Model I TRSDOS v2.3 BACKUP/CMD Explained

GOSUB to 4467H to send the message pointed to by HL to the video display. HL is preserved.

5219

JP 4030H C33040

JUMP to 4030H to ABORT.
NOTE: 4030H will load and execute the SYS1/SYS overlay with a Request Code of 2, so as to get the next command

521CH - Continuing if NOT from a BATCH File

521C

LD HL,5FD0H 21D05F

Point Register Pair HL to the message CLS + "TRSDOS DISK BACKUP UTILITY VER 2.3"

521F

GOSUB to 4467H.
NOTE: 4467H is a vector call to 44CFH which sends the message pointed to by HL to the video display. HL is preserved.

5222

POP HL E1

Restore the pointer to the next character on the DOS command line into HL.

5223

LD A,(4030H) 3A3040

Fetch the OPCODE at 4030H and store it into Register A. This would normally the the opcode LD A,n. 4030H is the is the address DOS jumps to at DOS READY to load the command interpreter (SYS1/SYS).

5226

CP C7H FEC7

Compare the value held in Register A against C7H. If Register A equals C7H the Z FLAG is set; otherwise the NZ FLAG is set.

5228

JR Z,5232H 2808

If the Z FLAG (Zero) has been set, continue with the BACKUP via a JUMP to 5232H to prompt the user for the SOURCE drive

522A

LD A,(HL) 7E

Fetch the next character from the DOS command line.

522B

CP 3AH FE3A

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

522D

JR NZ,5232H2003

If the NZ FLAG (Not Zero) has been set then we know we need to prompt the user for the source disk, so JUMP to 5232H to do that.

522F

INC HL 23

If we are here then we had a DOS command line of BACKUP :. INCrement the value stored in Register Pair HL by 1 to point to the next character on the DOS command line.

5230

JR 5240H 180E

JUMP to 5240H to skip over the prompt to enter in the SOURCE disk number and go directly to the code that would process the character given in answer to that question.

5232H - Determine the SOURCE disk drive number

As a pass through, we had enough of a BACKUP command to move us to trying to get the SOURCE drive number, but we need to prompt for it.

5232

LD HL,5FF8H 21F85F

Point Register Pair HL to the message 'SOURCE DRIVE NUMBER ? '

5235

GOSUB to 4467H.
NOTE: 4467H is a vector call to 44CFH which sends the message pointed to by HL to the video display. HL is preserved.

5238

LD HL,4B00H 21004B

Let Register Pair HL equal 4B00H to act as the required buffer location for a ROM call of 0040H to secure input from the user.

523B

LD B,01H 0601

Let Register B equal 01H to act as the required number of characters to fetch from the user, which must be in Register B before a CALL to 0040H to secure input from the user.

523D

CALL 0040H CD4000

GOSUB to 0040H in ROM to wait for keyboard input of B characters and store the characters at (HL).

At this point, (HL) points to a single character which we need to validate as a drive number. Either this was a pass through, in which case (HL) was populated by the ROM cal to 0040H or as a jump from 522F, where (HL) is pointing to the character on the DOS line following a :.

5240

LD A,(HL) 7E

Fetch the character at (HL) and put it into Register A.

5241

SUB 30H D630

SUBtract the value 30H from Register A. This will convert an ASCII number '0'-'9' to its decimal equivalent. The Carry Flag will be affected.

5243

JP C,5232H DA3252

If the C FLAG (Carry) has been set, then we did not get a number, so LOOP BACK to 5232H to prompt the use for SOURCE DRIVE NUMBER.

5246

CP 04H FE04

Compare the value held in Register A against 04H, which is 1 higher than the highest possible drive number. If Register A equals 04H, the Z FLAG is set. If Register A < 04H, the CARRY FLAG will be set. If Register A >= 04H, the NO CARRY FLAG will be set. This means that a valid drive number would set the CARRY FLAG.

5248

JP NC,5232H D23252

If the NC FLAG (No Carry) has been set, meaning we do not have a valid drive number, LOOP BACK to 5232H to prompt the use for SOURCE DRIVE NUMBER.

We now have a valid SOURCE disk drive number held in Register A as a number 0, 1, 2, or 3.

524B

INC HL 23

INCrement the value stored in Register Pair HL by 1. This will either move to the next character in the user input buffer, or the next character on the DOS command line, whichever was being processed.

But a 0, 1, 2, or 3 isn't usable. We need to convert this to C7H, CFH, D7H, or DFH which is a FDC Command code.

524C

RLCA 07

Rotate the bits of Register left (i.e., lower bit moves higher) one bit position. The contents of bit 7 are copied to the CARRY FLAG and to bit 0.

524D

RLCA 07

Rotate the bits of Register left (i.e., lower bit moves higher) one bit position. The contents of bit 7 are copied to the CARRY FLAG and to bit 0.

524E

RLCA 07

Rotate the bits of Register left (i.e., lower bit moves higher) one bit position. The contents of bit 7 are copied to the CARRY FLAG and to bit 0.

These 3 RLCAS turn a "0" into an 00H, "1" into an 08H, a "2" into a 10H, and a "3" into 0CH.

524F

OR C7H F6C7

OR Register A against C7H (1100 0111). This has the effect of turning on bits 7, 6, 2, 1, 0.

"0" is now C7H, "1" is now CFH, "2" is now D7H, and "3" is now DFH, which are all Floppy Disk Controller Command bytes.

5251

LD (5256H),A 325652

Store the value held in Register A into memory location 5256H, which is part of an OPCODE. This is one of two locations in the program where the SOURCE DRIVE NUMBER is stored, and this one is saved in the format which can be sent to the FDC as a command.

5254

XOR A AF

Set Register A to ZERO and clear all Flags.

The instruction at 5251H has actually replaced the OPCODE at 5255 with the SOURCE disk drive number, converted into C7H, CFH, D7H, or DFH

5255

SET 0,A CBC7

While this is coded as SET 0,A, the n actually changes to 0-3 based on the users entry of the SOURCE drive number. This means A becomes 01H for a "0", 02H for a "1", 04H for a "2", and 08H for a "3".

5257

LD (57C6H),A 32C657

Store the value held in Register A (01, 02, 04, or 08 corresponding to a choice of drive "0", "1", "2", or "3") into memory location 57C6H which is part of a routine which sends that code to the disk drive select latch address and RETurns. . This is one of two locations where the SOURCE DRIVE NUMBER is saved, and this one is saved in the format which can be sent to the FDC latch port to select the disk drive.

525A

LD A,(4030H) 3A3040

Fetch the OPCODE at 4030H and store it into Register A. This would normally the the opcode LD A,n. 4030H is the is the address DOS jumps to at DOS READY to load the command interpreter (SYS1/SYS).

525D

CP C7H FEC7

Compare the value held in Register A against C7H. If Register A equals C7H the Z FLAG is set; otherwise the NZ FLAG is set.

525F

JR Z,526DH 280C

If the Z FLAG (Zero) has been set, JUMP to 526DH prompt the user for the DESTINATION disk drive.

5261

LD A,(HL) 7E

Fetch the value held in the memory location pointed to by Register Pair HL (which is either the DOS command line or the fetched input) and store it into Register A.

5262

INC HL 23

INCrement the value stored in Register Pair HL by 1 to point to the next character location we wish to interrogate.

5263

CP 0DH FE0D

Compare the value held in Register A against 0DH (ASCII: CARRIAGE RETURN). If there was a CARRIAGE RETURN then the Z FLAG will be set, otherwise the NZ FLAG will be set.

5265

JR Z,526DH 2806

If we found a CARRIAGE RETURN, JUMP to 526DH prompt the user for the DESTINATION disk drive.

If we didn't find a ENTER here, then we need to still process the command line. The only valid character at this point would be a : or a SPACE as we now have BACKUP :n :

5267

CP 3AH FE3A

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

5269

JR NZ,5261H 20F6

If we did not find a :, the NZ FLAG (Not Zero) will be set, so LOOP BACK to 5261H to read the next character on the DOS command line.

526B

JR 527BH 180E

JUMP to 527BH to use THAT character as the current drive number (instead of prompting) and test to see if its valid.

526DH - Determine DESTINATION Drive Number

As a pass through, we had enough of a BACKUP command to move us to trying to get the DESTINATION drive number, but we need to prompt for it.

526D

LD HL,600FH 210F60

Let Register Pair HL equal 600FH to point to the message 'DESTINATION DRIVE NUMBER ? "+03H

5270

GOSUB to 4467H to display the message 'DESTINATION DRIVE NUMBER ? "+03H on screen. HL is preserved.

5273

LD HL,4B00H 21004B

Let Register Pair HL equal 4B00H to act as the required buffer location for a ROM call of 0040H to secure input from the user.

5276

LD B,01H 0601

Let Register B equal 01H, meaning that we need the ROM routine at 0040H to fetch ONE character from the user.

5278

CALL 0040H CD4000

GOSUB to 0040H in ROM to wait for keyboard input of B characters and store the characters at (HL).

At this point (HL) is pointing to a potential drive number. Either because the user entered it on the command line and, as a result, 526BH jumped here with (HL) pointing to the DOS command line -or- because the user typed it in, in which case (HL) points to the character just typed in. Either way, the code rejoins here to test it.

527B

LD A,(HL) 7E

Fetch the value held in the memory location pointed to by Register Pair HL and store it into Register A.

527C

SUB 30H D630

SUBtract the value 30H from Register A. This will convert an ASCII number '0'-'9' to its decimal equivalent. The Carry Flag will be affected.

527E

JP C,526DH DA6D52

If the C FLAG (Carry) has been set, then we did not get a number, so LOOP BACK to 526DH to prompt the use for DESTINATION DRIVE NUMBER.

5281

CP 04H FE04

5283

JP NC,526DH D26D52

If the NC FLAG (No Carry) has been set, meaning we do not have a valid drive number, LOOP BACK to 526DH to prompt the use for DESTINATION DRIVE NUMBER.

But a 0, 1, 2, or 3 isn't usable. We need to convert this to C7H, CFH, D7H, or DFH.

5286

RLCA 07

Rotate the bits of Register left (i.e., lower bit moves higher) one bit position. The contents of bit 7 are copied to the CARRY FLAG and to bit 0.

5287

RLCA 07

Rotate the bits of Register left (i.e., lower bit moves higher) one bit position. The contents of bit 7 are copied to the CARRY FLAG and to bit 0.

5288

RLCA 07

Rotate the bits of Register left (i.e., lower bit moves higher) one bit position. The contents of bit 7 are copied to the CARRY FLAG and to bit 0.

These 3 RLCAs turn a "0" into an 00H, "1" into an 08H, a "2" into a 10H, and a "3" into 0CH.

5289

OR nnH F6C7

OR Register A against nnH, which is the DESTINATION DRIVE but converted into Floppy Disk Controller Command bytes of either C7H, CFH, D7H, or DFH, corresponding to the drives 0-3 as selected by the user.

"0" is now C7H, "1" is now CFH, "2" is now D7H, and "3" is now DFH.

528B

LD (5290H),A 329052

Store the value held in Register A into memory location 5290H, which is part of an OPCODE. This is one of two locations in the program where the DESTINATION DRIVE NUMBER is stored, and this one is saved in the format which can be sent to the FDC as a command.

528E

XOR A AF

Set Register A to ZERO and clear all Flags.

The instruction at 528BH has actually replaced the OPCODE at 5290 with the DESTINATION disk drive number, converted into C7H, CFH, D7H, or DFH

528F

SET 0,A CBC7

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

5291

LD (5838H),A 323858

Store the value held in Register A (01, 02, 04, or 08 corresponding to a choice of drive "0", "1", "2", or "3") into memory location 57C6H which is part of a routine which sends that code to the disk drive select latch address and RETurns. This is one of two locations where the DESGINATION DRIVE NUMBER is saved, and this one is saved in the format which can be sent to the FDC latch port to select the disk drive.

5294

LD A,(4030H) 3A3040

Fetch the OPCODE at 4030H and store it into Register A. This would normally the the opcode LD A,n. 4030H is the is the address DOS jumps to at DOS READY to load the command interpreter (SYS1/SYS).

5297

CP C7H FEC7

Compare the value held in Register A against C7H. If Register A equals C7H the Z FLAG is set; otherwise the NZ FLAG is set.

5299

JR Z,52A9H 280E

If the Z FLAG (Zero) has been set, JUMP to 52A9H to prompt the user for th BACKUP DATE

529B

LD A,(4046H) 3A4640

Let Register Pair A equal the contents of RAM location 4046H (Decimal: 16454) which is the storage location for the current month

529E

OR A B7

Since a LD command does not set any FLAGS, Set FLAGS based on the contents of Register A (and, in all events, clear the CARRY FLAG).

529F

JR Z,52A9H 2808

If that RAM location held a 0, then JUMP to the next routine at 52A9H to prompt the user for th BACKUP DATE.

If we are here, then we are going to use the SYSTEM DATE held in 5AA5H as the backup date. This is put into HL similar to the above routines; either HL will be this pointer or it will be 8 fetched characters from the user. Either way, HL needs to be validated as a valid 8 character date.

52A1

LD HL,5AA5H 21A55A

Let Register Pair HL equal 5AA5H, which is the 8 character buffer set up to accept a date from the user.

52A4

CALL 4470H CD7044

GOSUB to 4470H which is the DISPLAY DATE ON VIDEO Vector.

52A7

JR 52CAH 1821

JUMP to 52CAH to continue with the BACKUP routine after successfully processing an accepted date.

52A9H - Determine the BACKUP DATE

52A9

LD HL,602BH 212B60

Let Register Pair HL equal 602BH so as to point to the message 'BACKUP DATE (MM/DD/YY) ' + 03H

52AC

GOSUB to 4467H to display the message 'BACKUP DATE (MM/DD/YY) ' + 03H to the video display. HL is preserved.

52AF

LD HL,5AA5H 21A55A

Let Register Pair HL equal 5AA5H as the buffer for a the KEYBOARD INPUT routine in ROM at 0040H.

52B2

LD B,08H 0608

Let Register B equal 08H, to indicate that the ROM routine at 0040H is fetching 8 characters from the user.

52B4

CALL 0040H CD4000

GOSUB to 0040H.
NOTE: 0040H is the Model I ROM routine to wait for keyboard input of B characters and store the characters at (HL).

52B7

LD A,B 78

Copy the contents of Register B, which is the number of characters actually fetched from the user, into Register A.

52B8

CP 08H FE08

Compare the value held in Register A against 08H, which is the number of characters we need this date field to be. If B is not 8, then we have a problem, and the NZ FLAG is set.

52BA

JR NZ,52A9H 20ED

If we didn't get 8 characters for the date, LOOP BACK to 52A9H to prompt for the BACKUP DATE.

If we are here, then we have 8 characters starting at (HL) which need to be vetted. We hope it will be a valid date as the BACKUP DATE.

52BC

LD A,(5AA7H) 3AA75A

Fetch the value held in memory location 5AA7H (which would be the third character in MM/DD/YY) and store it into Register A.

52BF

CP 2FH FE2F

Compare the value held in Register A against /). If it isn't a /, the NZ FLAG will be set.

52C1

JR NZ,52A9H 20E6

If the NZ FLAG (Not Zero) has been set, we didn't get a SLASH where we needed it, so LOOP BACK to 52A9H to prompt for the BACKUP DATE.

52C3

LD A,(5AAAH) 3AAA5A

Fetch the value held in memory location 5AAAH (which would be the FIFTH character in MM/DD/YY) and store it into Register A.

52C6

CP 2FH FE2F

Compare the value held in Register A against /). If it isn't a /, the NZ FLAG will be set.

