 43ABH, Register C is loaded with 0F3H which is the FLOPPY DISK CONTROLLER DATA port and Register B is set to 00H for a loop of 256.

58D8

LD A,81H

LET Register A = 81H (1000 0001) to send DRIVE 0 SELECT and DOUBLE-DENSITY to the FLOPPY DISK CONTROLLER.

58DA

OUTput the value held in Register A (1000 0001) 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)

58DC

LD D,A

Save the DRIVE SELECT code (held in Register A) into Register D.

58DD

LD HL,BOTINT

LET Register Pair HL = 43B7H to set the interrupt vector to jump to 43B7H once triggered.

58E0

LD (DOS+1),HL

Store the value held in Register HL into the memory location 404AH.
NOTE: 4049H is a NMI Vector.

58E3

LD A,C3H

LET Register A = C3H which is a JP instruction.

58E5

LD (DOS),A

Store the “JP instruction into the memory location 4049H.
NOTE: 4049H is the RST 0 NMI Vector.

58E8

Disable Maskable Interrupts.

58E9

LD A,C0H

LET Register A = C0H (1100 0000) to enable the DRQ and INTRQ interrupts.

58EB

OUT (E4H),A

OUTput the value held in Register A to port F4H.
NOTE: Port E4H is the Non-Maskable Interrupt Latch. Sending a Bit 6 and Bit 7 high to E4 will enable the DRQ and INTRQ Interrupts.

58ED

LD E,02H

LET Register E = 02H (0000 0010). This would be the CONTROLLER READY flag. This will be masked against a FDC Status Read in 43A6H so as to test for CONTROLLER READY there.

58EF

LD HL,BUFF2

LET Register Pair HL = 4D00H to point to a buffer in memory starting at 4D00H.

58F2

LD A,84H

LET Register A = 84H (1000 0100).

58F4

OUTput the value held in Register A to port F0H (the Floppy Disk Command/Status Register). In this case, 1000 = READ SECTOR with MULTIPLE RECORD FLAG set, 0 = SIDE COMPARE side 0, 1 = 15 ms delay, 00 = 6ms stepping.

58F6

CALL DELY

GOSUB to 43E0H to do some PUSHes and POPs and NOPs to cause a span delay.

“FSTDRQ”
58F9

IN A,(F0H)

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

58FB

AND E

MASK the value of Register A against the value held in Register E. Register E was set in 439AH to 0000 0010, so only Bit 1 will be set in Register A.

58FC

JR Z,FSTDRQ

If the Z FLAG (Zero) is set (because Bit was 0; meaning CONTROLLER NOT READY), LOOP BACK to 43A6H to poll the controller again.

If we are here, then the FDC returned CONTROLLER READY.

58FE

INI

Read a single byte from Port C (Set to F3H at 4382H) into the memory location pointed to by HL (Set to 4D00H at 439CH), then DECrement HL and INCrement B (Set to 00H at 4382H). Z is set if B = 0; otherwise it is reset.

5900

LD A,D

Put the DRIVE SELECT CODE of 81H (which was moved to Register D in 4389H) back into Register A.

5901

OR 40H

OR Register A against 40H (0100 0000) to turn on Bit 6 (which is the WAIT SYNC request). This will turn 1000 0001 into 1100 0001.

“RDLOOP”
5903

OUTput the value held in Register A to port F4H. This sends the following to the FDC: DRIVE 0 SELECT, SIDE 0 SELECT, disable WRITE PRECOMP, enable WAIT STATE, DOUBLE DENSITY.
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)

5905

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.

5907

JP RDLOOP

LOOP BACK 2 instructions (43B0H) to keep reading bytes. An Interrupt will exit the routine.

590AH – This routine was set as the INTERRUPT VECTOR at 438AH. This routine exits with Register A holding the FLOPPY DISK CONTROLLER status.

“BOTINT”
590A

POP HL

Since HL was set to jump to this specific routine, we need to restore HL. MOVE the value held at the stop of the STACK into Register Pair HL.

590B

XOR A

Set Register A to ZERO and clear all Flags.

590C

OUT (E4H),A

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.

590E

LD A,DOUBLE+1

LET Register A = 81H (1000 0001) to reselect the drive.

5910

OUTput the value held in Register A (1000 0001)to port F4H, which, in this case, is DRIVE 0 SELECT and DOUBLE-DENSITY.
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)

5912

CALL STILL

GOSUB to 43E6H (a VERY short delay routine; basically a PUSH, a POP, a NOP, and a RET.

5915

IN A,(F0H)

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

5917

RET

RETurn to the caller.

5918H – Routine to position the read head based on the contents of HL (H=Track, L=Sector)

“XSEEK”
5918

LD A,81H

LET Register A = 81H (1000 0001) to select the drive.

591A

OUTput the value held in Register A (1000 0001)to port F4H, which, in this case, is DRIVE 0 SELECT and DOUBLE-DENSITY.
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)

591C

LD HL,(TRKSAV)

Fetch the track and sector from memory location 43EAH and put it into Register HL.
NOTE: 43EAH is where the DISK SECTOR NUMBER and 43EBH is where the DISK TRACK NUMBER is stored.

591F

LD A,H

LET Register A = the TRACK NUMBER we want.

5920

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.

5922

LD A,1CH

LET Register A = 1CH (0001 1100). When sent to the FLOPPY DISK COMMAND register, 0001 is Seek, 1=Head Load at Beginning, 1=Verify On Destination Track, 00 = 6 ms stepping.

5924

OUTput the value held in Register A to port F0H (the Floppy Disk Command/Status Register).

