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

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

4306

LD HL,0002H

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

4309

LD (43EAH),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.

430C

XOR A

Set Register A to ZERO and clear all Flags.

430D

LD (43ECH),A

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

4310

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

4313

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.

4315

JR Z,4323H

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

4317

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.

4319

JR NZ,4302H

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 4302H.

431B

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.

431E

CALL 4335H

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

4321

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

4322

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.

4323

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).

4326

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.

4328

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.

4329

CALL 4335H

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.

432CH – Start of LOOP to read “B” bytes from the diskette and put them into a buffer starting at HL.

432C

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

432F

LD (HL),A

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

4330

INC HL

Bump Register Pair HL by 1.

4331

DJNZ 432CH

LOOP back to 432CH until Register B is ZERO.

4333

JR 4310H

JUMP to 4310H.

4335H – Get an Address from Diskette

Basically, this just reads 2 bytes into Register Pair HL; LSB into L and MSB into H.

4335

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

4338

LD L,A

LET Register L = Register A.

4339

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

433C

LD H,A

LET Register H = Register A.

433D

RET

RETurn to the caller.

433EH – Read a Byte from Diskette

433E

PUSH BC

PUSH Register BC to the top of the stack.

433F

PUSH HL

PUSH Register HL to the top of the stack.

4340

LD A,(43ECH)

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

4343

OR A

Set FLAGS based on the contents of Register A.

4344

JR NZ,4374H

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.

4346

LD B,09H

LET Register B = 09H.

4348H is the start of a loop of 9 tries to get a good sector read.

4348

PUSH BC

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

4349

CALL 437FH

GOSUB to 437FH to read a sector from diskette.

434C

POP BC

RESTORE Register Pair BC from the top of the STACK.

434D

AND 1DH

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

434F

JR Z,4364H

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

… uh oh, we have an error!

4351

LD A,D0H

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).

4353

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

4355

DJNZ 4348H

LOOP back to 4348H until Register B is ZERO.

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

4357

LD A,17H

LET Register A = 17H (ASCII: TRS-80 video control code for DOUBLE SIZED characters).

4359

CALL 0033H

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.

435C

LD HL,43EDH

LET Register Pair HL = 43EDH to point to the ERROR message in this BOOT/SYS.

435F

CALL 021BH

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

4362

JR 4362H

Keep executing this JUMP forever.

4364H – Sector Successfully Read

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

4364

LD HL,(43EAH)

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.

4367

INC L

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

4368

LD A,L

LET Register A = Register L for testing.

4369

CP 13H

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.

436B

JR C,4370H

If we are at less than 19 sectors the C FLAG (CARRY FLAG) 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).

436D

LD L,01H

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

436F

INC H

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

4370

LD (43EAH),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.

4373

XOR A

Set Register A to ZERO and clear all Flags.

4374

LD L,A

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

4375

LD H,4DH

LET Register H = 4DH.

4377

INC A

Bump Register A by 1 to point to the next byte index inside the sector-buffer loop.

4378

LD (43ECH),A

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

437B

LD A,(HL)

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

437C

POP HL

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

437D

POP BC

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

437E

RET

RETurn to the caller.

437FH – Read a Sector from Diskette

437F

CALL 43C5H

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.

4382

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.

4385

LD A,81H

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

4387

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)

4389

LD D,A

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

438A

LD HL,43B7H

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

438D

LD (404AH),HL

Store the value held in Register HL into the memory location 404AH.
NOTE: 404AH is the high byte of the RST 0 NMI Vector.

4390

LD A,C3H

LET Register A = C3H.

4392

LD (4049H),A

Store the value held in Register A into the memory location 4049H.
NOTE: 4049H is the low byte of the RST 0 NMI Vector.

4395

Disable Maskable Interrupts.

4396

LD A,C0H

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

4398

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.

439A

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.

439C

LD HL,4D00H

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

439F

LD A,84H

LET Register A = 84H (1000 0100).

43A1

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.

43A3

CALL 43E0H

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

43A6

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

43A8

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.

43A9

JR Z,43A6H

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.

43AB

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 INCrement HL and DECrement B (Set to 00H at 4382H). Z is set if B = 0; otherwise it is reset.

43AD

LD A,D

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

43AE

OR 40H

OR Register A against 40H (0100 0000) to turn on Bit 6. This will turn 1000 0001 into 1100 0001.

43B0

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)

43B2

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.

43B4

JP 43B0H

LOOP BACK 2 instructions (43B0H) to keep reading bytes. The NMI handler at 43B7H terminates the transfer loop.

43B7H – Interrupt Vector Handler

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

43B7

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.

43B8

XOR A

Set Register A to ZERO and clear all Flags.

43B9

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.

43BB

LD A,81H

LET Register A = 81H (1000 0001).

43BD

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)

43BF

CALL 43E6H

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

43C2

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

43C4

RET

RETurn to the caller.

43C5H – Position the Read Head

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

43C5

LD A,81H

LET Register A = 81H (1000 0001).

43C7

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)

43C9

LD HL,(43EAH)

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.

43CC

LD A,H

LET Register A = Register H. Register H holds the TRACK NUMBER we want.

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.

43CF

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.

43D1