52C8

JR NZ,52A9H 20DF

If the NZ FLAG (Not Zero) has been set, we didn't get a SLASH where we needed it, so LOOP BACK to 52A9H to prompt for the BACKUP DATE.

At this point, we know the 8 characters are at least __/__/__, which is all the BACKUP routine actually tests for. HI/TH/ER is a valid answer! Now we are going to relocate part of the BACKUP/CMD program to 4200H. This will clobber DOS, which is why the system reboots after a BACKUP.

52CA

LD HL,559BH 219B55

Let Register Pair HL equal 559BH, which is part of this BACKUP/CMD routine.

52CD

LD DE,4200H 110042

Let Register Pair DE equal 4200H which will be a buffer to hold this routine.

52D0

LD BC,0B00H 01000B

Let Register Pair BC equal 0B00H to indicate that we are going to move 2816 bytes.

52D3

LDIR EDB0

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.

52D5

XOR A AF

Set Register A to zero and clear all flags. This zero will be used to initialize the "last prompted drive" indicator.

Things get a little rough from this point. The same code is now in 2 RAM locations. Relocated code below bears both addresses.
For a little clarity:

Source drive select code is at 47C6H and 442BH
Destination drive select code is at 449DH and 5838H
The CP instruction with these codes is at 44E7H and 5582H
The LD instruction with these codes is at 55EEH and 5589H

52D6

LD (44E8H),A32E844

Store zero into 44E8H. This is the "nn" operand of the CP nn instruction at 44E7H. Initializing it to zero means: "no drive has been prompted for yet." When the 44E7H routine runs, it compares the incoming drive code against this value. If they match, it knows that drive's disk is already inserted and skips the prompt.

52D9

LD A,(442BH)3A2B44

Load the SOURCE drive select code (01H, 02H, 04H, or 08H) into Register A. This value was placed at 57C6H earlier (at 5257H) and now also exists at 442BH after relocation.

52DC

LD HL,449DH219D44

Point HL to the location holding the DESTINATION drive select code. This value was placed at 5838H earlier (at 5291H) and now also exists at 449DH after relocation.

52DF

SUB (HL) 96

Subtract DESTINATION drive code from SOURCE drive code. Result in A = (SOURCE - DESTINATION). If source and destination are the same drive, result = 0. If they are different drives, result ≠ 0.

52E0

LD (44EFH),A32EF44

Store the drive difference into 44EFH. This is the "nn" operand of LD A,nn at 44EEH. The 44E7H-44EEH routine uses this to determine if this is a single-drive backup (value=0, requiring disk swaps) or a two-drive backup (value≠0, no swaps needed between source/dest).

52E3

CALL 442AH CD2A44

Call the "Insert Source Disk" prompt routine. This routine: (1) loads the SOURCE drive code from 442BH, (2) ORs it with 80H to set the high bit (identifying it as source), (3) displays "INSERT SOURCE DISK (ENTER)", (4) waits for ENTER, and (5) returns.

52E6

Call the FDC RESTORE routine. This sends a RESTORE command to the Floppy Disk Controller to move the read/write head to Track 0, then polls the FDC status until the drive is no longer BUSY. This initializes the drive head position.

52E9

LD A,(37ECH) 3AEC37

Fetch the FDC's status (which is mirrored to 37ECH) and store it into Register A.

7	6	5	4	3	2	1	0
Not Ready	Protected	Head Loaded	Seek Error	CRC Error	Track 00	Index	Busy

52EC

LD HL,6045H 214560

Let Register Pair HL equal 6045H to point to the message 'SOURCE DRIVE NOT READY!' + 0DH.

52EF

BIT 7,A CB7F

Test Bit 7 of the FDC status (the "Not Ready" flag). Z flag set = drive IS ready (bit=0). NZ flag set = drive is NOT ready (bit=1).

7	6	5	4	3	2	1	0
Not Ready	Protected	Head Loaded	Seek Error	CRC Error	Track 00	Index	Busy

52F1

If the NZ FLAG (Not Zero) has been set, then the drive is returning NOT READY, so JUMP to 43A3H to Display the message pointed to by HL and prompt for ENTER; keep polling the keyboard until ENTER, and then HALT.

52F4

LD HL,605DH 215D60

Let Register Pair HL equal 605DH to point to the message 'SOURCE DRIVE NOT IN SYSTEM!' + 0DH.

52F7

BIT 2,A CB57

Test Bit Number 2 of Register A, which will test to see if the Read/Write head is at Track 0 or not. Z FLAG will be set if that bit is 0 (not positioned at track 0), and NZ FLAG will be set if that bit is 1 (positioned at track 0).

52F9

If the Z FLAG (Zero) has been set, JUMP to 43A3H to Display the message pointed to by HL and prompt for ENTER; keep polling the keyboard until ENTER, and then HALT.

Read the source disks GAT (Track 0, Sector 0) into a buffer at 4B00H

52FC

LD DE,0000H 110000

Let Register Pair DE equal 0000H.

52FF

LD BC,4B00H 01004B

Let Register Pair HL equal 4B00H to act as the required buffer location.

5302

GOSUB to 453CH (Disk I/O Executive) which performs the drive selection, seeks to the track specified by DE, issues the Read command, and runs the data transfer loop to fill the buffer pointed to by BC.

5305

LD HL,45E8H 21E845

Let Register Pair HL equal 45E8H to point to the message 0A + 'SOURCE DISK READ ERROR!' + 0DH.

5308

If the NZ FLAG (Not Zero) has been set, JUMP to 43A3H to Display the message pointed to by HL and prompt for ENTER; keep polling the keyboard until ENTER, and then HALT.

Extract the directory track number from the +02H offset (i.e., 4B02H) and store it at 4709H

530B

LD A,(4B02H) 3A024B

Fetch the value held in memory location 4B02H, which is the GAT +02H entry, or the DIRECTORY TRACK, and store it into Register A.

530E

LD (4709H),A 320947

Store the directory track into memory location 4709H.

Read the SOURCE disks's directory track, Sector 0 (the GAT of the directory track) into a buffer at 4900H.

5311

LD A,(4709H) 3A0947

Fetch the value held in memory location 4709H (which will be the GAT in the directory) and store it into Register A.

5314

LD D,A 57

Copy the contents of Register A into Register D.

5315

LD E,00H 1E00

Let Register E equal 00H, so now DE will be a 16 bit number.

5317

LD BC,4900H 010049

Let Register Pair BC equal 4900H which will be the buffer where the GAT is stored.

531A

531D

LD HL,45E8H 21E845

Let Register Pair HL equal 45E8H to point to the message 0A + 'SOURCE DISK READ ERROR!' + 0DH.

5320

If the NZ FLAG (Not Zero) has been set, JUMP to 43A3H to Display the message pointed to by HL and prompt for ENTER; keep polling the keyboard until ENTER, and then HALT.

5323

CALL 449CH CD9C44

GOSUB to 449CH to turn on Bit 6 in A, prompt for the Destination Disk and make sure its not write protected, and RETurn

5326

GOSUB to 441AH to send a RESTORE/LOAD HEAD to the Floppy Drive and then keep checking the floppy drive controller status until the drive is not BUSY.

5329

LD A,(37ECH) 3AEC37

Fetch the FDC's status (which is mirrored to 37ECH) and store it into Register A.

7	6	5	4	3	2	1	0
Not Ready	Protected	Head Loaded	Seek Error	CRC Error	Track 00	Index	Busy

532C

LD HL,6079H 217960

Let Register Pair HL equal 6079H to point to the message 'DESTINATION DRIVE NOT READY!' + 0DH.

532F

BIT 7,A CB7F

Test Bit Number 7 of Register A, which is currently holding the Floppy Drive Controller status. This then tests a drive status of NOT READY. Z FLAG will be set if that bit is 0 (meaning the drive is ready), and NZ FLAG will be set if that bit is 1 (meaning the drive is NOT ready).

5331

If the NZ FLAG (Not Zero) has been set, JUMP to 43A3H to Display the message pointed to by HL and prompt for ENTER; keep polling the keyboard until ENTER, and then HALT.

5334

LD HL,6096H 219660

Let Register Pair HL equal 6096H to point to the message 'DESTINATION DRIVE NOT IN SYSTEM!' + 0DH.

5337

BIT 2,A CB57

5339

If the Z FLAG (Zero) has been set, JUMP to 43A3H to Display the message pointed to by HL and prompt for ENTER; keep polling the keyboard until ENTER, and then HALT.

533C

LD HL,463EH 213E46

Let Register Pair HL equal 463EH to point to the message 0A + 'DESTINATION DISK IS WRITE PROTECTED!' + 0DH.

533F

BIT 6,A CB77

Test Bit Number 6 of Register A to see if the FDC returned a PROTECTED error code. Z FLAG will be set if that bit is 0 (=NOT write protected), and NZ FLAG will be set if that bit is 1 (=Write protected).

7	6	5	4	3	2	1	0
Not Ready	Protected	Head Loaded	Seek Error	CRC Error	Track 00	Index	Busy

5341

If the NZ FLAG (Not Zero) has been set, then the disk is not Write Protected, so continue on via a JUMP to 43A3H to Display the message pointed to by HL and prompt for ENTER; keep polling the keyboard until ENTER, and then HALT.

At this point, we have a disk and it is not write protected, read the DESTINATION disks' GAT (Track 0, Sector 0) into a buffer at 4B00H.

5344

LD BC,4B00H 01004B

Let Register Pair HL equal 4B00H to act as the required buffer location.

5347

LD DE,0000H 110000

Let Register Pair DE equal 0000H.

534A

This is where the FORMAT vs BACKUP decision is made. If the read was good, and exited with Z, it is going to backup. If that read routine failed and returned a NZ, then the FORMAT routines are called.

534D

JP NZ,5386H C28653

If the NZ FLAG (Not Zero) has been set, JUMP to 5386H (part of the FORMATTING routine) to fill a buffer at 4A00H with 35 (representing then number of tracks) FCH's and then 203 FFH's.

If we are here, then we are NOT going to format, and instead we are going to proceed to the next step, which is to read the directory tracks, check the Pack ID's to make sure they match, and if they do match, jump to 5553H to copy.

5350

LD A,(4709H) 3A0947

Load the directory track number (typically 11H/17 decimal for TRSDOS 2.3) into Register A. This value was extracted earlier from the source disk's GAT at offset +02H.

5353

LD D,A 57

Copy the directory track number into Register D. D now holds the track number for the disk read.

5354

LD E,00H 1E00

Set Register E to 00H. E holds the sector number (Sector 0). DE together = Track/Sector pair for the directory track's first sector (the GAT sector of the directory).

5356

LD BC,4A00H 01004A

Set BC to 4A00H as the destination buffer for the sector read. This will hold the DESTINATION disk's directory GAT sector.

5359

GCall the Disk I/O Executive to read the destination disk's directory track, sector 0 into buffer at 4A00H. This reads the GAT/lockout table from the destination disk's directory track.

535C

LD HL,4601H 210146

Let Register Pair HL equal 4601H to point to the message DEFM 0A + 'DESTINATION DISK READ ERROR!' + 0DH.

535F

If the NZ FLAG (Not Zero) has been set, JUMP to 43A3H to Display the message pointed to by HL and prompt for ENTER; keep polling the keyboard until ENTER, and then HALT.

5362

LD HL,49CEH 21CE49

Point HL to offset +CEH (206 decimal) within the SOURCE directory GAT buffer (4900H). This is the location of the Pack ID/Disk Name in the GAT sector.

5365

LD DE,4ACEH 11CE4A

Point DE to offset +CEH (206 decimal) within the DESTINATION directory GAT buffer (4A00H). Same offset - the Pack ID location.

5368

LD B,0AH 060A

Set loop counter B to 10 (0AH). The Pack ID is 10 bytes long.

Top of a loop to check the diskette pack ID's, which must match. This routine will check the byte in DE against the byte in HL, and if they do not match, error out; otherwise continues to the next byte pair, for a total of 10 bytes.

536A

LD A,(DE) 1A

Fetch one byte of the DESTINATION disk's Pack ID into A.

536B

CP (HL) BE

Compare it against the corresponding byte of the SOURCE disk's Pack ID.

536C

JR NZ,5380H 2012

If bytes don't match (NZ), jump to 5380H to display "BACKUP REJECTED, DISKETTES CONTAIN DIFFERENT PACK IDS!" and halt.

536E

INC DE 13

Move to next byte in destination Pack ID.

536F

INC HL 23

Move to next byte in source Pack ID.

5370

DJNZ 536AH 10F8

LOOP back to 536AH, 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 have identical Pack ID's so we can continue. Next we move 35 bytes from 4A60H to 4A00H

5372

LD HL,4A60H 21604A

Point HL to offset +60H within the destination directory GAT buffer (the track lockout table portion).

5375

LD DE,4A00H 11004A

Point DE to the start of the buffer at 4A00H.

5378

LD BC,0023H 012300

Set byte count to 35 (23H) - one byte per track on a 35-track disk.

537B

LDIR EDB0

Block copy 35 bytes from 4A60H to 4A00H. This copies the destination disk's track lockout table to the beginning of the buffer. This lockout info will be used during copying to know which tracks to skip.

537D

JP 5553H C35355

Jump to the COPYING routine at 5553H to begin the actual sector-by-sector copy process.

5380H - Process the situation where the disks contain different pack IDs

5380

LD HL,612DH 212D61

Let Register Pair HL equal 612DH to point to the message 'BACKUP REJECTED, DISKETTES CONTAIN DIFFERENT PACK IDS!' + 0DH.

5383

JP 43A3H C3A343

JUMP to 43A3H to Display the message pointed to by HL and prompt for ENTER; keep polling the keyboard until ENTER, and then HALT.

5386H - Part of the FORMATTING routine. This will fill a buffer at 4A00H with 35 (representing the track lockout table) FCH's and then 203 FFH's.

5386

LD HL,4A00H 21004A

Let Register Pair HL equal 4A00H to be used as a buffer to hold the track lockout status.

5389

XOR A AF

Set Register A to ZERO and clear all Flags.

538A

LD (HL),FCH 36FC

Store the value FCH into the memory location pointed to by Register Pair HL.

538C

INC HL 23

INCrement the value stored in Register Pair HL by 1.

538D

INC A 3C

INCrement the value stored in Register A by 1.

538E

CP 23H FE23

Compare the value held in Register A against 23H (Decimal: 35), which is the highest number of tracks + 1. If A is not 23H then the NZ FLAG will be set.

5390

JR NZ,538AH 20F8

If the NZ FLAG (Not Zero) has been set, then we have not yet hit the highest number of tracks + 1, so LOOP BACK to 538AH to keep filling with FC's.

5392

CP CBH FECB

Compare the value held in Register A against CBH (Decimal: 203). When A hits CBH the Z FLAG will be set

5394

JR Z,539CH 2806

If the Z FLAG (Zero) has been set, JUMP to 539CH to continue with the FORMATTING. This is the routine's exit.

5396

LD (HL),FFH 36FF

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

5398

INC HL 23

INCrement the value stored in Register Pair HL by 1.

5399

INC A 3C

INCrement the value stored in Register A by 1.

539A

JR 5392H 18F6

LOOP BACK to 5392H to keep filling with FF's

539CH - Drive Ready and Disk Verification Checks

539C

LD BC,00CDH 01CD00

Let Register Pair BC equal 00CDH (Decimal: 205).

539F

LD HL,60B7H 21B760

Let Register Pair HL equal 60B7H to point to the message 'NO DISKETTE IN DESTINATION DRIVE!' + 0DH.