5926

CALL STILL

GOSUB to 43E6H (a VERY short delay routine; basically a PUSH, a POP, a NOP, and a RET.

“XSEEK1”
5929

IN A,(F0H)

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

592B

BIT 0,A

Test Bit Number 0 of Register A. Bit 0 of the result of polling 0F0H is the BUSY bit.
Z FLAG will be set if that bit is 0 (=NOT BUSY), and NZ FLAG will be set if that bit is 1 (=BUSY).

592D

JR NZ,XSEEK1

If the NZ FLAG (Not Zero) is set, meaning that the FDC returned a BUSY status, LOOP BACK 2 instructions (to 43D6H) and keep polling the FDC.

592F

LD A,L

LET Register A = the SECTOR NUMBER we want.

5930

OUT (F2H),A

OUTput the value held in Register A to Port F2H.
NOTE: Port F2H is the Floppy Disk Controller SECTOR Register.

5932

RET

RETurn to the caller.

5933H – Routine delay Of 2 Lengths (Depending on Entry). Used primarily as a short delay between OUTputting to the FDC and INputting from the FDC.

“DELY”
5933

PUSH AF

Save Register Pair AF to the top of the stack.

5934

POP AF

Put the value held at the top of the STACK into Register Pair AF, and then remove the entry from the stack.

5935

PUSH AF

Save Register Pair AF to the top of the stack.

5936

POP AF

Put the value held at the top of the STACK into Register Pair AF, and then remove the entry from the stack.

5937

PUSH AF

Save Register Pair AF to the top of the stack.

5938

POP AF

Put the value held at the top of the STACK into Register Pair AF, and then remove the entry from the stack.

“STILL”
5939

PUSH AF

Save Register Pair AF to the top of the stack.

593A

POP AF

Put the value held at the top of the STACK into Register Pair AF, and then remove the entry from the stack.

593B

NOP

No Operation (Do Nothing).

593C

RET

RETurn to the caller.

593DH – MESSAGE and BYTE STORAGE AREA.

“TRKSAV”
593D

DEFW 02H 00H

“EOFBYT”
593F

DEFB 00H

“DSKERR”
5940

DEFB 17H

5941

DEFM …

“ERROR + 0DH”.

“S1SER”
5947

DEFB 256-$+ENT-3%0

“S1SIDE”
5950

DEFB 00H

“TYPES”
5951

DEFB 13H

TRSDOS v1.3

“TRACKS”
5952

DEFB 28H

= 40 Tracks

“SEC3”
5953

DEFB 01

“LEFT”
5954

DEFB RIGHT

This is 21

5955

DEFW NOSYS

This is 00 4D

5957H – Display NOT A SYSTEM DISK and Freeze the System.

“NOSYS”
5957

LD A,17H

LET Register A = 17H (ASCII: Double-Sized Characters).

5959

CALL DSP
[0033H]

GOSUB to 0033H.
NOTE: 0033H is the Model III ROM character print routine; displays the character held in Register A at the current cursor position.

595C

LD HL,SYSTMG

LET Register Pair HL = 4D0DH to point to the message “NOT A SYSTEM DISK“.

595F

CALL PRINT
[021BH]

GOSUB to 021BH.
NOTE: 021BH is the Model III ROM routine to display the character at (HL) until a 03H is found.

“NOSYS1”
5962

JR NOSYS1
[5962H]

Infinite JUMP to this Location.

5964H – MESSAGE and BYTE STORAGE AREA.

“SYSTMG”
5964

DEFM …

“NOT A SYSTEM DISK” + 0DH

5976

DEFB 02 02 00 4D

Execute at 4D00H

597A

DEFB E5H x 217

217 E5’s to fill the rest of a sector.

5A53H – Write the Bootstrap, System, and Directory to Diskette.

“BOTWRI”
5A53

LD DE,0001H

LET Register Pair DE = 0001H, which is the bootstrap sector location (Track 00, Sector 01).

5A56

LD HL,BOOT

LET Register Pair HL = 5853H, which is a pointer to the bootstrap image.

5A59

LD A,(FDRIVD)

Fetch the destination drive number from memory location 4E7FH and put it into Register A.

5A5C

LD C,A

LET Register C = the destination drive number.

5A5D

CALL XWRITE
[45F7H]

GOSUB to 45F7H to write the bootstrap to diskette.

5A60

JP NZ,FATAL
[4E00H]

If the NZ FLAG (Not Zero) is set, meaning that the prior CALL routine returned because of an ERROR, JUMP to 4E00H.

5A63

INC E

INCrement the value stored in Register E by 1 so as to point to Sector 02.

5A64

LD HL,SYST

LET Register Pair HL = 5953H, which is a pointer to a fake system image for disks with NO SYStem to write to diskette.

5A67

CALL XWRITE
[45F7H]

GOSUB to 45F7H to write the fake system image to diskette.

5A6A

JP NZ,FATAL
[4E00H]

If the NZ FLAG (Not Zero) is set, meaning that the prior CALL routine returned because of an ERROR, JUMP to 4E00H.

5A6D

LD DE,1101H

LET Register Pair DE = 1101H, meaning Register D will hold 11H (Track 17), and Register E will hold 01H (Sector 1).

5A70

LD HL,FGAT

LET Register Pair DE = 6100H as the destination for a transfer of the GAT SECTOR’s RAM Buffer.

5A73

LD A,(FDRIVD)

Fetch the destination drive number from memory location 4E7FH and put it into Register A.

5A76

LD C,A

LET Register C = the destination drive number.