Set up for the next disk-in-drive-and-ready test. This will count down from 204 to 0 to test for a DISKETTE IN DRIVE.

53A2

DEC BC 0B

DECrement the value stored in Register Pair BC by 1.

53A3H
53A3

LD A,B
OR C78

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.

53A5

If the Z FLAG (Zero) has been set then we have unsuccessfully timed out, JUMP to 43A3H to Display the message pointed to by HL and prompt for ENTER; keep polling the keyboard until ENTER, and then HALT.

53A8

LD A,(37ECH) 3AEC37

Fetch the FDC's status (which is mirrored to 37ECH) and store it into Register A.

7	6	5	4	3	2	1	0
Not Ready	Protected	Head Loaded	Seek Error	CRC Error	Track 00	Index	Busy

53AB

AND 02H E602

MASK the value of Register A against 02H (0000 0010). This has the effect of turning off bits 7, 6, 5, 4, 3, 2, 0, leaving only bit 1 active. Bit 1 is the INDEX FOUND response.

53AD

JR NZ,53A2H 20F3

If the NZ FLAG (Not Zero) has been set, meaning it found the index hole, LOOP BACK to to 53A2H to DECrement the counter in BC and try again.

Set up for the next disk-in-drive-and-ready test. This will count down from 6557 to 0 to test for DOOR OPEN.

53AF

LD BC,199DH 019D19

Let Register Pair BC equal 199DH (Decimal: 6557) which will be the counter for the number if passes.

53B2

LD HL,60D9H 21D960

Let Register Pair HL equal 60D9H, to point to the message 'DOOR NOT CLOSED ON DESSTINATION DRIVE!' + 0DH.

53B5

DEC BC 0B

DECrement the counter stored in Register Pair BC by 1.

53B6H
53B6

LD A,B
OR C78

53B8

If the Z FLAG (Zero) has been set then we ran out tries, so JUMP to 43A3H to Display the message pointed to by HL and prompt for ENTER; keep polling the keyboard until ENTER, and then HALT.

53BB

LD A,(37ECH) 3AEC37

Fetch the FDC's status (which is mirrored to 37ECH) and store it into Register A.

7	6	5	4	3	2	1	0
Not Ready	Protected	Head Loaded	Seek Error	CRC Error	Track 00	Index	Busy

53BE

AND 02H E602

MASK the value of Register A against 02H (0000 0010). This has the effect of turning off bits 7, 6, 5, 4, 3, 2, 0, leaving only bit 1 active. Bit 1 is the INDEX FOUND response.

53C0

JR Z,53B5H 28F3

If the Z FLAG (Zero) has been set, meaning it DID NOT FIND the INDEX, JUMP to 53DFH to decrement the BC counter and try again.

Set up for the next disk-in-drive-and-ready test. This will count down from 186 to 0 to test for a DISKETTE IN DRIVE.

53C2

LD DE,0000H 110000

Let Register Pair DE equal 0000H. DE will be a counter up

53C5

LD BC,00BAH 01BA00

Let Register Pair BC equal 00BAH (Decimal: 186). BC will be a try counter down..

53C8

LD HL,60B7H 21B760

Let Register Pair HL equal 60B7H to point to the message 'NO DISKETTE IN DESTINATION DRIVE!' + 0DH.

53CB

DEC BC 0B

DECrement the value stored in Register Pair BC by 1.

53CCH
53CC

LD A,B
OR C78

53CE

53D1

INC DE 13

INCrement the value stored in Register Pair DE by 1.

53D2

LD A,(37ECH) 3AEC37

Fetch the FDC's status (which is mirrored to 37ECH) and store it into Register A.

7	6	5	4	3	2	1	0
Not Ready	Protected	Head Loaded	Seek Error	CRC Error	Track 00	Index	Busy

53D5

AND 02H E602

MASK the value of Register A against 02H (0000 0010). This has the effect of turning off bits 7, 6, 5, 4, 3, 2, 0, leaving only bit 1 active. Bit 1 is the INDEX FOUND response.

53D7

JR NZ,53CBH 20F2

If the NZ FLAG (Not Zero) has been set, meaning it found the index hole, LOOP BACK to to 53CBH to keep polling.

Set up for the next disk-in-drive-and-ready test. This will count down from 5920 to 0 to test for DOOR OPEN.

53D9

LD BC,1720H 012017

Let Register Pair BC equal 1720H / (Decimal: 5920).

53DC

LD HL,60D9H 21D960

Let Register Pair HL equal 60D9H to point to the message 'DOOR NOT CLOSED ON DESTINATION DRIVE!' + 0DH.

53DF

DEC BC 0B

DECrement the try counter stored in Register Pair BC by 1.

53E0H
53E0

LD A,B
OR C78

53E2

If the Z FLAG (Zero) then we ran out tries, so JUMP to 43A3H to Display the message pointed to by HL and prompt for ENTER; keep polling the keyboard until ENTER, and then HALT.

53E5

INC DE 13

INCrement the counter stored in Register Pair DE by 1.

53E6

LD A,(37ECH) 3AEC37

Fetch the FDC's status (which is mirrored to 37ECH) and store it into Register A.

7	6	5	4	3	2	1	0
Not Ready	Protected	Head Loaded	Seek Error	CRC Error	Track 00	Index	Busy

53E9

AND 02H E602

MASK the value of Register A against 02H (0000 0010). This has the effect of turning off bits 7, 6, 5, 4, 3, 2, 0, leaving only bit 1 active. Bit 1 is the INDEX FOUND response.

53EB

JR Z,53DFH 28F2

If the Z FLAG (Zero) has been set, meaning no INDEX MARKER FOUND, LOOP BACK to 53DFH to decrement the BC counter and try again.

53ED

LD HL,EACBH 21CBEA

Let Register Pair HL equal to EACBH as the start of an offset.

53F0

ADD HL,DE 19

LET Register Pair HL = Register Pair HL + Register DE. The Carry Flag will be affected.

53F1

LD HL,60FFH 21FF60

Let Register Pair HL equal 60FFH to point to the message 'DESTINATION DISKETTE UNUSABLE! HARD SECTORED!' + 0DH.

53F4

JP NC,43A3H D2A343

If EACBH (held in HL) + DE does NOT trigger a CARRY, then NC FLAG (No Carry) will have been set, so JUMP to 43A3H to Display the message pointed to by HL and prompt for ENTER; keep polling the keyboard until ENTER, and then HALT.

53F7

XOR A AF

>Set Register A to ZERO and clear all Flags. A will be holding the current track being formatted.

53F8

LD (5435H),A 323554

Store the value held in Register A, which will be the track counter, into memory location 5435H, which is a LD C,nn opcode.

Main Formatting Track Loop

RAM	Contents	Explanation
5435H	Current track number (0-34)	Track counter for the format loop. Also the "nn" operand of LD C,nn at 5434H
4709H	Directory track number (typically 11H/17 decimal)	Read from source disk's GAT at offset +02H earlier in the program
545FH	The "nn" operand of LD C,nn at 545EH	Holds the Data Address Mark (FAH or FBH) to be written into each sector
6176H	The "nn" digits in "FORMATTING TRACK nn" message	Two ASCII bytes for track number display
6293H-629DH	Sector interleave table: 00,05,01,06,02,07,03,08,04,09,FF	Defines the order sectors are written (2:1 interleave pattern), terminated by FFH. 6293H: 00 (Sector 0) 6294H: 05 (Sector 5) 6295H: 01 (Sector 1) 6296H: 06 (Sector 6) 6297H: 02 (Sector 2) 6298H: 07 (Sector 7) 6299H: 03 (Sector 3) 629AH: 08 (Sector 8) 629BH: 04 (Sector 4) 629CH: 09 (Sector 9) 629DH: FF (Terminator)
6300H	Start of format pattern buffer	Where the track format data is built before being written to disk

53FB

LD A,(5435H) 3A3554

Fetch the value held in memory location 5435H and store it into Register A as the current track being formatted.

53FE

CALL 4402H CD0244

GOSUB to 4402H to Set Register C to the ASCII Number corresponding to the TENS digit of Register A, and Register B to the ONES digit of Register A.

5401

LD (6176H),BC ED437661

Store the value held in Register Pair BC into memory location 6176H which is the nn in the message 1DH + "FORMATTING TRACK nn' + 03H.

5405

LD HL,6164H 216461

Let Register Pair HL equal 6164H to point to the message 1DH + "FORMATTING TRACK 00' + 03H

5408

CALL 457BH CD7B45

GOSUB to 457BH to display the message pointed to by HL on the screen, and RETurn. HL is not affected..

540B

LD A,(5435H) 3A3554

Fetch the value held in memory location 5435H (the current track being formatted) and store it into Register A. 5435H is the nn in an opcode LD C,nn.

540E

LD B,A 47

Copy the contents of Register A (the current track being formatted) into Register B.

540F

LD A,(4709H) 3A0947

Fetch the directory track number (typically 11H/17 decimal) from 4709H into A. This value was read from the source disk's GAT sector (offset +02H) earlier at 530B-530EH.

5412

CP B B8

Compare directory track vs. current track. If they are equal (we're formatting the directory track), the Z flag is set.

5413

LD B,FAH 06FA

Preload B with FAH (the Data Address Mark for directory sectors).

5415

JR Z,5419H 2802

If Z flag set (current track IS the directory track), skip the next instruction and keep FAH as the Data Address Mark.

5417

LD B,FBH 06FB

If Z flag NOT set (current track is NOT the directory track), change B to FBH (the Data Address Mark for normal data sectors).

5419

LD A,B 78

Copy the selected Data Address Mark (FAH or FBH) from B into A.

541A

LD (545FH),A 325F54

Store the DAM into 545FH, which is the operand byte of the LD C,nn instruction at 545EH. This self-modifying code ensures each sector's data field is written with the correct address mark.

This code sets up the FORMAT pattern in RAM. 6300H-6293H = 109 in decimal. The format track is modified many times.

541D

LD HL,6300H 210063

Point HL to 6300H, the start of the format pattern buffer. This is where we'll build the raw bytes to be written to the track.

5420

LD DE,6293H 119362

Point DE to the sector interleave table at 6293H. DE will walk through this table to get sector numbers in interleaved order.

5423

LD B,0EH 060E

Set B = 14 (0EH). This is the count for writing the initial GAP bytes.

5425

Call routine to write 14 FFH bytes (GAP 0) to the format buffer at (HL). HL advances past the written bytes.

This is the top of the sector loop.

5428

LD B,06H 0606

Set B = 6 for writing 6 bytes.

542A

LD C,00H 0E00

Set C = 00H (sync byte value).

542C

CALL 4411H CD1144

GOSUB to 4411H to write 6 x 00H bytes (sync field before ID Address Mark) to format buffer.

542F

LD C,FEH 0EFE

Set C = FEH (the ID Address Mark that precedes the sector ID field)..

5431

GOSUB to 4417H to write 1 x FEH byte (ID Address Mark) to format buffer.

5434

LD C,00H 0E00

Set C = 00H (track number placeholder - but note: 5435H gets modified!).

5436

GOSUB to 4417H to write the track number byte to format buffer. (The actual track number is at 5435H which is this instruction's operand).

5439

LD C,00H 0E00

Set C = 00H (side number - always 0 for single-sided).

543B

GOSUB to 4417H to write side number byte (00H) to format buffer.

543E

LD A,(DE) 1A

Fetch the current sector number from the interleave table pointed to by DE.

543F

LD C,A 4F

Copy sector number into C so it can be written via the next routine.

5440

GOSUB to 4417H to write the sector number byte to format buffer.

5443

LD C,01H 0E01

Set C = 01H (sector size code: 01 = 256 bytes).

5445

GOSUB to 4417H to write the sector size byte to format buffer.

5448

LD C,F7H 0EF7

Set C = F7H (CRC generator command - tells FDC to write CRC bytes here).

544A

GOSUB to 4417H to write CRC command byte to format buffer. Note, the FDC will insert the actual CRC here.

544D

LD C,FFH 0EFF

Let Register C equal FFH which is the gap after ID field.

544F

GOSUB to 4417H to write 1 x FFH byte (gap after ID field) to format buffer.

5452

LD B,0BH 060B

Let Register B equal 0BH, which is the gap between ID and data fields.

5454

GOSUB to 440FH to write 11 x FFH bytes (GAP 2 - gap between ID and data fields) to format buffer.

5457

LD B,06H 0606

Let Register B equal 06H.

5459

LD C,00H 0E00

Let Register C equal 00H.

545B

CALL 4411H CD1144

GOSUB to 4411H to write 6 x 00H bytes (sync field before Data Address Mark) to format buffer.

545E

LD C,nnH 0E00

Load C with the Data Address Mark (FAH or FBH). The "nn" at 545FH was set at 541AH.

5460

GOSUB to 4417H to write the Data Address Mark (FAH for directory track, FBH otherwise) to format buffer.

5463

LD B,00H 0600

Let Register B equal 00H to set up a loop of 256 times.

5465

LD C,E5H 0EE5

Let Register C equal E5H, which is the standard "empty sector" fill byte.

5467

CALL 4411H CD1144

GOSUB to 4411H to write 256 x E5H bytes (the actual sector data area, pre-filled with E5H) to format buffer.

546A

LD C,F7H 0EF7

Set C = F7H (CRC generator command - tells FDC to write CRC bytes here).

546C

GOSUB to 4417H to write the CRC command byte for data field to format buffer.

546F

LD C,FFH 0EFF

Let Register C equal FFH.

5471

GOSUB to 4417H to put FFH into format buffer..

5474

LD B,0BH 060B

Let Register B equal 0BH (Decimal: 11) as a counter.

5476

GOSUB to 440FH to write 11 x FFH bytes, which will be "GAP 3", the gap after data field, to the format buffer..

5479

INC DE 13

Advance to next entry in sector interleave table (next sector number)..

547A

LD A,(DE) 1A

Fetch the next sector number from the interleave table.

547B

INC A 3C

Increment A by 1 to test if it was FFH (the table terminator). If it was FFH, adding 1 makes it 00H and sets the Z flag.

547C

JR NZ,5428H 20AA

If NOT at end of table (not FFH), loop back to 5428H to build the next sector's format pattern.

At this point, all 10 sectors have been set up.

547E

LD B,5BH 065B

Let Register B equal 5BH (Decimal: 91).

5480

GOSUB to 440FH to write 91 x FFH bytes, which will be "GAP 4", the final gap to fill remainder of track, to format buffer.

5483

CALL 54A3H CDA354

GOSUB to 54A3H to select the destination drive, issue the "Write Track" command (F4H), and write the entire format buffer to the physical disk track.

5486

LD A,(5435H) 3A3554

Fetch the current track number from track counter.

5489

INC A 3C

Increment track counter (we just finished formatting this track, move to next).

548A

LD (5435H),A 323554

Store the incremented track counter back into memory location 5435H, so now 5435H holds the NEXT track to be formatted.

548D

CP 23H FE23

Compare the track counter against 23H (decimal 35). TRSDOS 2.3 disks have 35 tracks numbered 0-34. If A=35, we've gone past the last track, meaning all tracks have been formatted. Z flag is set if A equals 35.

548F

JR Z,54EDH 285C

If Z flag is set (track counter = 35), all tracks are formatted! Jump to 54EDH which is the "Last Routine of FORMAT" - it verifies the format, writes out the directory structure, and finishes up.

Next we issue a STEP IN command to move the read/write head one track inward (toward higher track numbers.

5491

LD A,5BH 3E5B

If we didn't jump away, we have more to format. Let Register A equal 5BH (Binary:0101 1011) which tells the floppy disk controller to STEP IN with HEAD LOAD, 30ms Stepping.

5493

LD (37ECH),A 32EC37

Send the command held in Register A to the FDC (37ECH is the Floppy Disk Controller's Command Register).

Bit 7	Bit 6	Bit 5	Bit 4	Bit 3	Bit 2	Bit 1	Bit 0	Function Description
0	0	0	0	h	V	r1	r0
0	1	0	u	h	V	r1	r0	Command=Step In Bit 7-5: Command Code (010) u: 1=Update Track Register, 0=No update h: 1=Enable Head Load/Settle, 0=No delay V: 1=Verify Destination Track ID, 0=No verification r1, r0: Stepping Motor Rate (00=3ms, 01=6ms, 10=10ms, 11=15ms)

Standard code to delay to compensate for the slow FDC to make sure it had time to set the command.

5496

PUSH AF F5

Save the contents of Register Pair AF to the top of the stack.

5497

POP AF F1

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

5498

PUSH AF F5

Save the contents of Register Pair AF to the top of the stack.

5499

POP AF F1

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

549A

LD A,(37ECH) 3AEC37

Fetch the FDC's status (which is mirrored to 37ECH) and store it into Register A.

7	6	5	4	3	2	1	0
Not Ready	Protected	Head Loaded	Seek Error	CRC Error	Track 00	Index	Busy

549D

RRCA 0F

Rotate the bits in A right (i.e., higher bit moves lower), with Bit 0 being copied to both the CARRY FLAG and Bit 7. This means that the CARRY flag will be the same as BIT 0 (which is the BUSY return code)

549E

JR C,549AH 38FA

If Carry is set (FDC was BUSY), loop back to 549AH to keep polling. The head is still stepping to the next track.

54A0

JP 53FBH C3FB53

If we are here then the head has arrived at the new track. Jump back to 53FBH (the top of the main format loop) to format this next track.

54A3H - Write the format pattern to disk.

54A3

CALL 54A7H CDA754

Call the write routine at 54A7H. This is a clever trick: if the routine at 54A7H returns with NZ (indicating the destination disk wasn't ready or was swapped), execution continues to 54A6 which will fall through and call 54A7H again. This allows for a retry.

54A6

RET Z C8

If Z flag is set (write completed successfully), return to caller (5486H). If NZ, execution falls through to call 54A7H again as a retry.

54A7

CALL 449CH CD9C44

GOSUB to 449CH to call the "Insert Destination Disk" routine. This routine: (1) Loads the destination drive select code from 449DH, (2) ORs it with 40H to set bit 6 (identifying it as destination), (3) Displays "INSERT DESTINATION DISK (ENTER)" if needed, (4) Waits for user to press ENTER, (5) Checks that the disk is not write-protected. Returns Z if OK, NZ if there was a problem.

54AA

LD BC,6300H 010063

Point BC to the format pattern buffer at 6300H. BC will be used as the source pointer for the bytes to write to disk.

54AD

LD HL,37ECH 21EC37

Point HL to the FDC Command/Status Register at 37ECH. HL will be used to read the FDC status during the write loop.

54B0

LD (HL),F4H 36F4

Store the value held in F4H (Binary: 1111 0100) into the memory location pointed to by Register Pair HL. This sends: Write Track (1111 0n00), n = 15ms Delay (1)

Bit 7	Bit 6	Bit 5	Bit 4	Bit 3	Bit 2	Bit 1	Bit 0	Function Description
1	1	1	1	0	1	0	0
1	1	1	1	0	1	0	0	Command=Write Track (Format) Bit 7-4: Write Command (1111) Bit 3-0: Specific Code (0100) 0111 = Write CRC 1000-1011 = DAMs 1100=Index Address Mark 1110=ID Address Mark

54B2

LD DE,37EFH 11EF37

Let Register Pair DE equal 37EFH.
NOTE: 37EFH is the Floppy Disk Controller's Data Register (i.e., the byte to be written/read from disk).

54B5

PUSH BC C5

4 Instruction Delay

54B6

POP BC C1

4 Instruction Delay

54B7

PUSH BC C5

4 Instruction Delay

54B8

POP BC C1

4 Instruction Delay

54B9

DI F3

Disable Interrupts.

54BA

JR 54BFH 1803

Continue via a JUMP to 54BFH which will write out all that code to the diskette.

54BCH - Check that the drive is ready, and write the byte pointed at by BC to the diskette

54BC

RRCA 0F

Rotate the bits in A right (i.e., higher bit moves lower), with Bit 0 being copied to both the CARRY FLAG and Bit 7. This will have the effect of moving the FDC's BUSY FLAG to the CARRY FLAG

54BD

JR NC,54CAH 300B

If NOT busy, something's wrong, so JUMP out of this loop to the next routine 54CAH because the disk might be moving at the wrong speed. That routine will either declare drive too fast/slow, or just REturn.

54BF

LD A,(HL) 7E

Fetch the value held in the memory location pointed to by Register Pair HL and store it into Register A. HL should be pointing to the Floppy Disk Controller Status, so this fetches the status of the drive.

7	6	5	4	3	2	1	0
Not Ready	Protected	Head Loaded	Seek Error	CRC Error	Track 00	Index	Busy

54C0

BIT 1,A CB4F

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

54C2

JR Z,54BCH 28F8

If the Z FLAG (Zero) has been set, then there is no index found, so JUMP to 54BCH to move Bit 1 to Bit 0, and Bit 0 (the BUSY status) to the CARRY FLAG, and to then test the CARRY FLAG.

54C4

LD A,(BC) 0A

If we are here then Index was found and the FDC is busy - fetch next byte from format buffer.

54C5

LD (DE),A 12

Store the value held in Register A from the above table to the Floppy Drive by writing it to 37EFH (held in Register Pair DE).

54C6

INC BC 03

INCrement the value stored in Register Pair BC by 1 to point to the next byte in the format buffer.

54C7

JP 54BFH C3BF54

KEEP WRITING via a JUMP BACK to 54BFH.

54CAH - Jumped here from 54BDH if the INDEX/BUSY status isn't quite right to either declare "TRANSFER RATE TOO FAST," "MOTOR FAST," "MOTOR SLOW," and HALT in those cases, or to just REturn.

54CA

LD A,(HL) 7E

Fetch the value from the Floppy Disk Controller Command/Status (pointed to by Register Pair HL) and store it into Register A.

7	6	5	4	3	2	1	0
Not Ready	Protected	Head Loaded	Seek Error	CRC Error	Track 00	Index	Busy

54CB

LD HL,6179H 217961

Let Register Pair HL equal 6179H to point to the message 0AH + 'CAN'T FORMAT, DISK TRANSFER RATE TOO FAST!' + 0DH.

54CE

BIT 2,A CB57

Test Bit Number 2 of Register A to see if the disk was at TRACK 0 or not (1=Yes, 0=No). Z FLAG will be set if that bit is 0, and NZ FLAG will be set if that bit is 1.

54D0

If the disk is NOT at Track 0 then the NZ FLAG (Not Zero) will have been set, JUMP to 43A3H to Display the message pointed to by HL and prompt for ENTER; keep polling the keyboard until ENTER, and then HALT.

54D3

LD HL,9D00H 21009D

Let Register Pair HL equal 9D00H.

54D6

ADD HL,BC 09

LET Register Pair HL = Register Pair HL + Register BC. The Carry Flag will be affected.

54D7

EX DE,HL EB

EXchange the value stored in Register Pair HL with the value stored in Register Pair DE.

54D8

LD HL,F413H 2113F4

Let Register Pair HL equal F413H.

54DB

ADD HL,DE 19

LET Register Pair HL = Register Pair HL + Register DE. The Carry Flag will be affected.

54DC

LD HL,61A5H 21A561

Let Register Pair HL equal 61A5H to point to the message 0AH + 'CAN'T FORMAT, MOTOR SPEED TOO SLOW!' + 0DH.

54DF

JP NC,43A3H D2A343

If the NC FLAG (No Carry) has been set, JUMP to 43A3H to Display the message pointed to by HL and prompt for ENTER; keep polling the keyboard until ENTER, and then HALT.

54E2

LD HL,F397H 2197F3

Let Register Pair HL equal F397H.

54E5

ADD HL,DE 19

LET Register Pair HL = Register Pair HL + Register DE. The Carry Flag will be affected.

54E6

LD HL,61CAH 21CA61

Let Register Pair HL equal 61CAH to point to the message 0AH + 'CAN'T FORMAT, MOTOR SPEED TOO FAST!' + 0DH.

54E9

JP C,43A3H DAA343

If the C FLAG (Carry) has been set, JUMP to 43A3H to Display the message pointed to by HL and prompt for ENTER; keep polling the keyboard until ENTER, and then HALT.

54EC

RET C9

RETurn to the caller.

54EDH - Last Routine of FORMAT. This routine verifies the format, writes out the directory, and finished up.

We are done formatting, so now we need to display a CARRIAGE RETURN to move off of the "FORMATTING TRACK nn" overwriting message.

The main variables are:

4A00H-4A22H	Track status bytes (35 bytes)	FCH = track OK (verify it), FFH = track locked out (skip it)
45AAH	Start of "VERIFYING TRACK nn, SECTOR nn" message	Status message with embedded track/sector placeholders
45BBH	Track number position in message (2 bytes)	ASCII digits for track number (tens, ones)
45C6H	Sector number position in message (2 bytes)	ASCII digits for sector number (tens, ones)
4B00H	Sector read buffer	256-byte buffer for reading sectors during verification
4709H	Directory track number	Typically 11H (17 decimal) for TRSDOS 2.3
D	Current track number (0-34)
E	Current sector number (0-9)
HL	Pointer into track status buffer at 4A00H
BC	Sector read buffer address (4B00H) and ASCII conversion results

54ED

LD A,0DH 3E0D

Load A with 0DH (ASCII carriage return). This will move the cursor to the start of the next line, leaving the "FORMATTING TRACK nn" display behind.

54EF

CALL 0033H CD3300

Call the Model I ROM character output routine at 0033H to display the carriage return. This moves the cursor down so the verify messages don't overwrite the format messages.

54F2

LD D,00H 1600

Initialize D = 0 (starting with Track 0). D will be the track counter for the verification loop.

Start of the TRACK loop.

54F4

LD L,D 6A

Copy the track number from D into L. This sets up HL to point to the track status byte.

54F5

LD H,4AH 264A

Set H = 4AH. Now HL = 4A00H + track number, pointing to the status byte for this track in the buffer.

54F7

LD A,(HL) 7E

Fetch the track status byte from the buffer at (HL). This byte indicates whether the track should be verified (FCH = OK) or skipped (FFH = locked out).

54F8

INC A 3C

INCrement the value stored in Register A by 1 to make it easier to test to see if it was FFH or not.

54F9

JP Z,553CH CA3C55

If Z flag set (track status was FFH = locked out), skip verification of this track entirely - jump to 553CH which increments to the next track.

54FC

LD A,D 7A

Copy the current track number from D into A for ASCII conversion.

54FD

CALL 4402H CD0244

Call the binary-to-ASCII conversion routine. Converts the track number in A to two ASCII digits: C = tens digit, B = ones digit.

5500

LD (45BBH),BC ED43BB45

Store the ASCII track digits into 45BBH (the "nn" position for track number in the "VERIFYING TRACK nn, SECTOR nn" message). Note: Z80 stores BC as C first, then B (little-endian), so tens digit goes to 45BBH, ones to 45BCH.

5504

LD E,00H 1E00

Initialize E = 0 (starting with Sector 0). E will be the sector counter for the inner verification loop.

Start of the SECTOR loop.

5506

PUSH DE D5

Save DE (track/sector) to the stack. DE will be needed for the disk read but also needs to be preserved across the message display routine.

5507

LD A,D 7A

Copy the current track number into A. (Note: This instruction seems redundant as the track number was already converted above, but it may be here for re-entry from a different code path.)

Something is really bad here. In the FORMAT/CMD program, Register A is sent to the drive select latch before the next command overwrites A. It is odd to have 2 commands, back to back, where the first command is just simply ignored.

5508

LD A,E 7B

Copy the current sector number from E into A for ASCII conversion.

5509

CALL 4402H CD0244

Call the binary-to-ASCII conversion routine. Converts the sector number in A to two ASCII digits: C = tens digit, B = ones digit.

550C

LD (45C6H),BC ED43C645

Store the ASCII sector digits into 45C6H (the "nn" position for sector number in the "VERIFYING TRACK nn, SECTOR nn" message).

5510

LD HL,45AAH 21AA45

Point HL to the start of the "VERIFYING TRACK nn, SECTOR nn" message at 45AAH.

5513

CALL 457BH CD7B45

Call the display message routine at 457BH to show the verification progress message on screen. The message starts with 1DH (cursor home) so it overwrites the previous message at the same screen position.

5516

POP DE D1

Restore DE from the stack. D = track number, E = sector number - ready for the disk read.

5517

LD BC,4B00H 01004B

Point BC to the sector read buffer at 4B00H. This is where the disk read routine will store the sector data during verification.

551A

Read the sector (track D, sector E) into buffer at BC (4B00H). On exit, Register A holds the FDC Status.

Test for "Record Not Found" (bit 4) - if set, lock out the track

551D

LD HL,61EFH 21EF61

Let Register Pair HL equal 61EFH to point to the message 3AH + 'NOT FOUND! TRACK LOCKED OUT!' + 0DH

5520

BIT 4,A CB67

Test Bit Number 4 of Register A which would be the FDC status of "RECORD NOT FOUND" if high. Z FLAG will be set if that bit is 0, and NZ FLAG will be set if that bit is 1.

5522

JP NZ,43F5H C2F543

If the NZ FLAG (Not Zero) has been set, JUMP to 43F5H to display the message pointed to by Register Pair HL, put a FFH into the in-RAM GAT at 5Cnn (nn = the contents of Register D), and then JUMP to 55BDH.

Test for "CRC Error" (bit 3) or "Lost Data" (bit 2) - display error and halt

5525

LD HL,620EH 210E62

Let Register Pair HL equal 620EH to point to the message 3AH + 'CRC ERROR! TRACK LOCKED OUT!' + 0DH

5528

BIT 3,A CB5F

Test Bit Number 3 of Register A which would be the FDC status of "CRC ERROR" if high. Z FLAG will be set if that bit is 0, and NZ FLAG will be set if that bit is 1.

552A

JP NZ,43F5H C2F543

552D

LD HL,6260H 216062

Let Register Pair HL equal 6260H to point to the message 0AH + 'CAN'T VERIFY FORMAT, DISK TRANSFER RATE TOO FAST!' + 0DH

5530

BIT 2,A CB57

Test Bit Number 2 of Register A which would be the FDC status of "LOST DATA" if high. Z FLAG will be set if that bit is 0, and NZ FLAG will be set if that bit is 1.

5532

If the NZ FLAG (Not Zero) has been set, JUMP to 43A3H to Display the message pointed to by HL and prompt for ENTER; keep polling the keyboard until ENTER, and then HALT.

5535

INC E 1C

Increment sector, loop back to 5506H if still fewer than 10 sectors.

5536

LD A,E 7B

Copy the contents of Register E into Register A for testing.

5537

CP 0AH FE0A

Compare the value held in Register A against 0AH (Decimal: 10). If Register A equals 10, the Z FLAG is set; otherwise the NZ FLAG is set.

5539

JP NZ,5506H C20655

If the NZ FLAG (Not Zero) has been set then we have not yet hit 10 sectors on the track, so JUMP BACK to 5506H.