5A77

CALL XWRITE
[45F7H]

GOSUB to 45F7H to write the GAT SECTOR to diskette.

5A7A

JP NZ,FATAL
[4E00H]

If the NZ FLAG (Not Zero) is set, meaning that the prior CALL routine returned because of an ERROR, JUMP to 4E00H.

“BOTWR1”
5A7D

INC E

INCrement the value stored in Register E by 1 so as to point to the next sector.

5A7E

LD A,E

LET Register A = the current sector number.

5A7F

CP TRACK+1

Compare the value held in Register A against 13H (Decimal: 19) to see if we are done with the directory track. Results:

If Register A equals 13H, the Z FLAG is set.
If A < sector 19, the CARRY FLAG will be set.
if A >= sector 19, the NO CARRY FLAG will be set.

5A81

RET NC

If we have advanced Register E to sector 19 (which is one too many), the the NC FLAG (No Carry) is set; RETurn to the caller.

5A82

LD HL,DUM1

LET Register Pair HL = 5F00H to point to a dummy directory sector image.

5A85

CALL XWRITE
[45F7H]

GOSUB to 45F7H to write the SECTOR DATA to diskette.

5A88

JP NZ,FATAL
[4E00H]

If the NZ FLAG (Not Zero) is set, meaning that the prior CALL routine returned because of an ERROR, JUMP to 4E00H.

5A8B

LD A,E

LET Register A = the current sector number (held in Register E).

5A8C

CP 02H

Compare the value held in Register A against 02H, which would be the HIT Table. Results: If Register A 02H, the Z FLAG is set; otherwise the NZ FLAG is set.

5A8E

JR NZ,BOTWR1
[5A7DH]

If the NZ FLAG (Not Zero) is set, then we have not yet found the HIT sector; so JUMP to 5A7DH.

5A90

PUSH BC

Save Register Pair BC to the top of the stack.

5A91

PUSH DE

Save Register Pair DE to the top of the stack.

5A92

PUSH HL

Save Register Pair HL to the top of the stack.

5A93

LD HL,DUM1+224

LET Register Pair HL = 5FE0H, to prepare to zero out the SYSTEM EXTENTS area to default values.

5A96

LD DE,DUM1+225

LET Register Pair DE = HL + 1.

5A99

LD BC,000FH

LET Register Pair BC = 000FH (Decimal: 15) for a LDIR count of 15 bytes.

5A9C

LD (HL),00H

Store a 00H into the memory location pointed to by Register Pair HL.

5A9E

LDIR

5AA0

LD HL,COPY

5AA3

LD DE,DUM1+240

LET Register Pair DE = 5FF0H to point to the applicable notice area in the sector image.

5AA6

LD BC,0010H

LET Register Pair BC = 0010H (Decimal: 16) for the number of bytes to copy.

5AA9

LDIR

5AAB

POP HL

Put the value held at the top of the STACK into Register Pair HL, and then remove the entry from the stack.

5AAC

POP DE

Put the value held at the top of the STACK into Register Pair DE, and then remove the entry from the stack.

5AAD

POP BC

Put the value held at the top of the STACK into Register Pair BC, and then remove the entry from the stack.

5AAE

JR BOTWR1
[5A7DH]

LOOP back to 5A7DH to move to the next sector.

5AB0H – Write a Track of Data.

“TRKWRT”
5AB0

LD BC,00F3H

LET Register Pair BC = 00F3H, which sets Register C to be the FDC Data Port of F3H.

5AB3

LD A,(FDRIVE)

Fetch the drive select code (stored at memory location 4E7DH) and put it into Register A.

5AB6

CALL DSEL0
[44E0H]

GOSUB to 44E0H to select the drive.

5AB9

OR 40H

OR Register A against 40H (0100 0000) to turn on bit 6, which is the WAIT SYNC bit.

5ABB

LD D,A

LET Register D = the masked drive select code (held in Register A).

5ABC

LD A,(DDSID)

Fetch the SIDE of the diskette we want to format (stored at memory location 5BD6H) and put it into Register A.

5ABF

OR A

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

5AC0

JR Z,TRKWR1
[5AC6H]

If the Z FLAG (Zero) is set, then we are formatting side 0, so JUMP to 5AC6H.

5AC2

LD A,10H

If we are here, then we are formatting side 1. LET Register A = 10H (0001 0000) to act as a mask to signify that we are writing to side 1.

5AC4

OR D

OR Register D against Register A to set that bit. The results are stored in Register A.

5AC5

LD D,A

LET Register D = the masked drive select code (held in Register A).

“TRKWR1”
5AC6

CALL BOTNMI
[459CH]

GOSUB to 459CH to set up interrupts.

5AC9

LD A,0C0H

LET Register A = 0C0H (1100 0000). When applied to port C0H it enables DRQ Interrupt and enables INTRQ Interrupt.

5ACB

OUT (0E4H),A

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.

5ACD

LD E,02H

LET Register E = 02H (0000 0010) to be a “data request” mask when applied to port 0F0H

5ACF

LD A,0F0H

LET Register A = 0F0H (1111 0000) which is a track write command.

5AD1

When this bit pattern in sent to Port F0H (the Floppy Disk Control Port) and bits 7-4 are 1111, this is a WRITE TRACK command.

5AD3

CALL DELLY
[4400H]

GOSUB to 4400H for a short delay which is needed between issuing a command to the FDC and polling it.

“IMDRQ1”
5AD6

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

5AD8

AND E

MASK the value of Register A (which we just got from the FDC) against 0000 0010 (held in Register E) to keep only bit 1 to see if it was a data request.