While this can be a pass through, it is also the jump point if the byte in the in-RAM GAT was FFH

553C

INC D 14

Increment track, loop back to 54F4H if fewer than 35 tracks.

553D

LD A,D 7A

Copy the contents of Register D into Register A.

553E

CP 23H FE23

Compare the value held in Register A against 23H (Decimal: 35). If Register A equals 35, the Z FLAG is set; otherwise the NZ FLAG is set.

5540

JP NZ,54F4H C2F454

If the NZ FLAG (Not Zero) has been set, then we have not yet hit 35 tracks, so JUMP BACK to 54F4H to keep reading.

At this point, we have verified all tracks, now lets display a carriage return, copy 35 bytes from 4A00H (destination disk's track allocation status) to 4A60H (normally the track lockout table offset in the GAT buffer).

5543

LD A,0DH 3E0D

Let Register A equal CARRIAGE RETURN.

5545

CALL 0033H CD3300

GOSUB to 0033H in the Model I ROM to display the character held in Register A at the current cursor position.

5548

LD HL,4A00H 21004A

Let Register Pair HL equal 4A00H, which is the source RAM area.

554B

LD DE,4A60H 11604A

Let Register Pair DE equal 4A60H, which is the DEstination RAM area.

554E

LD BC,0023H 012300

Let Register Pair BC equal 0023H (Decimal: 35) to transfer 35 bytes of data; one byte for each track.

5551

LDIR EDB0

5553H-5571H is a Lockout Table Compatibility Check. It will [1] Compare the source and destination disk lockout tables, [2] For each track, verify that any granules USED on the source disk are NOT locked out on the destination, and [3] If the destination has bad sectors where the source needs good ones, displays "BACKUP REJECTED DUE TO FLAWS ON DESTINATION DISK!" and halt.

5553

LD HL,4A60H 21604A

Point HL to the DESTINATION disk's track lockout table at 4A60H (offset +60H in the GAT buffer at 4A00H). This table has 35 bytes, one per track, where each bit indicates if a granule is locked out.

5556

LD DE,4960H 116049

Point DE to the SOURCE disk's track lockout table at 4960H (offset +60H in the GAT buffer at 4900H). This tells us which granules the source disk needs to be good on the destination.

5559

LD B,23H 0623

Set loop counter B = 35 (23H). There are 35 tracks on a TRSDOS 2.3 disk (tracks 0-34), so we need to check all 35 lockout bytes.

Start of a loop

555B

LD A,(DE) 1A

Top of a loop to fetch the SOURCE disk's lockout byte for this track from (DE). Each bit that is SET (=1) indicates a granule that is locked out on the source disk.

555C

CPL 2F

Complement (invert) all bits in A. Now each bit that is SET indicates a granule that is GOOD (usable) on the source disk. These are the granules we need to exist on the destination.

555D

LD C,A 4F

Save the inverted source lockout mask into C. C now holds a mask where 1 = "source needs this granule to be good".

555E

PUSH DE D5

Save DE (pointer into source lockout table) to the stack, as we need to temporarily modify E.

555F

LD A,E 7B

Copy the low byte of the source pointer (E) into A. E ranges from 60H to 82H as we walk through the lockout table.

5560

SUB 60H D660

Subtract 60H from A to convert the lockout table offset to a track allocation table offset. This changes range from 60H-82H to 00H-22H.

5562

LD E,A 5F

Put the adjusted offset into E. DE now points to the SOURCE disk's track ALLOCATION byte (at 4900H-4922H) instead of the lockout byte.

5563

LD A,(DE) 1A

Fetch the SOURCE disk's track allocation byte from (DE). Each bit pattern indicates granule usage: FC=both free, FD/FE=partial, FF=both allocated or track locked out.

5564

POP DE D1

Restore DE to point back to the source lockout table position.

5565

AND C A1

AND the source allocation byte with the inverted source lockout mask (C). Result: bits set only where source has ALLOCATED granules that are NOT locked out. These are granules we actually need on the destination.

5566

AND (HL) A6

AND the result with the DESTINATION disk's lockout byte at (HL). If any bit remains set, it means the source needs a granule that is LOCKED OUT on the destination - a fatal flaw!

5567

PUSH HL E5

Save HL (destination lockout table pointer) to the stack before loading the error message pointer.

5568

LD HL,622DH 212D62

Point HL to the error message at 622DH: "BACKUP REJECTED DUE TO FLAWS ON DESTINATION DISK!" in case we need to display it.

556B

JP NZ,43A3H C2A343

If NZ (any bit was set after the AND operations), the destination disk has a locked-out granule where the source needs good space. JUMP to error handler at 43A3H to display the message and HALT.

556E

POP HL E1

Restore HL to point to the destination lockout table position.

556F

INC DE 13

Increment DE to point to the next byte in the source lockout table.

5570

INC HL 23

Increment HL to point to the next byte in the destination lockout table.

5571

DJNZ 555BH 10E8

Decrement B and loop back to 555BH if not zero. Continue checking all 35 tracks.

If we have a single-drive backup, we need to calculate the RAM Buffer Size.
If 44EFH is not 0, then this is a 2 drive backup, and we skip this routine and move right to copy.
Otherwise, we need to determine how much RAM is available. To do this, 4K pages are tested from high memory downward. Once that is determined the calculation as to how many complete tracks can be buffered in RAM at a time, and stores that count in 4713H so that the copy routine knows how many tracks to load before prompting for a disk swap.

5573

LD A,(44EFH) 3AEF44

Fetch the DRIVE DIFFERENCE value from 44EFH. This was calculated earlier as (SOURCE drive - DESTINATION drive). Zero means single-drive backup (same drive for source and destination).

5576

OR A B7

Set flags based on A. Z flag set if A=0 (single-drive backup), NZ if A≠0 (two-drive backup).

5577

JP NZ,4200H C20042

If NZ (two-drive backup), jump to 4200H to begin the actual backup copy operation. No need to calculate available RAM since we have two drives.

557A

LD HL,0FFFH 21FF0F

For single-drive backup, we need to calculate how much RAM is available for buffering tracks. Start HL at 0FFFH (4095), which will be adjusted to find the top of usable RAM.

557D

LD A,H 7C

Top of the RAM Test Loop. Copy H into A. H starts at 0FH and will be decremented by 10H each iteration to test different 4K pages.

557E

SUB 10H D610

Subtract 10H from A. This moves down by 4K (1000H) in memory. Testing if RAM exists at progressively lower addresses.

5580

LD H,A 67

Put the decremented value back into H. HL now points 4K lower in memory.

5581

LD A,(HL) 7E

Read the byte at the current test address (HL) into A. This is the original value at this memory location.

5582

LD B,A 47

Save the original byte value into B so we can restore it after the test.

5583

CPL 2F

Complement (invert) all bits in A. If we can write this inverted value and read it back, RAM exists at this address.

5584

LD (HL),A 77

Write the complemented (inverted) value to the memory location being tested.

5585

CP (HL) BE

Compare A with the value we just wrote. If RAM exists, the value should match (Z flag set). If no RAM or ROM, the value won't match (NZ flag set).

5586

LD (HL),B 70

Restore the original byte value (saved in B) back to the memory location, preserving memory contents.

5587

JR NZ,557DH 20F4

If NZ (RAM test failed - no RAM at this address), loop back to 557DH to test the next lower 4K page.

5589

LD A,H 7C

RAM was found! Copy H (the high byte of the highest usable RAM address) into A.

558A

SUB 4DH D64D

Subtract 4DH from A. 4D00H is the start of the track buffer area. This calculates how many 256-byte pages are available above the buffer area.

558C

LD B,00H 0600

Initialize B = 0. B will count how many complete tracks (10 sectors = 2560 bytes ≈ 0AH pages) can fit in available RAM.

Track Count Loop Start

558E

INC B 04

Increment track counter B.

558F

SUB 0AH D60A

Subtract 0AH (10 decimal) from A. Each track requires 10 sectors (10 x 256 = 2560 bytes ≈ 0AH pages of 256 bytes).

5591

JR NC,558EH 30FB

If No Carry (still have room for more tracks), loop back to 558EH to count another track.

5593

LD A,B 78

Copy the track count from B into A. B now holds how many complete tracks can be buffered in RAM.

5594

DEC A 3D

Decrement A by 1. We counted one too many (the loop increments before checking), so adjust the count.

5595

LD (4713H),A 321347

Store the number of tracks that can be buffered into memory location 4713H. This value will be used during the single-drive backup to know how many tracks to read before prompting for a disk swap.

5598

JP 4200H C30042

Jump to 4200H to begin the actual backup copy operation. The routine at 4200H will prompt for the destination disk, perform the copy using the calculated buffer size, and handle disk swapping for single-drive backups.

4200H - BACKUP COPY ROUTINE: Initialize Destination Disk

This is the main BACKUP copy routine, relocated from 559BH to run at 4200H. It operates in three phases: (1) Initialize destination disk with marker byte, (2) Read source tracks into RAM buffer, (3) Write buffered data to destination. For single-drive backups, phases 2 and 3 repeat with disk swaps until all 35 tracks are copied.

Key Memory Locations

Address	Purpose	Description
4713H	Tracks per buffer	Number of complete tracks that can fit in RAM buffer (calculated for single-drive backup)
4714H	Tracks loaded counter	How many tracks have been loaded into the buffer so far
44E8H	Last prompted drive	Drive code of disk currently in drive (used to determine if disk swap is needed)
44EFH	Drive difference	Source - Destination drive; 0 = single-drive backup requiring swaps
4800H-4822H	FDC status array	Stores FDC status for each track (0-34), indexed by track number
4900H-49FFH	Source GAT buffer	GAT sector from source disk
4A00H-4AFFH	Dest GAT buffer	GAT sector from destination disk
4B00H-4BFFH	General buffer	Single-sector read/write buffer
4D00H+	Multi-track buffer	Large buffer for single-drive backup (holds multiple tracks)

Key Subroutines

Address	Purpose
4402H	Convert binary in A to two ASCII digits (tens in C, ones in B)
4437H	Prompt for source disk if needed, verify correct source via Pack ID
43B3H	Write sector to disk (A=FDC command, BC=buffer, DE=track/sector)
44B4H	Prompt for destination disk if needed, verify write-enabled
449CH	Prompt for destination disk, check write protection
453CH	Disk I/O Executive - read sector (BC=buffer, DE=track/sector)
457BH	Display message pointed to by HL

4200H
559BH

CALL 449CH CD 9C 44

Call the "Insert Destination Disk" routine. This prompts the user to insert the destination disk (if needed), sets Bit 6 in the drive code to mark it as destination, verifies the disk is not write-protected, and returns.

4203H
559EH

Issue a RESTORE command to the FDC to move the read/write head to Track 0, then poll until the drive is no longer BUSY.

4206H
55A1H

LD BC,4B00H 01 00 4B

Set BC to point to 4B00H, the general-purpose 256-byte sector buffer. This will receive the GAT sector data from the destination disk.

4209H
55A4H

LD DE,0000H 11 00 00

Set DE to Track 0, Sector 0 - the GAT (Granule Allocation Table) sector location.

420CH
55A7H

Call the Disk I/O Executive to read the destination disk's GAT sector (Track 0, Sector 0) into the buffer at 4B00H.

420FH
55AAH

LD HL,4601H 210146

Point HL to the error message "DESTINATION DISK READ ERROR!" in case the read failed.

4212H
55ADH

If the read failed (NZ flag set), jump to the error handler at 43A3H which displays the error message and HALTs.

Now we mark the destination disk's GAT with 76H in the first byte. This serves as a "backup in progress" marker. The value 76H is the HALT opcode, so if this sector is accidentally executed, the system will safely halt.

4215H
55B0H

LD A,76H 3E 76

Load A with 76H, the "backup marker" value. This will be written to the first byte of the destination GAT.

4217H
55B2H

LD (4B00H),A 32 00 4B

Store the 76H marker into the first byte of the GAT buffer. This marks the destination disk as being part of a backup set.

421AH
55B5H

LD A,A8H 3E A8

Load A with A8H (10101000 binary) - the FDC "Write Sector" command with head load enabled. This will be passed to the write routine.

421CH
55B7H

LD BC,4B00H 01 00 4B

Set BC to point to the modified GAT buffer at 4B00H (source of data to write).

421FH
55BAH

Call the Write Sector routine. A=command (A8H), BC=buffer (4B00H), DE=track/sector (still 0000H from earlier). This writes the modified GAT back to the destination disk with the 76H marker.

4222H
55BDH

LD HL,461FH 21 1F 46

Point HL to the error message "DESTINATION DISK WRITE ERROR!" in case the write failed.

4225H
55C0H

If the write failed (NZ flag set), jump to error handler to display the message and HALT.

4228H
55C3H

CALL 4437H CD 37 44

Call the "Swap to Source Disk" routine. If this is a single-drive backup, this prompts the user to insert the source disk, verifies it's the correct source via Pack ID comparison, and returns.

422BH
55C6H

Issue RESTORE command to move head to Track 0 on the (now) source disk.

422EH - PHASE 2: Load Source Tracks (LOADING)

This phase reads all tracks from the source disk into the RAM buffer. Register D holds the current track number (0-34), Register E holds the current sector number (0-9). For single-drive backups, 4714H counts how many tracks have been loaded; when it equals 4713H (tracks per buffer), we switch to writing.

422EH
55C9H

XOR A AF

Set A to zero and clear all flags. This zero will initialize the "tracks loaded" counter.

422FH
55CAH

LD (4714H),A 32 14 47

Initialize the "tracks loaded" counter at 4714H to zero. This counts how many complete tracks have been read into the buffer during single-drive backups.

4232H
55CDH

LD D,00H 16 00

Initialize Register D to 0 - this is the track counter. We'll read tracks 0 through 34.

4234H
55CFH

LD BC,4D00H 01 00 4D

Top of the Track loop. Set BC to 4D00H, the start of the multi-track sector buffer. Each sector is 256 bytes, so this buffer will fill with sector data as we read.

4237H
55D2H

LD A,(37EDH) 3A ED 37

Fetch the current FDC Track Register value from 37EDH. This is the physical track position of the read/write head.

423AH
55D5H

LD (42ADH),A 32 AD 42

Store the current head position into 42ADH. This is a self-modifying code location - the operand of a LD A,nn instruction at 42ACH, used later during the write phase.

423DH
55D8H

LD A,D 7A

Copy the current track number (from D) into A for self-modifying code.

423EH
55D9H

LD (42B2H),A 32 B2 42

Store the starting track number into 42B2H. This is the operand of a LD D,nn instruction at 42B1H, used during the write phase to know which track to start writing.

Top of the INNER TRACK loop. For each track, we check the source disk's track allocation to see if this track needs to be copied. We skip locked-out tracks.

4241H
55DCH

PUSH BC C5

Top of the TRACK part of the loop. Save the current buffer pointer (BC) to the stack before we modify registers.

4242H
55DDH

LD H,49H 26 49

Set H to 49H, pointing to the source disk's GAT buffer at 4900H.

4244H
55DFH

LD L,D 6A

Set L to the current track number (D). HL now points to 49xxH where xx is the track number - this is the track allocation byte in the source GAT.

4245H
55E0H

LD C,(HL) 4E

Fetch the source disk's track allocation byte for this track into C. Each bit pair indicates granule status: 11=allocated, 00=free, etc.

4246H
55E1H

LD A,D 7A

Copy the track number into A for calculating the lockout table offset.

4247H
55E2H

ADD A,60H C6 60

Add 60H to the track number to get the lockout table offset. The lockout table starts at GAT+60H.

4249H
55E4H

LD L,A 6F

Set L to the lockout table offset. HL now points to the source disk's lockout byte for this track (4960H + track).

424AH
55E5H

LD A,(HL) 7E

Fetch the source disk's lockout byte for this track. A bit=1 means that granule is locked out (bad sector).

424BH
55E6H

CPL 2F

Complement (invert) the lockout byte. Now bit=1 means the granule is GOOD and needs to be copied.

424CH
55E7H

AND C A1

AND with the track allocation byte (C). Result: bits set only where source has ALLOCATED granules that are NOT locked out.

424DH
55E8H

LD H,4AH 26 4A

Point H to the destination disk's GAT buffer at 4A00H.

424FH
55EAH

LD C,(HL) 4E

Fetch the destination disk's lockout byte for this track into C (HL points to 4A60H + track).

4250H
55EBH

OR C B1

OR with destination lockout. A now has bits set for: (1) source allocated non-locked granules OR (2) destination locked granules.

4251H
55ECH

CP C B9

Compare result against destination lockout. If equal (Z flag), this track is entirely locked out on destination - skip reading it.

4252H
55EDH

POP BC C1

Restore the buffer pointer from the stack.

4253H
55EEH

JP Z,429BH CA 9B 42

If Z flag set (track fully locked out), skip to 429BH to advance to the next track without reading this one.

Track needs to be read. Call 4437H to ensure source disk is inserted (for single-drive backup), then read all 10 sectors.

4256H
55F1H

CALL 4437H CD 37 44

Call the "Verify Source Disk" routine. For single-drive backups, this ensures the source disk is inserted and verifies it's the correct disk via Pack ID.

4259H
55F4H

LD E,00H 1E 00

Initialize sector counter E to 0. We'll read sectors 0-9 from this track.

Top of the SECTOR part of the loop. Read all 10 sectors from the current track, displaying "LOADING TRACK nn, SECTOR nn" for each.

425BH
55F6H

PUSH BC C5

Save the buffer pointer to the stack.

425CH
55F7H

LD A,D 7A

Copy the track number into A for conversion to ASCII.

425DH
55F8H

Convert track number in A to two ASCII digits: tens digit in C, ones digit in B.

4260H
55FBH

LD (45DAH),BC ED 43 DA 45

Store the ASCII track digits into the "LOADING TRACK nn" message at offset 45DAH (the "nn" placeholder).

4264H
55FFH

LD A,E 7B

Copy the sector number into A for conversion to ASCII.

4265H
5600H

Convert sector number in A to two ASCII digits: tens digit in C, ones digit in B.

4268H
5603H

LD (45E5H),BC ED 43 E5 45

Store the ASCII sector digits into the "LOADING" message at offset 45E5H (the second "nn" placeholder).

426CH
5607H

POP BC C1

Restore the buffer pointer from the stack.

426DH
5608H

PUSH DE D5

Save the track/sector (DE) to the stack before the display call.

426EH
5609H

LD HL,45C9H 21 C9 45

Point HL to the "LOADING TRACK nn, SECTOR nn" message at 45C9H.

4271H
560CH

Display the "LOADING TRACK nn, SECTOR nn" message. The 1DH at the start homes the cursor so each update overwrites the previous.

4274H
560FH

POP DE D1

Restore track/sector from the stack.

4275H
5610H

Call the Disk I/O Executive to read one sector from the source disk. D=track, E=sector, BC=buffer address.

4278H
5613H

LD HL,45E8H 21 E8 45

Point HL to "SOURCE DISK READ ERROR!" message in case of failure.

427BH
5616H

If read failed (NZ), jump to error handler to display message and HALT.

427EH
5619H

INC E 1C

Increment sector counter. BC automatically advanced by 256 during the read, so it now points to the next buffer slot.

427FH
561AH

LD A,E 7B

Copy sector number into A for comparison.

4280H
561BH

CP 0AH FE 0A

Compare sector number against 10 (0AH). Have we read all 10 sectors (0-9)?

4282H
561DH

JR NZ,425BH 20 D7

If not all 10 sectors read yet (NZ), loop back to 425BH to read the next sector.

All 10 sectors of this track have been read. Store the FDC status, increment the track counter, and check if we need to switch to writing.

4284H
561FH

LD A,(37ECH) 3A EC 37

Fetch the FDC status register from 37ECH into A. This records the result of the last disk operation.

4287H
5622H

LD L,D 6A

Set L to the track number.

4288H
5623H

LD H,48H 26 48

Set H to 48H. HL now points to the FDC status array at 4800H + track. Each track has its status stored here.

428AH
5625H

LD (HL),A 77

Store the FDC status for this track into the status array. This is checked later during the write phase.

428BH
5626H

LD HL,4714H 21 14 47

Point HL to the "tracks loaded" counter at 4714H.

428EH
5629H

INC (HL) 34

Increment the tracks loaded counter. We've successfully loaded one more track into the buffer.

428FH
562AH

LD A,(44EFH) 3A EF 44

Fetch the "drive difference" value from 44EFH. Zero = single-drive backup, non-zero = two-drive backup.

4292H
562DH

OR A B7

Set flags based on A. Z flag means single-drive backup (need to check buffer capacity).

4293H
562EH

JR NZ,42A9H 20 14

If two-drive backup (NZ), skip buffer capacity check and jump to 42A9H to continue with the write phase.

Single-drive backup: check if the buffer is full by comparing tracks loaded (4714H) against tracks per buffer (4713H).

4295H
5630H

LD A,(4713H) 3A 13 47

Fetch the "tracks per buffer" value from 4713H. This was calculated earlier based on available RAM.

4298H
5633H

CP (HL) BE

Compare tracks per buffer against tracks loaded (HL still points to 4714H). Z flag = buffer is full.

4299H
5634H

JR Z,42A9H 28 0E

If buffer is full (Z flag), jump to 42A9H to switch to the WRITE phase (write buffered tracks to destination).

Move to the next track since the buffer is not full yet (or we have a two-drive backup).

429BH
5636H

INC D 14

429CH
5637H

LD A,D 7A

Copy track number into A for comparison.

429DH
5638H

CP 23H FE 23

Compare against 35 (23H). Have we processed all 35 tracks (0-34)?

429FH
563AH

JP NZ,4241H C2 41 42

If not all tracks done (NZ), loop back to 4241H to process the next track.

All 35 tracks have been read. Check if anything was actually loaded (for edge case where all tracks were locked out).

42A2H
563DH

LD A,(4714H) 3A 14 47

Fetch the tracks loaded counter.

42A5H
5640H

OR A B7

Set flags. Z flag = no tracks were loaded (all locked out).

42A6H
5641H

JP Z,434FH CA 4F 43

If nothing was loaded (Z), skip the write phase entirely and jump to 434FH to finish up (copy date, write final GAT).

42A9H - PHASE 3: Write to Destination (COPYING)

This phase writes the buffered sectors to the destination disk. For single-drive backups, after writing all buffered tracks, it loops back to read more from the source. Register D = track number, Register E = sector number, BC = buffer pointer.

Start of write routine

42A9H
5644H

LD BC,4D00H 01 00 4D

42ACH
5647H

LD A,00H 3E 00

Load A with the saved FDC track position. NOTE: The 00H here is self-modified - the actual value was stored at 42ADH during the read phase (at 423AH).

42AEH
5649H

LD (37EDH),A 32 ED 37

Restore the FDC Track Register to the saved head position. This ensures we seek from the correct starting point.

42B1H
564CH

LD D,00H 16 00

Load D with the starting track number. NOTE: The 00H here is self-modified - the actual value was stored at 42B2H during the read phase (at 423EH).

Top of the write track loop. For each track that was read, check if it needs to be written and perform the copy.

42B3H
564EH

PUSH BC C5

Save the buffer pointer to the stack.

42B4H
564FH

LD H,49H 26 49

Set H to 49H, pointing to the source disk's GAT buffer at 4900H.

42B6H
5651H

LD L,D 6A

Set L to the current track number. HL points to source track allocation byte.

42B7H
5652H

LD C,(HL) 4E

Fetch source track allocation byte into C.

42B8H
5653H

LD A,D 7A

Copy track number to A for lockout table calculation.

42B9H
5654H

ADD A,60H C6 60

Add 60H to get lockout table offset.

42BBH
5656H

LD L,A 6F

Point HL to source lockout byte for this track.

42BCH
5657H

LD A,(HL) 7E

Fetch source lockout byte.

42BDH
5658H

CPL 2F

Complement - now 1=good granule.

42BEH
5659H

AND C A1

AND with allocation - now 1=allocated good granule.

42BFH
565AH

LD H,4AH 26 4A

Point H to destination GAT buffer at 4A00H.

42C1H
565CH

LD C,(HL) 4E

Fetch destination lockout byte into C.

42C2H
565DH

OR C B1

OR source allocation with destination lockout.

42C3H
565EH

LD L,D 6A

Point HL to destination lockout byte (4A60H + track).

42C4H
565FH

LD (HL),A 77

Update destination lockout table with combined status.

42C5H
5660H

CP C B9

Compare result with destination lockout. Z = track fully locked, skip writing.

42C6H
5661H

POP BC C1

Restore buffer pointer from stack.

42C7H
5662H

JP Z,433FH CA 3F 43

If track fully locked out (Z), skip to 433FH to advance to next track without writing.

Track needs to be written. First ensure destination disk is inserted (for single-drive backup).

42CAH
5665H

CALL 44B4H CD B4 44

Call "Swap to Destination Disk" routine. For single-drive backups, prompts user to insert destination disk and verifies it's correct.

Check if this is track 0 (special handling for the GAT sector that has our 76H marker).

42CDH
5668H

LD E,00H 1E 00

Initialize sector counter E to 0.

42CFH
566AH

LD A,D
OR E 7A B3

Test if both D and E are zero (Track 0, Sector 0). The Z-80 can't directly test DE=0, so we OR D with E after loading D into A.

42D1H
566CH

JR NZ,42DAH 20 07

If not Track 0 Sector 0 (NZ), skip the special GAT handling and jump to 42DAH for normal sector write.

Special handling for Track 0, Sector 0 (the GAT sector). We need to restore the original first byte that we replaced with 76H marker.

42D3H
566EH

LD A,(BC) 0A

Fetch the original first byte of the source disk's GAT from the buffer (pointed to by BC).

42D4H
566FH

LD (4390H),A 32 90 43

Save the original GAT first byte at 4390H. This is a self-modifying location - the operand of a LD A,nn at 438FH used later to restore it.

42D7H
5672H

LD A,76H 3E 76

Load A with the 76H marker value again.

42D9H
5674H

LD (BC),A 02

Store the 76H marker as the first byte of the GAT in the buffer. This ensures the destination disk keeps the "backup in progress" marker until the final write.

Top of the sector write loop. Write all 10 sectors of this track, displaying "COPYING TRACK nn, SECTOR nn".

42DAH
5675H

PUSH BC C5

Save the buffer pointer twice - once for the display routine, once for the write.

42DBH
5676H

PUSH BC C5

Save buffer pointer again (second copy on stack).

42DCH
5677H

LD A,D 7A

Copy track number into A for ASCII conversion.

42DDH
5678H

Convert track number to ASCII digits (tens in C, ones in B).

42E0H
567BH

LD (459CH),BC ED 43 9C 45

Store ASCII track digits into the "COPYING TRACK nn" message at offset 459CH.

42E4H
567FH

LD A,E 7B

Copy sector number into A for ASCII conversion.

42E5H
5680H

Convert sector number to ASCII digits (tens in C, ones in B).

42E8H
5683H

LD (45A7H),BC ED 43 A7 45

Store ASCII sector digits into the "COPYING" message at offset 45A7H.

42ECH
5687H

POP BC C1

Restore first copy of buffer pointer.

42EDH
5688H

PUSH DE D5

Save track/sector to stack before display call.

42EEH
5689H

LD HL,458BH 21 8B 45

Point HL to the "COPYING TRACK nn, SECTOR nn" message at 458BH.

42F1H
568CH

Display the "COPYING TRACK nn, SECTOR nn" status message.

42F4H
568FH

POP DE D1

Restore track/sector from stack.

Now build the FDC write command based on the stored FDC status from when we read this track. We extract bits 5-6 (step rate info) and merge with A8H (write sector command).

42F5H
5690H

LD L,D 6A

Set L to track number to index into the FDC status array.

42F6H
5691H

LD H,48H 26 48

Point HL to the FDC status array at 4800H + track. This was stored during the read phase.

42F8H
5693H

LD A,(HL) 7E

Fetch the FDC status that was recorded when this track was read.

42F9H
5694H

AND 60H E6 60

Mask to keep only bits 5 and 6 (01100000). These bits contain drive timing/status information.

42FBH
5696H

RLCA 07

Rotate left 3 times to move bits 5-6 to bits 0-1 (via bit 7 and carry).

42FCH
5697H

RLCA 07

Second rotate left.

42FDH
5698H

RLCA 07

Third rotate left. Original bits 5-6 are now in positions 0-1.

42FEH
5699H

OR A8H F6 A8

OR with A8H (10101000) to form the complete "Write Sector" FDC command. A8H = Write Sector with head load and side 0.

4300H
569BH

Call the Write Sector routine. A=FDC command, BC=buffer (second copy from stack), DE=track/sector.

4303H
569EH

LD HL,461FH 21 1F 46

Point HL to "DESTINATION DISK WRITE ERROR!" message in case write failed.

4306H
56A1H

If write failed (NZ), jump to error handler to display message and HALT.

4309H
56A4H

INC E 1C

Increment sector counter. The write routine advanced BC by 256, so it now points to the next sector's data.

430AH
56A5H

LD A,E 7B

Copy sector number into A for comparison.

430BH
56A6H

CP 0AH FE 0A

Compare against 10. Have we written all 10 sectors (0-9)?

430DH
56A8H

JR NZ,42DBH 20 CC

If not all sectors written (NZ), loop back to 42DBH to write the next sector.

All 10 sectors of this track written. Clean up stack and prepare for next track. For VERIFYING phase display.

430FH
56AAH

POP BC C1

Pop the second buffer pointer copy that was saved at 42DAH. BC now points past all 10 sectors we just wrote.

4310H
56ABH

LD E,00H 1E 00

Reset sector counter to 0 for the VERIFY read-back loop.

Top of the VERIFY loop. Read back each sector to verify the write was successful. Display "VERIFYING TRACK nn, SECTOR nn".

4312H
56ADH

PUSH BC C5

Save buffer pointer to stack.

4313H
56AEH

LD A,D 7A

Copy track number into A for ASCII conversion.

4314H
56AFH

Convert track number to ASCII digits.

4317H
56B2H

LD (45BBH),BC ED 43 BB 45

Store ASCII track digits into the "VERIFYING TRACK nn" message at offset 45BBH.

431BH
56B6H

LD A,E 7B

Copy sector number into A for ASCII conversion.

431CH
56B7H

Convert sector number to ASCII digits.

431FH
56BAH

LD (45C6H),BC ED 43 C6 45

Store ASCII sector digits into the "VERIFYING" message at offset 45C6H.

4323H
56BEH

POP BC C1

Restore buffer pointer from stack.

4324H
56BFH

PUSH DE D5

Save track/sector before display call.

4325H
56C0H

LD HL,45AAH 21 AA 45

Point HL to the "VERIFYING TRACK nn, SECTOR nn" message at 45AAH.

4328H
56C3H

Display the "VERIFYING TRACK nn, SECTOR nn" status message.

432BH
56C6H

POP DE D1

Restore track/sector from stack.

432CH
56C7H

Call Disk I/O Executive to READ the sector back for verification. D=track, E=sector, BC=buffer.