5AD9

JP Z,IMDRQ1
[5AD6H]

If the Z FLAG (Zero) is set then the FDC is not yet ready for data so JUMP to 5AD6H.

5ADC

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.

“IMDRQ2”
5ADE

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

5AE0

AND E

MASK the value of Register A against 0000 0010 (held in Register E) to check for a data request.

5AE1

JP Z,IMDRQ2
[5ADEH]

If the Z FLAG (Zero) is set then the FDC is not yet ready for data so JUMP to 5ADEH.

5AE4

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.

5AE6

LD A,D

LET Register A = the drive select code (with the WAIT BIT turned on).

“WRLP”
5AE7

OUT (0F4H),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)

5AE9

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.

5AEB

JP 5AE7H
[5AE7H]

JUMP to 5AE7H and keep jumping until an Interrupt is triggered.

5AEEH – Verify One Track.

“VERFIT”
5AEE

LD D,12H

LET Register D = 12H (Decimal: 18) to signify that we want to verify 18 sectors.

5AF0

LD A,(FDRIVE)

Fetch the disk select code (stored at memory location 4E7DH) and put it into Register A.

5AF3

CALL DSEL0
[44E0H]

GOSUB to 44E0H to select the disk drive.

“VER1”
5AF6

LD A,D

LET Register A = the sector number to verify (held in Register D).

5AF7

SUB 13H

SUBtract the value 13H (Decimal: 19) from Register A so that sectors are verified in ascending order.

5AF9

NEG

Negate the subtracted value so that it is a positive number

5AFB

LD (FSVSEC),A

Store the ascending sector number (held in Register A) of the sector being checked (held in Sector D) into the memory location 5B1BH.

5AFE

PUSH BC

Save Register Pair BC to the top of the stack.

5AFF

PUSH DE

Save Register Pair DE to the top of the stack.

5B00

PUSH HL

Save Register Pair HL to the top of the stack.

5B01

LD HL,VERASC

LET Register Pair HL = 5B9AH in preparation for the next call to convert it to ASCII.

5B04

CALL ASCII
[5B55H]

GOSUB to convert the sector number into text.

5B07

LD A,(CURSOR)

Fetch the value stored at memory location 4020H and put it into Register A.
NOTE: 4020H is the storage location for the CURSOR POSITION ON SCREEN.

5B0A

AND 0DFH

MASK the value of Register A against 0DFH (1101 1111). This has the effect of turning off bit 5.

5B0C

OR 1FH

OR Register A against 1FH (0001 1111). This has the effect of turning on bits 4, 3, 2, 1, 0. Now Register A should be pointing to column 31.

5B0E

LD (CURSOR),A

Store the value held in Register A into the memory location 4020H.
NOTE: 4020H is the storage location for the CURSOR POSITION ON SCREEN.

5B11

LD HL,VERMSG

LET Register Pair HL = 5B89H to point to the message “Verifying Sector xx“.

5B14

CALL PRINT
[021BH]

GOSUB to 021BH.
NOTE: 021BH is the Model III ROM routine to display the character at (HL) until a 03H is found.

5B17

LD HL,BUFF

LET Register Pair HL = 6200H, which will be the buffer in RAM

5B1A

LD E,(FSVSEC)

LET Register E = the value (i.e., the sector number in ascending order) put into 5B1BH elsewhere in this overlay; which is the sector number.

5B1C

LD A,(DDTRK)

Fetch the track number (stored at memory location 5BD3H) and put it into Register A.

5B1F

LD D,A

LET Register D = the track number (held in Register A).

5B20

LD A,(FDRIVD)

Fetch the destination drive number from memory location 4E7FH and put it into Register A.

5B23

LD C,A

LET Register C = destination drive number (held in Register A).

5B24

LD A,(DDSID)

Fetch the side number (stored at memory location 5BD6H) and put it into Register A.

5B27

LD (CSIDE),A

Store the source side number (held in Register A) into the memory location 4463H.
NOTE: 4463H is the storage location for the old system disk side number.

5B2A

CALL XREAD
[4675H]

GOSUB to 4675H to read the sector.

5B2D

POP HL

Put the value held at the top of the STACK into Register Pair HL, and then remove the entry from the stack.

5B2E

POP DE

Put the value held at the top of the STACK into Register Pair DE, and then remove the entry from the stack.

5B2F

POP BC

Put the value held at the top of the STACK into Register Pair BC, and then remove the entry from the stack.

5B30

RET NZ

If the NZ FLAG (Not Zero) is set then we had an error from the call to read the sector, so RETurn to the caller.

“VERFI2”
5B31

DEC D

DECrement the value stored in Register D by 1 for the next sector to check.

5B32

JR NZ,VER1
[5AF6H]

If the NZ FLAG (Not Zero) is set, keep processing the sectors in the track by JUMPing to 5AF6H.

5B34

RET

RETurn to the caller.

5B35H – Position the Diskette Head to the Track held in Register C.

“STEPR”
5B35

LD A,(FDRIVE)

Fetch the drive select code (stored at memory location 4E7DH) and put it into Register A.

5B38

CALL DSEL0
[44E0H]

GOSUB to 44E0H to select the disk.

5B3B

LD A,58H

LET Register A = 58H (0101 1000), which, when sent to 0F0H, sends: STEP IN (010), Track register needs to be updated (1), Load head at the beginning (1), no Verify (0), 6ms stepping (00).

5B3D

Send the bit pattern in sent to Port F0H (the Floppy Disk Control Port). When activated one step pulse is sent in the direction of track 76. Once stepped, and interrupt is generated.

5B3F

CALL CHKBSY
[4547H]

GOSUB to 4547H.
NOTE: 4547H is a routine to wait for the FDC.

5B42

PUSH BC

Save Register Pair BC to the top of the stack.

5B43

PUSH DE

Save Register Pair DE to the top of the stack.

5B44

PUSH HL

Save Register Pair HL to the top of the stack.

5B45

LD A,(FDRIVD)

Fetch the destination drive number from memory location 4E7FH and put it into Register A.

5B48

LD E,A

LET Register E = the destination drive number (held in Register A).

5B49

LD D,00H

LET Register D = 00H so that Register Pair DE is now the Offset in the disk head position table.

5B4B

LD HL,SEKTRK

LET Register Pair HL = 445BH to point to the DOS TRACK TABLE.

5B4E

ADD HL,DE

LET Register Pair HL = Register Pair HL + Register DE so that HL now points to the appropriate entry in the DOS TRACK TABLE.

5B4F

LD (HL),C

Store the head position (held in Register C) into the appropriate entry in the DOS TRACK TABLE.

5B50

POP HL

Put the value held at the top of the STACK into Register Pair HL, and then remove the entry from the stack.

5B51

POP DE

Put the value held at the top of the STACK into Register Pair DE, and then remove the entry from the stack.

5B52

POP BC

Put the value held at the top of the STACK into Register Pair BC, and then remove the entry from the stack.

“STEPR1”
5B53

XOR A

Set Register A to ZERO and clear all Flags.

5B54

RET

RETurn to the caller.

5B55H – Convert Register A into a Two Digit Decimal to be stored at (HL).

“ASCII”
5B55

PUSH BC

Save Register Pair BC to the top of the stack.

5B56

LD C,30H

LET Register C = 30H (ASCII: 0) so that the tens digit is a “0” unless changed.

“SUB”
5B58

SUB 0AH

SUBtract the value 0AH from Register A to see if Register A is still positive after subtracting 10.

5B5A

JR C,ADD
[5B5FH]

If the C FLAG (Carry) is set then subtracting 10 made it negative, so JUMP to 5B5FH.

5B5C

INC C

INCrement the tens digit (stored in Register C) by 1.

5B5D

JR SUB
[5B58H]

Loop to 5B58H until Register A is less than 10.

5B5FH – Continuation of the routine to convert Register A into a Two Digit Decimal to be stored at (HL), jumped here once we have a remainder.

“ADD”
5B5F

ADD A,3AH

LET Register A = Register A + 3AH to form the one’s digit.

5B61

LD B,A

LET Register B = the one’s digit (the Register A).

5B62

LD (HL),C

Store the ten’s digit (held in Register C) into the memory location pointed to by Register Pair HL.

5B63

INC HL

INCrement the value stored in Register Pair HL by 1 to move to the next memory location.

5B64

LD (HL),B

Store the one’s digit (held in Register B) into the memory location pointed to by Register Pair HL.

5B65

POP BC

Put the value held at the top of the STACK into Register Pair BC, and then remove the entry from the stack.

5B66

RET

RETurn to the caller.

5B67H – Display a Message.

“PDATA”
5B67

PUSH DE

Save Register Pair DE to the top of the stack.

5B68

CALL PRINT
[021BH]

GOSUB to 021BH.
NOTE: 021BH is the Model III ROM routine to display the character at (HL) until a 03H is found.

5B6B

POP DE

Put the value held at the top of the STACK into Register Pair DE, and then remove the entry from the stack.

5B6C

RET

RETurn to the caller.

5B6DH – MESSAGE and BYTE STORAGE.

“WRTDIR”
5B6D

DEFM …

“0AH + “Writing System Information” + 0DH.

“VERMSG”
5B89

DEFM …

““Verifying Sector “.

“VERASC”

DEFM …

““00” + 03H.

“FORMSG”
5B9D

DEFM …

“1DH + “Formatting Track “.

“FORASC”

DEFM …

““00” + 03H.

5BB5H – This is the phrase address list.

“TRKNN”
5BB5

DEFW C5H 5BH

Start of Track Data for tracks 00 – MAXTRK, found at “DDTAB” (5BC5H) below.

5BB7

DEFW C8H 5BH

Start of Sector Data, found at “DDREP” (5BC8H) below.

5BB9

DEFW 07H 5CH

Start of End-of-Track Data, found at “DDTRL” (5C07H) below.

5BBB

DEFW 0AH 5CH

Start of Sector Table, found at “SECTAB” (5C0AH) below.

“DDST”
5BBD

DEFW D9H 5BH

Start of Track Data, found at “DDSEC” (5BD9H) below.

5BBF

DEFB 01H

Beginning Sector Number in a Track

5BC0

DEFB 12H

Ending Sector Number in a Track (=18 Decimal)

5BC1

DEFW D3H 5BH

Track Number Structure Address, found at “DDTRK” (5BD3H) below.

5BC3

DEFW ECH 5BH

Double Density Data Sector Table Entry Address, found at “DDDATA” (5BECH) below.

“DDTAB”
5BC5

DEFW 40 00 4E

64 x ‘4E’. This is the track header.

“DDREP”
5BC8

DEFW 08 00 00

08 x ’00’. This is the part of the Sector ID Header.

5BCB

DEFW 03 00 F5

03 x ‘F5’. This is the part of the Sector ID Header.

5BCE

DEFW 01 00 FE

01 x ‘FE’. This is an ID Address Mark.

5BD1

DEFW 01 00

01 x …

“DDTRK”
5BD3

DEFB 00

… ’00’. This is the track number.