432FH
56CAH

LD HL,4601H 210146

Point HL to "DESTINATION DISK READ ERROR!" message (verify is a read operation).

4332H
56CDH

If verify read failed (NZ), jump to error handler to display message and HALT.

4335H
56D0H

INC E 1C

Increment sector counter for next verify iteration.

4336H
56D1H

LD A,E 7B

Copy sector number into A for comparison.

4337H
56D2H

CP 0AH FE 0A

Compare against 10. Have we verified all 10 sectors?

4339H
56D4H

JR NZ,4312H 20 D7

If not all sectors verified (NZ), loop back to 4312H to verify the next sector.

All sectors of this track verified. Decrement tracks-loaded counter and advance to next track.

433BH
56D6H

LD HL,4714H 21 14 47

Point HL to the "tracks loaded" counter.

433EH
56D9H

DEC (HL) 35

Decrement the tracks-loaded counter. One track has been written from the buffer.

433FH
56DAH

INC D 14

Increment track counter D.

4340H
56DBH

LD A,D 7A

Copy track number into A for comparison.

4341H
56DCH

CP 23H FE 23

Compare against 35. Have we processed all tracks?

4343H
56DEH

JR Z,434FH 28 0A

If all 35 tracks processed (Z), jump to 434FH to finish the backup (copy date, write final GAT).

Not all tracks done. For single-drive backup, check if buffer is empty (need to read more).

4345H
56E0H

LD A,(4714H) 3A 14 47

Fetch the tracks-loaded counter.

4348H
56E3H

OR A B7

Set flags. Z = buffer is empty (no more buffered tracks to write).

4349H
56E4H

JP NZ,42B3H C2 B3 42

If buffer still has tracks (NZ), loop back to 42B3H to write the next buffered track.

434CH
56E7H

JP 4234H C3 34 42

Buffer is empty - jump back to 4234H to swap to source disk and load more tracks. The single-drive backup cycle continues until all 35 tracks are copied.

434FH - Finish Phase: Finalize Backup and Restore Boot Sector

Entry point after all tracks have been copied. The destination disk still has the 76H "backup in progress" marker at both the GAT (directory track sector 0) byte 0 and the BOOT sector (Track 0, Sector 0) byte 0. The original BOOT byte 0 was saved at self-modifying location 4390H during the copy phase. Now we finalize the backup by: (1) writing the backup date to the GAT, (2) writing the final GAT to the destination, and (3) restoring the original BOOT sector byte 0.

Memory Locations Referenced

Address	Contents	Purpose
470AH	8-byte date	User-entered backup date (MM/DD/YY format + 2 spaces)
9D8H	GAT+D8H	Date field within source GAT buffer
4900H	256 bytes	Source GAT buffer (allocation, lockout, Pack ID, date)
4B00H	256 bytes	General purpose sector buffer
4709H	1 byte	Directory track number (typically 11H/17 decimal)
4390H	1 byte	Saved original BOOT sector byte 0 (self-modifying)
37ECH	FDC Status/Command	WD1771 command/status register
37EDH	FDC Track	FDC track register
37EEH	FDC Sector	FDC sector register
37EFH	FDC Data	FDC data register
3840H	Keyboard Row 7	`ENTER`, `CLEAR`, `BREAK`, `arrows`, `SPACE`

434FH
56EAH

LD HL,470AH 21 0A 47

Point HL to the 8-byte backup date buffer at 470AH. This contains the user-entered date in MM/DD/YY format (or the system date if none was entered). The date was validated earlier during program initialization.

4352H
56EDH

LD DE,49D8H 11 D8 49

Point DE to offset D8H within the source GAT buffer (4900H + D8H = 49D8H). In the GAT structure, offset D8H-DFH is the date field where the backup date is stored.

4355H
56F0H

LD BC,0008H 01 08 00

Set byte count to 8 (the length of the date field: MM/DD/YY).

4358H
56F3H

LDIR ED B0

Block transfer: Copy 8 bytes from (HL) to (DE). This transfers the backup date from the user input buffer (470AH-4711H) to the GAT buffer's date field (49D8H-49DFH). The source GAT buffer now contains the complete backup metadata: allocation table, lockout table, Pack ID, and the backup date.

The source GAT buffer at 4900H now has the backup date embedded. Next we write this complete GAT to the destination disk's directory track.

435AH
56F5H

CALL 44B4H CD B4 44

Call the destination disk swap routine. This prompts for the destination disk if needed (single-drive backup), selects the destination drive, and verifies that the 76H marker is present (confirming this is the correct backup-in-progress disk).

435DH
56F8H

LD A,(4709H) 3A 09 47

Fetch the directory track number from 4709H into A. This is typically 11H (17 decimal) for TRSDOS 2.3, but was read from the source disk's GAT earlier to support non-standard disk formats.

4360H
56FBH

LD D,A 57

D = directory track number. This will be the track parameter for the sector write.

4361H
56FCH

LD E,00H 1E 00

E = sector 0. The GAT is always sector 0 of the directory track. DE now contains the full address: directory track, sector 0.

4363H
56FEH

LD BC,4900H 01 00 49

BC = source GAT buffer address. This 256-byte buffer contains the complete GAT with: track allocation (bytes 00-22H), lockout table (bytes 60-82H), Pack ID (bytes CE-D7H), and now the backup date (bytes D8-DFH).

4366H
5701H

LD A,A9H 3E A9

A = FDC Write Sector command with verify (A9H = 1010 1001). Bit 7-5: 101 = Write Sector command. Bit 3: 1 = enable side select compare. Bit 0: 1 = write Data Address Mark FB (normal data).

4368H
5703H

Call the Write Sector subroutine. This writes the source GAT buffer (BC=4900H) to the destination disk at directory track sector 0 (DE). The routine returns Z flag set if successful, NZ if error occurred.

436BH
5706H

LD HL,461FH 21 1F 46

Preload HL with address of error message: "DESTINATION DISK WRITE ERROR!" This is loaded before testing the result so the message is ready if needed.

436EH
5709H

If write failed (NZ), jump to the error handler at 43A3H which displays the message pointed to by HL, prompts "HIT ENTER TO CONTINUE", waits for ENTER, and HALTs.

GAT write successful. Now verify by reading it back.

4371H
570CH

LD BC,4B00H 01 00 4B

BC = general purpose buffer address for read verification. We read the sector back into a different buffer to verify the write succeeded.

4374H
570FH

Call the Disk I/O Executive to read the sector. DE still contains (directory track, sector 0) from the write operation. This reads the GAT we just wrote back into buffer 4B00H.

4377H
5712H

LD HL,4601H 21 01 46

Preload HL with address of error message: "DESTINATION DISK READ ERROR!"

437AH
5715H

If read-back failed (NZ), jump to error handler. The GAT write cannot be verified, so the backup may be corrupt.

GAT verified. Now we must restore the BOOT sector (Track 0, Sector 0). During the copy phase, we wrote 76H to byte 0 as a "backup in progress" marker and saved the original value at 4390H.

437DH
5718H

LD BC,4B00H 01 00 4B

BC = buffer address for reading the BOOT sector.

4380H
571BH

LD DE,0000H 11 00 00

DE = Track 0, Sector 0. This is the BOOT sector (BOOT/SYS) - the first sector on the disk that the ROM loads during boot.

4383H
571EH

Read the BOOT sector into buffer 4B00H. This sector currently has 76H at byte 0 (the backup-in-progress marker we wrote earlier).

4386H
5721H

LD HL,4601H 21 01 46

Preload error message: "DESTINATION DISK READ ERROR!"

4389H
5724H

If read failed, jump to error handler. We cannot restore the BOOT sector.

BOOT sector read into buffer at 4B00H. Byte 0 of the buffer contains 76H. Now restore the original byte 0 value that was saved during the copy phase.

438CH
5727H

LD BC,4B00H 01 00 4B

BC points to byte 0 of the buffer (the first byte of the BOOT sector).

438FH
572AH

LD A,00H 3E 00

The operand at 4390H was modified during the copy phase (at 42E3H-42EFH). It contains the original byte 0 from the source disk's BOOT sector. The 00H shown here is just the initial value; at runtime it holds the actual original byte.

4391H
572CH

LD (BC),A 02

Store the original byte 0 value into the buffer, overwriting the 76H backup-in-progress marker. The buffer now contains the correct BOOT sector data.

4392H
572DH

LD A,A8H 3E A8

A = FDC Write Sector command (A8H = 1010 1000). Similar to A9H but with Data Address Mark selection = 0.

4394H
572FH

Call Write Sector subroutine. Write the corrected BOOT sector (BC=4B00H) back to Track 0, Sector 0 (DE=0000H). The 76H marker is now replaced with the original byte.

4397H
5732H

LD HL,461FH 21 1F 46

Preload error message: "DESTINATION DISK WRITE ERROR!"

439AH
5735H

If write failed, jump to error handler. The destination disk will still have the 76H marker and will be recognized as an incomplete backup.

Backup is now complete! The destination disk has: all data tracks copied, the GAT with backup date, and the BOOT sector restored. Now display success message and terminate.

439DH
5738H

CALL 441AH CD 1A 44

Send RESTORE command to the FDC to position the head at Track 0, and wait for the controller to become ready. This is a clean-up step before displaying the completion message.

43A0H
573BH

LD HL,46DEH 21 DE 46

Point HL to the success message: 0AH + "BACKUP COMPLETE" + 0DH (with line feed prefix and carriage return suffix).

43A3H - Message Display, ENTER Prompt, and HALT (Also Error Handler Entry)

This routine serves dual purpose: (1) Error handler entry point - jumped to after loading error message into HL, (2) Normal completion - falls through from 43A0H with "BACKUP COMPLETE" message. Either way, it displays the message in HL, prompts for ENTER, waits for keypress, then HALTs.

43A3H
573EH

Call the message display routine. Displays the null-terminated message pointed to by HL. For success: "BACKUP COMPLETE". For errors: the error message loaded before jumping here. HL is preserved.

43A6H
5741H

LD HL,46EFH 21 EF 46

Point HL to prompt message: 0AH + 0AH + "HIT 'ENTER' TO CONTINUE" + 0DH. The double 0AH provides blank line separation.

43A9H
5744H

Display the "HIT ENTER TO CONTINUE" prompt.

Now poll the keyboard waiting for the ENTER key. The TRS-80 keyboard is memory-mapped with each row at a specific address. Row 7 at 3840H contains ENTER (bit 0), CLEAR, BREAK, and arrow keys.

43ACH
5747H

LD A,(3840H) 3A 40 38

Read keyboard row 7 (address 3840H). Bit 0 = ENTER key (1 when pressed, 0 when not pressed). Other bits: Bit 1=CLEAR, Bit 2=BREAK, Bit 3=UP, Bit 4=DOWN, Bit 5=LEFT, Bit 6=RIGHT, Bit 7=SPACE.

43AFH
574AH

RRCA 0F

Rotate A right: Bit 0 (ENTER) shifts into the Carry flag, while Bit 7 receives Carry. After this: Carry=1 if ENTER pressed, Carry=0 if not pressed.

43B0H
574BH

JR NC,43ACH 30 FA

If No Carry (ENTER not pressed), jump back to 43ACH to continue polling. This is a tight loop: read keyboard, test ENTER, repeat until pressed. Offset FAH = -6 bytes, target = 43B0H + 2 - 6 = 43ACH.

43B2H
574DH

HALT 76

Stop CPU execution. The backup operation is complete (or terminated due to error). The system must be reset to continue. Note: The HALT opcode is 76H, the same value used as the "backup in progress" marker!

43B3H - Write Sector Subroutine (Entry Point)

Low-level sector write routine. Called from multiple places in the backup program to write sectors to disk.
A = FDC write command byte, BC = buffer address (256 bytes), DE = track (D) and sector (E).
Z flag set if successful, NZ if error. HL = buffer address (from BC).

43B3H
574EH

LD (43D4H),A 32 D4 43

Store the FDC command byte at address 43D4H. This location is the operand of a later instruction that will send the command to the FDC. Different callers use different command bytes (A8H, A9H) for various write modes.

43B6H
5751H

PUSH BC C5

Save the buffer address (BC) on the stack. Will be retrieved into HL after the write attempt.

43B7H
5752H

CALL 43BEH CD BE 43

Call the FDC write sequence at 43BEH. This performs the actual disk write: programs FDC registers, issues seek command, transfers data. Returns Z flag indicating success/failure.

43BAH
5755H

POP HL E1

Retrieve the buffer address from stack into HL. (Originally in BC, now in HL for the caller's convenience or for retry logic.)

43BBH
5756H

RET Z C8

If Z flag is set (write succeeded), return to caller. HL contains the buffer address. The caller can continue with verification or the next operation.

43BCH - Write Sector Retry / FDC Write Sequence

If the write at 43B7H failed (returned NZ), execution falls through here. This copies the buffer address from HL back to BC and falls into 43BEH to retry the write operation.

43BCH
5757H

LD B,H 44

Copy H to B (high byte of buffer address).

43BDH
5758H

LD C,L 4D

Copy L to C (low byte of buffer address). BC now equals HL (buffer address restored to BC for retry). Falls through to 43BEH to attempt the write again.

FDC Write Sequence begins here. Programs the FDC registers, seeks to the target track, and prepares for data transfer.

43BEH
5759H

LD (37EEH),DE ED 53 EE 37

Store DE to FDC sector register (37EEH). Note: This 16-bit store writes E to 37EEH (sector register) and D to 37EFH (data register). The data register write is harmless; the important thing is getting E (sector number) into the sector register.

43C2H
575DH

PUSH DE D5

Save track/sector values on stack for later use.

43C3H
575EH

CALL 4527H CD 27 45

Call drive select and wait routine: (1) Poll FDC status, (2) Fetch drive code from 44E8H, (3) Write to drive select latch, (4) If drive not ready, wait up to 1 second, (5) Return when ready.

43C6H
5761H

LD HL,37ECH 21 EC 37

HL = FDC command/status register address (37ECH). This register is used to send commands and read status from the WD1771 controller.

43C9H
5764H

LD (HL),1BH 36 1B

Send SEEK command (1BH = 0001 1011) to FDC. Bits 7-4: 0001 = Seek command. Bit 3 (h): 1 = enable head load delay. Bit 2 (V): 0 = no track verify. Bits 1-0 (r): 11 = 15ms stepping rate (slowest, most reliable). The FDC will seek to the track number in its track register.

Bit 7	Bit 6	Bit 5	Bit 4	Bit 3	Bit 2	Bit 1	Bit 0	Function Description
0	0	0	1	h	V	r1	r0
0	0	0	1	1	0	1	1	Command=Seek (1BH) Bit 7-4: Command Code (0001 = Seek) h=1: Enable Head Load and 15ms settle delay V=0: No destination track verification r1,r0=11: 15ms stepping motor rate (slowest)

After issuing the SEEK command, we must wait for the FDC to complete the operation. The delay sequence below allows the command to process before we check status.

43CBH
5766H

PUSH AF F5

Delay instruction 1: Push AF to waste time (11 T-states).

43CCH
5767H

POP AF F1

Delay instruction 2: Pop AF back (10 T-states).

43CDH
5768H

PUSH AF F5

Delay instruction 3: Push AF again (11 T-states).

43CEH
5769H

POP AF F1

Delay instruction 4: Pop AF back (10 T-states). Total delay: ~42 T-states ≈ 24 microseconds at 1.77 MHz. This gives the FDC time to begin processing the seek.

Now poll the FDC status register waiting for the BUSY bit (bit 0) to clear, indicating the seek is complete.