5BD4

DEFW 01 00

01 x …

“DDSID”
5BD6

DEFB 00

… ’00’. This is the side number.

5BD7

DEFW 01 00

01 x …

“DDSEC”
5BD9

DEFB 00

… ’00’. This is the sector number.

5BDA

DEFW 01 00 00

01 x ’00’. This is the sector number.

5BDD

DEFW 01 00 01

01 x ’01’. This is an 01H.

5BD1

DEFW 01 00 F7

01 x ‘F7’. This the code to write two CRC bytes to the Sector ID Header.

5BE0

DEFW 16 00 4E

22 x ‘4E’. This is part of the user data header.

5BE3

DEFW 0C 00 00

12 x ’00’. This is part of the user data header.

5BE6

DEFW 03 00 F5

03 x ‘F5’. This is part of the user data header.

5BE9

DEFW 01 00 FB

01 x ‘FB’. This is a Data Address Mark.

“DDDATA”
5BEC

DEFW F0 00 E5

240 x ‘E5’. This fills the sector with the empty byte filler of E5.

5BEF

DEFW FF FF

“COPY”
5BF1

This is the copyright message.

5C01

DEFW 01 00 F7

01 x ‘F7’. This the code to write two CRC bytes to the Sector ID Header.

5C04

DEFW 18 00 4E

24 x ‘4E’. This is the trailer bytes.

“DDTRL”
5C07

DEFW 82 03 4E

898 x ‘4E. This is the end of track data.

5C0AH – MESSAGE and BYTE STORAGE – Sector Number Table.

“SECTAB”
5C0A

DEFB …

01 04 07 0A 0D 10 – Interleaved sector numbers.

5C10

DEFB …

02 05 08 0B 0E 11 – Interleaved sector numbers.

5C16

DEFB …

03 06 09 0C 0F 12 – Interleaved sector numbers.

5C1C

DEFB 00H

End of Table.

“CSECTB”
5C1D

DEFW 0AH 5CH

Pointer into the sector number table

5C1FH – Generate the Track Image Area into the Buffer. On Entry, C contains the Track Number.

“TRKIMG”
5C1F

PUSH BC

Save Register Pair BC to the top of the stack.

5C20

PUSH DE

Save Register Pair DE to the top of the stack.

5C21

PUSH HL

Save Register Pair HL to the top of the stack.

5C22

PUSH AF

Save Register Pair AF to the top of the stack.

5C23

PUSH IY

Save Special Index Register IY to the top of the stack.

5C25

LD H,(IX+TRKADD+MSB)

Fetch the MSB of the TRACK NUMBER in track format image into Register H.

5C28

LD L,(IX+TRKADD+LSB)

Fetch the LSB of the TRACK NUMBER in track format image into Register H.

5C2B

LD (HL),C

Store the track number (held in Register C) into the table (i.e. the memory location pointed to by Register Pair HL).

5C2C

LD A,C

LET Register A = the track number.

5C2D

LD (SAVEA),A

Store the track number (held in Register A) into the memory location 5C4DH.

5C30

LD DE,BUFF

LET Register Pair DE = 6200H, which will be the buffer in RAM for the track image.

5C33

CALL TBLBMP
[5C7BH]

GOSUB to 5C7BH to get the addresses for the table (Part 1).

“TRKIM1”
5C36

CALL EXPAND
[5C8CH]

GOSUB to 5C8CH to generate the track header field from the table entry.

5C39

JR C,TRKIM1
[5C36H]

If the HL is less than BC, the C FLAG (Carry) is set, so LOOP back to 5C36H as there is more to generate.

5C3B

CALL TBLBMP
[5C7BH]

GOSUB to 5C7BH to get the addresses for the table (Part 2).

5C3E

PUSH HL

Save the address of the current table entry (held in Register Pair HL) to the top of the stack.

5C3F

LD HL,SECTAB

LET Register Pair HL = 5C0AH to point to the SECTOR NUMBER TABLE (in interleaved order)

5C42

LD (CSECTB),HL

Reset the SECTOR NUMBER TABLE (held in Register HL) into the memory location 5C1DH (which is the storage location for the pointer into the sector number table).

“TRKIM2”
5C45

LD HL,(CSECTB)

Fetch the position in the sector number table (stored at memory location 5C1DH) and put it into Register HL.

5C48

LD A,(HL)

Fetch the sector number to use based on where HL is pointing and put it into Register A.

5C49

OR A

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

5C4A

JR Z,TRKIM4
[5C6BH]

If the Z FLAG (Zero) is set then we have hit the 00H terminator for the SECTOR NUMBER TABLE, so JUMP to 5C6BH.

5C4C
+1=”SAVEA”

LD A,(SAVEA)

LET Register A = the track number which was put into 5C4DH elsewhere in this overlay.

5C4E

CP 40H

Compare the value held in Register A against 28H (Decimal: 40). Results:

If Register A equals track 40, the Z FLAG is set.
If A < track 40, the CARRY FLAG will be set.
if A >= track 40, the NO CARRY FLAG will be set.

5C50

LD A,(HL)

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

5C51

JR C,TRIM21
[5C55H]

If we are < track 40, then the C FLAG (Carry) is set and we are done, so JUMP to 5C55H.

If we are here, then we are at track 40, which Frank Durda IV referred to as “the secret track”.

5C53

OR 80H

Set the correct sector number for “secret track” by ORing Register A against 80H (1000 0000) to turn on bit 7 of the sector number held in Register A.

“TRIM21”
5C55

INC HL

INCrement the value stored in Register Pair HL by 1 so that it will point to the next entry in the sector number table.

5C56

LD (CSECTB),HL

Store the current pointer to the SECTOR NUMBER TABLE (held in Register HL) into the memory location 5C1DH (which is the storage location for the pointer into the sector number table).

5C59

LD H,(IX+SECADD+MSB)

Fetch the MSB of the SECTOR NUMBER TABLE into Register H.

5C5C

LD L,(IX+SECADD+LSB)

Fetch the LSB of the SECTOR NUMBER TABLE into Register H.

5C5F

LD (HL),A

Store the sector number (i.e., the value held in Register A) into the SECTOR NUMBER TABLE at the memory location pointed to by Register Pair HL.

5C60

POP HL

Put the pointers (held at the top of the STACK) into Register Pair HL, and then remove the entry from the stack.

5C61

PUSH HL

Save the pointers (held in Register Pair HL) to the top of the stack.

“TRKIM3”
5C62

CALL EXPAND
[5C8CH]

GOSUB to 5C8CH to generate the sector image field.

5C65

JR C,TRKIM3
[5C62H]

If the C FLAG (Carry) is set then we still have more to process, so jump back to 5C62H.

5C67

POP HL

Put the pointers (held at the top of the STACK) into Register Pair HL, and then remove the entry from the stack.

5C68

PUSH HL

Save the pointers (held in Register Pair HL) to the top of the stack.

5C69

JR TRKIM2
[5C45H]

LOOP BACK to 5C45H to process the next sector.

5C6BH – Continuation of the Generate the Track Image Area; jumped here when we have reached the end of the SECTOR NUMBER TABLE.

“TRKIM4”
5C6B

POP HL

Clear the Stack.

5C6C

CALL TBLBMP
[5C7BH]

GOSUB to 5C7BH to get the addresses for the table (Part 3).

“TRKIM5”
5C6F

CALL EXPAND
[5C8CH]

GOSUB to 5C8CH to generate the trailer field.

5C72

JR C,TRKIM5
[5C6FH]

If the C FLAG (Carry) is set then we have more to go, so loop back to 5C6FH.

5C74

POP IY

Put the value held at the top of the STACK into Special Index Register IY, and then remove the entry from the stack.

5C76

POP AF

Put the value held at the top of the STACK into Register Pair AF, and then remove the entry from the stack.

5C77

POP HL

Put the value held at the top of the STACK into Register Pair HL, and then remove the entry from the stack.

5C78

POP DE

Put the value held at the top of the STACK into Register Pair DE, and then remove the entry from the stack.

5C79

POP BC

Put the value held at the top of the STACK into Register Pair BC, and then remove the entry from the stack.

5C7A

RET

RETurn to the caller.

5C7BH – Retrieve the Table Addresses.

“TBLBMP”
5C7B

LD H,(IY+BEGADD+MSB)

Fetch the MSB of the START OF THE IMAGE DATA BLOCK into Register H.

5C7E

LD L,(IY+BEGADD+LSB)

Fetch the LSB of the START OF THE IMAGE DATA BLOCK into Register L.

5C81

LD B,(IY+ENDADD+MSB)

Fetch the MSB of the START OF THE NEXT BLOCK into Register B.

5C84

LD C,(IY+ENDADD+LSB)

Fetch the LSB of the START OF THE NEXT BLOCK into Register C.

5C87
5C89

INC IY
INC IY

INCrement the pointer stored in Special Index Register IY by 2 to point to the next table entry.

5C8B

RET

RETurn to the caller.

5C8CH – Use the Track Building Table to fill up the table pointed to by DE.

On entry, HL points to the beginning of the table. On exit, DE = DE + the length of the fill, HL points to the NEXT table entry, and flags are set.

“EXPAND”
5C8C

LD (WORK),A

Store the current sector number (held in Register A) into the memory location 5CBCH.

5C8F

PUSH BC

Save the TABLE END POINTER (held in Register Pair BC) to the top of the stack.

5C90

LD C,(HL)

Fetch the LSB of the Byte Count from the memory location pointed to by Register Pair HL and put it into Register C.

5C91

INC HL

INCrement the value stored in Register Pair HL by 1.

5C92

LD B,(HL)

Fetch the MSB of the Byte Count from the memory location pointed to by Register Pair HL and put it into Register B.

5C93

INC HL

INCrement the value stored in Register Pair HL by 1.

5C94

INC BC

INCrement the value stored in Register Pair BC by 1.

5C95
5C96

LD A,B
OR C

Test for the copyright message. Since the Z-80 cannot test Register Pair BC against zero, the common trick is to set Register A to equal to Register B, and then OR A against Register C. Only if both Register B and Register C were zero can the Z FLAG be set.

5C97

DEC BC

DECrement the value stored in Register Pair BC by 1.

5C98

JR NZ,NOCOPY
[5CA2H]

If the NZ FLAG (Not Zero) is set, then this is not the copyright message, so JUMP to 5CA2H.

5C9A

LD BC,0010H

LET Register Pair BC = 0010H (Decimal: 16) for the length of the copyright message.

5C9D

LDIR

Transfers a byte of data from the memory location pointed to by HL to the memory location pointed to by DE (i.e., the sector buffer). 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.

5C9F

PUSH HL

Save the image data pointer (held in Register Pair HL) to the top of the stack.

5CA0

JR EXPND1
[5CB1H]

JUMP to 5CB1H to exit.

5CA2H – Continuation of the routine to Use the Track Building Table to fill up the table pointed to by DE; jumped here if we did not hit the copyright message.

“NOCOPY”
5CA2

LD A,(HL)

Fetch a byte of data from the table (stored at memory location pointed to by Register Pair HL) and put it into Register A.

5CA3

INC HL

INCrement the pointer (stored in Register Pair HL) by 1.

5CA4

PUSH HL

Save the pointer (stored in Register Pair HL) to the top of the stack.

5CA5

LD (DE),A

Store the data byte (held in Register A) into the track buffer image (pointed to by Register Pair DE).

5CA6

INC DE

INCrement the pointer in the track image (stored in Register Pair DE) by 1.

5CA7

DEC BC

DECrement the value stored in Register Pair BC (i.e., how many bytes to move) by 1.

5CA8
5CA9

LD A,B
OR C

5CAA

JR Z,EXPND1
[5CB1H]

IF all the bytes have been moved then the Z FLAG (Zero) will be set; so JUMP to 5CB1H.

5CAC
5CAD

PUSH DE
POP HL

Let Register Pair HL = the address of the next character in the track buffer (pointed to by Register Pair DE).

5CAE

DEC HL

DECrement the value stored in Register Pair HL by 1 to point to the prior stored character.

5CAF

LDIR

Fill the track buffer with the character via LDIR which transfers 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.

“EXPND1”
5CB1

POP HL

Put the current table entry address (held at the top of the STACK) into Register Pair HL, and then remove the entry from the stack.

5CB2

POP BC

Put the table end address (held at the top of the STACK) into Register Pair BC, and then remove the entry from the stack.

5CB3

PUSH HL

Put the current table entry address (held in Register Pair HL) to the top of the stack.

5CB4

OR A

Set FLAGS based on the contents of Register A and, in all cases, clear the CARRY FLAG..

5CB5

SBC HL,BC

Test to see (and set the CARRY FLAG with the answer) if we are done with this image block, by subtracting the END OF TABLE value (stored in Register Pair BC and the carry flag) from the CURRENT TABLE ADDRESS (value stored in Register Pair HL).

5CB7

POP HL

Restore the current table entry address from the top of the stacke into Register Pair HL.

5CB8

LD A,(WORK)

Fetch current sector number from its safe storage place in 5CBCH into Register A.

5CBB

RET

RETurn to the caller.

“WORK”
5CBC

DEFB 00H

Safe storage place for the current sector number in the above routine.

5CBDH – FORMAT and VERIFY a DISK.

First we need to initialize the hash index buffer to 0’s. A common trick is to set up a pointer to a buffer, the destination is 1 byte higher, and the to put a “0” in the first byte, so the 0 just keeps getting copied from a memory location to the memory location + 1.

“FORM”
5CBD

LD HL,DUM1

LET Register Pair HL = 5F00H to point to a buffer in RAM.

5CC0

LD DE,DUM1 + 1

LET Register Pair DE = one byte beyond HL.

5CC3

LD BC,SECTOR-32

LET Register Pair BC = 00E0H (Decimal = 224) which is the number of bytes we will fill with a 00H in a dummy directory sector.

5CC6

LD (HL),00H

Store a 00H into the memory location pointed to by Register Pair HL.

5CC8

LDIR

Next, initialize the system extents with a bunch of FFH’s.

5CCA

LD BC,001FH

The prior routine filled 224 bytes with 00’s. Now we need to deal with the remaining 32 bytes. LET Register Pair BC = 001FH (Decimal: 31) for a counter.

5CCD

LD (HL),0FFH

Store a FFH into the memory location pointed to by Register Pair HL.

5CCF

LDIR

Next, initialize the track counter.

5CD1

LD C,0FFH

LET Register C = 0FFH (Decimal: 255).

5CD3

LD B,28H

LET Register B = 28H (Decimal: 40 tracks).

5CD5

LD HL,FLAWD

LET Register Pair HL = 6160H, which is a buffer in RAM which will be used to store the GAT Flaws

5CD8

XOR A

Set Register A to ZERO and clear all Flags.

5CD9

LD (FLAWC),A

Store a 00H (held in Register A) into the memory location 4E80H to indicate that we have NO flawed tracks at the moment.

5CDC

LD IX,DDST

LET Special Index Register IX = 5BBDH, which is the SECTOR/TRACK DATA TABLE.

5CE0

LD IY,TRKNN

LET Special Index Register IY = 5BB5H, which is the DOUBLE DENSITY TRACK TABLE.

“FORMDD”
5CE4

INC C

INCrement the track number (stored in Register C) by 1 so as to position the disk head to the next track.

5CE5

CALL NZ,STEPR
[5B35H]

If we have not hit track 0, GOSUB to 5B35H to Position the Diskette Head to the Track held in Register C.

5CE8

JP NZ,ABRT
[4E4AH]

If the NZ FLAG (Not Zero) is set, meaning that the prior CALL routine returned because of an ERROR, JUMP to 4E4AH.

5CEB

LD D,04H

LET Register D = 04H, for up to 4 retries.

“RETRY2”
5CED

CALL TRKIMG
[5C1FH]

GOSUB to 5C1FH to make the track image in RAM to write to diskette.

5CF0

PUSH BC

Save Register Pair BC to the top of the stack.

5CF1

PUSH DE

Save Register Pair DE to the top of the stack.

5CF2

PUSH HL

Save Register Pair HL to the top of the stack.

5CF3

LD A,0D0H

LET Register A = 0D0H (1101 0000). When 1101 0000 is sent to the FDC, we are instructing it to Force Interrupt (1101) on a Not-Ready-to-Ready transition (0000).

5CF5