43CFH
576AH

LD A,(HL) 7E

Read FDC status register (HL = 37ECH). Bit 0 = BUSY flag (1 = controller busy, 0 = ready).

43D0H
576BH

RRCA 0F

Rotate A right: Bit 0 (BUSY) moves to Carry flag. Carry=1 means FDC is still busy.

43D1H
576CH

JR C,43CFH 38 FC

If Carry set (FDC busy), loop back to 43CFH and keep polling. Offset FCH = -4 bytes, target = 43D1H + 2 - 4 = 43CFH. This tight loop waits until the seek completes.

Seek complete. Now clear any pending FDC state and prepare for the data transfer phase.

43D3H
576EH

LD (HL),00H 36 00

Write 00H to FDC command register. This is a "Force Interrupt" command (Type IV) that terminates any pending command and resets the FDC's internal state machine. Ensures clean state before write operation.

43D5H
5770H

LD DE,37EFH 11 EF 37

DE = FDC data register address (37EFH). This will be the destination for the data transfer loop that follows. Data bytes from the buffer will be written here one at a time.

Another delay sequence to let the Force Interrupt command complete.

43D8H
5773H

PUSH BC C5

Delay instruction 1: Push BC (11 T-states). Also preserves BC (buffer address) on stack.

43D9H
5774H

POP BC C1

Delay instruction 2: Pop BC back (10 T-states).

43DAH
5775H

PUSH BC C5

Delay instruction 3: Push BC again (11 T-states).

43DBH
5776H

POP BC C1

Delay instruction 4: Pop BC back (10 T-states). BC still contains the buffer address.

Now entering the critical data transfer section. Interrupts must be disabled to ensure we don't miss the FDC's DRQ (Data Request) signals.

43DCH
5777H

DI F3

Disable interrupts. Critical: The write data transfer is timing-sensitive. Missing even one DRQ signal will cause data corruption. Interrupts must stay disabled until all 256 bytes are transferred.

43DDH
5778H

JR 43E2H 18 03

Jump forward to 43E2H to enter the data transfer loop. Offset 03H → target = 43DDH + 2 + 3 = 43E2H. The transfer loop sends the write command and pumps 256 bytes to the FDC data register.

43DFH - Write Sector Data Transfer Loop

This is the critical tight loop that transfers 256 bytes from RAM to the FDC during a sector write. HL=37ECH (status), BC=buffer, DE=37EFH (data reg). Interrupts are disabled. The loop polls DRQ (bit 1) to send each byte, and checks BUSY (bit 0) to detect completion.

43DFH
577AH

RRCA 0F

Check for BUSY by rotatin FDC status right, moving Bit 0 (BUSY flag) into Carry. If BUSY=1, the FDC is still processing and we should keep polling. If BUSY=0, the transfer is complete.

43E0H
577BH

JR NC,43EDH 30 0B

If No Carry (BUSY=0), the FDC has finished the sector write. Exit the routine via a jump to 43EDH to read final status and check for errors.

43E2H
577DH

LD A,(HL) 7E

Main entry point (from 43DDH and 43EAH). Read FDC status register (HL=37ECH) into A to check if controller is ready for next data byte.

43E3H
577EH

BIT 1,A CB 4F

est Bit 1 (Data Request). DRQ=1 means FDC is ready to accept a byte. DRQ=0 means either still processing or transfer complete.

43E5H
5780H

JR Z,43DFH 28 F8

We need to wait for a DRQ. If Z flag (DRQ=0), FDC not ready. Jump to 43DFH to check BUSY flag. Loop: check DRQ → check BUSY → repeat until either DRQ set or BUSY clear.

43E7H
5782H

LD A,(BC) 0A

If we are here we have the DRQ, so we need to get the data byte. Fetch the next byte from the RAM buffer pointed to by BC.

43E8H
5783H

LD (DE),A 12

Write the data byte to the FDC data register (DE=37EFH). The byte will be written to disk and DRQ will clear.

43E9H
5784H

INC BC 03

Point BC to the next byte in the buffer. After 256 iterations, one full sector has been transferred.

43EAH
5785H

JP 43E2H C3 E2 43

Jump back to poll FDC status and transfer next byte. Uses JP (10 T-states) for predictable timing in this critical loop.

43EDH - Write Completion and Error Check

The write data transfer loop has completed (FDC BUSY=0). Now check the final status for any errors. Returns Z=success, NZ=error.

43EDH
5788H

LD A,(HL) 7E

Read the FDC status register (HL=37ECH). This contains the result of the write operation including any error flags.

43EEH
5789H

AND 7CH E6 7C

AND with 0111 1100 to isolate write error flags: Bit 6 (Write Protect), Bit 5 (Write Fault), Bit 4 (Record Not Found), Bit 3 (CRC Error), Bit 2 (Lost Data). Result=0 means success.

43F0H
578BH

POP DE D1

Restore original track (D) and sector (E) values from the stack (pushed at 454BH before write).

43F1H
578CH

RET Z C8

If Z flag set (no error bits), the write succeeded. Return to caller with Z flag indicating success.

43F2H - Write Error Recovery

Error detected during write. Issue Force Interrupt to reset FDC, then return with NZ flag.

43F2H
578DH

LD (HL),D0H 36 D0

A write error occurred! Issue Force Interrupt command (D0H = 1101 0000) to the FDC to immediately terminate any pending operation and reset the controller state.

43F4H
578FH

RET C9

Return to caller with NZ flag set (from AND result) indicating write failure. Error bits remain in A for caller to examine.

43F5H - Format/Verify Error Handler - Mark Track as Bad

Called when a sector write or verify fails during formatting. Displays the error message, marks the entire track as bad in the destination lockout table (4A00H+track), and continues to the next track.

43F5H
5790H

PUSH DE D5

Preserve DE (D=track number) before calling display routine. Need track number to mark it as bad.

43F6H
5791H

Display the error message pointed to by HL (e.g., "NOT FOUND! TRACK LOCKED OUT!" or "CRC ERROR! TRACK LOCKED OUT!"). HL is preserved.

43F9H
5794H

POP DE D1

Get track number back into D. Need this to calculate lockout table offset.

43FAH
5795H

LD L,D 6A

We need to build the address. Copy track number to L. Destination lockout table is at 4A00H, so address = 4A00H + track.

43FBH
5796H

LD H,4AH 26 4A

Set H=4AH. HL now = 4A00H + track = address in destination lockout table buffer.

43FDH
5798H

LD (HL),FFH 36 FF

Mark the track bad by writing FFH to the lockout table. FFH = all 5 granules (10 sectors) locked out. This track will be skipped during backup.

43FFH
579AH

JP 553CH C3 3C 55

Jump to 553CH to increment track counter and continue format verification loop with next track.

4402H - SUBROUTINE to Set Register C to the ASCII Number corresponding to Register A (TENS Digit in ASCII to B).

4402H
579DH

LD C,30H 0E 30

Let Register C equal 30H (ASCII: 0).

4404H
579FH

SUB 0AH D6 0A

SUBtract the value 0AH (Decimal: 10) from Register A. The Carry Flag will be affected.

4406H
57A1H

JR C,57A6H 38 03

If the C FLAG (Carry) has been set, then the subtraction of 10 from A went negative, so JUMP to 57A6H to process the ONES digit into B.

4408H
57A3H

INC C 0C

INCrement the value stored in Register C by 1 since the subtraction didn't go negative.

4409H
57A4H

JR 579FH 18 F9

JUMP to 579FH to keep subtracting 10 and bumping Register C.

440BH - SUBROUTINE to Set Register B to the ASCII Number corresponding to Register A (Ones Digit in ASCII to B).

440BH
57A6H

ADD 3AH C6 3A

At this point A is negative, so LET Register A = Register A + 3AH to turn Register A into the ASCII Equivalent of the Ones Digit.

440DH
57A8H

LD B,A 47

Copy the contents of Register A into Register B.

440EH
57A9H

RET C9

RETurn to the caller.

440FH - Part of the FORMATTING routine. Set C = FFH and then do 'put the value held in Register C into (HL) and then BUMP HL by 1' Register B times

440FH
57AAH

LD C,FFH 0E FF

Let Register C equal FFH (Decimal: 255).

4411H
57ACH

CALL 4417H CD 17 44

GOSUB to 4417H to store the value held in Register C into (HL) and then BUMP HL by 1

4414H
57AFH

DJNZ 57ACH 10 FB

LOOP back to 57ACH, 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.

4416H
57B1H

RET C9

RETurn to the caller.

4417H - Part of the FORMATTING routine. Put C into (HL) and then BUMP HL by 1

4417H
57B2H

LD (HL),C 71

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

4418H
57B3H

INC HL 23

INCrement the value stored in Register Pair HL by 1.

4419H
57B4H

RET C9

RETurn to the caller.

441AH - Subroutine to send a RESTORE/LOAD HEAD to the Floppy Drive and then keep checking the floppy drive controller status until the drive is not BUSY

441AH
57B5H

LD A,0BH 3E 0B

Let Register A equal 0BH (Binary:0000 1011) to represent RESTORE (0000), Load Head at the Beginning (1), No Verify (0), 30 ms stepping rate (11).

441CH
57B7H

LD (37ECH),A 32 EC 37

Send the command held in Register A to the FDC (37ECH is the Floppy Disk Controller's Command Register).

Bit 7	Bit 6	Bit 5	Bit 4	Bit 3	Bit 2	Bit 1	Bit 0		Function Description
0	0	0	0	h	V	r1	r0		Command=Restore Bit 7-4: Command Code (0000) h: 1=Enable Head Load/Settle, 0=No delay V: 1=Verify Track 0 ID, 0=No verification r1, r0: Stepping Motor Rate (00=3ms, 01=6ms, 10=10ms, 11=15ms)

Standard code to delay to compensate for the slow FDC to make sure it had time to set the command.

441FH
57BAH

PUSH AF F5

Save the contents of Register Pair AF to the top of the stack.

4420H
57BBH

POP AF F1

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

4421H
57BCH

PUSH AF F5

Save the contents of Register Pair AF to the top of the stack.

4422H
57BDH

POP AF F1

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

Standard code to delay to compensate for the slow FDC to make sure it had time to set the command.

4423H
57BEH

LD A,(37ECH) 3A EC 37

Fetch the FDC's status (which is mirrored to 37ECH) and store it into Register A.

7	6	5	4	3	2	1	0
Not Ready	Protected	Head Loaded	Seek Error	CRC Error	Track 00	Index	Busy

4426H
57C1H

RRCA 0F

Test Bit 0 (i.e., "BUSY") by first rotating the bits in A right (i.e., higher bit moves lower), with Bit 0 being copied to both the CARRY FLAG and Bit 7.

4427H
57C2H

JR C,57BEH 38 FA

If the C FLAG (Carry), which used to be Bit 0, has been set, then the drive is BUSY, so LOOP BACK to 57BEH to send the command again.

4429H
57C4H

RET C9

RETurn to the caller.

442AH - Disk Insertion Prompt. Turn on the high bit in Register A, prompt the user with the message stored in HL, and return once the user hits ENTER.

442AH
57C5H

LD A,nnH 3E 00

Let Register A equal nnH. This is where the SOURCE DRIVE has been saved in the format of 01, 02, 04, or 08 (which are drive select codes for the drive select latch).

442CH
57C7H

OR 80H F6 80

The value in A is logically OR'd with 80H (1000 0000 binary). This will set up an operation code (like clearing a specific directory bit or activating an extended function) before calling a system handler.

442EH
57C9H

PUSH HL E5

Save the contents of Register Pair HL to the top of the stack so that HL can be used in the next instruction to display a message.

442FH
57CAH

LD HL,4664H 21 64 46

Let Register Pair HL equal 4664H to point to the message 1D + 'INSERT SOURCE DISK (ENTER)' + 1D + 03H.

4432H
57CDH

CALL 44E7H CD E7 44

GOSUB to 44E7H to displays the message pointed to by HL and wait for the user to press the ENTER key before returning.

4435H
57D0H

POP HL E1

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

4436H
57D1H

RET C9

RETurn to the caller.

4437H - SECTION TITLE. CALLED from 4432H above with HL pointing to the message 1D + 'INSERT SOURCE DISK (ENTER)' + 1D + 03H

4437H
57D2H

LD A,(44E8H) 3A E8 44

Fetch the value held in memory location 44E8H, which is the nn in a CP nn opcode in the 44E7H routine, and store it into Register A. This was a copy of the Floppy Disk Controller status register.

443AH
57D5H

BIT 7,A CB 7F

443CH
57D7H

JR NZ,57C5H 20 EC

If the NZ FLAG (Not Zero) has been set, JUMP to 57C5H which is the DISKETTE NOT READY routine (the error handling branch for the 4437H routine) ending ina message and a HALT.

443EH
57D9H

CALL 442AH CD 2A 44

If the drive IS READY, execution proceeds to call the subroutine at 442AH (a disk I/O preparation/setup routine).

4441H
57DCH

LD A,(44EFH) 3A EF 44

Load the Accumulator (A) with a status byte stored at memory location 44EFH. This value is likely the return code or status from the 442AH call

4444H
57DFH

OR A B7

Since a LD command does not set any FLAGS, Set FLAGS based on the contents of Register A (and, in all events, clear the CARRY FLAG).

4445H
57E0H

RET NZ C0

. If the status byte loaded from 44EFH was NZ, then we had an error from the 442AH routine, so RETurn to the calling routine.

If we are here, then we had success on the test, and will move forward to read a block of data from a disk and performing a disk change check

4446H
57E1H

CALL 441AH CD 1A 44

GOSUB to 441AH which issues the FDC RESTORE command (move head to Track 0) and polls the FDC status until the drive is not BUSY. This initializes the disk head position.

4449H
57E4H

PUSH BC C5

Save the contents of Register Pair BC to the top of the stack.

444AH
57E5H

PUSH DE D5

Save the contents of Register Pair DE to the top of the stack.

444BH
57E6H

PUSH HL E5

Save the contents of Register Pair HL to the top of the stack.

444CH
57E7H

LD BC,4B00H 01 00 4B

Sets BC (the Buffer Pointer) to 4B00H. This is the destination memory address where the disk data will be read into.

444FH
57EAH

LD DE,0000H 11 00 00

Sets DE to 0000H. This is the Track/Sector address to be read.

4452H
57EDH

4455H
57F0H

LD HL,45E8H 21 E8 45

Let Register Pair HL equal 45E8H to point to the message 0A + 'SOURCE DISK READ ERROR!' + 0DH.

4458H
57F3H

If the NZ (Not Zero) flag is set (signaling an error from 453CH), the routine jumps to 43A3H to display the error and halt the system.

445BH
57F6H

POP HL E1

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

445CH
57F7H

POP DE D1

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

445DH
57F8H

POP BC C1

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

445EH
57F9H

LD A,(4B00H) 3A 00 4B

Reads the first byte of the buffer into A. Since this is being tested for 0 or NZ, it is likely that this byte (4B00H) is a flag or counter related to the disk.

4461H
57FCH

OR A B7

Since a LD command does not set any FLAGS, Set FLAGS based on the contents of Register A (and, in all events, clear the CARRY FLAG).

4462H
57FDH

JR NZ,5831H 20 32

If the byte at 4B00H is not zero, it jumps to 5831H (4496H's XOR A) which clears a flag and redirects flow to the disk-present prompt loop (57D2H).

This block is skipped if the jump at 4462H is taken. If execution continues here, it performs a second read operation into the same buffer, possibly to read a different initial sector or as a retry mechanism, and includes its own error check.

4464H
57FFH

PUSH BC C5

Save the contents of Register Pair BC to the top of the stack.

4465H
5800H

PUSH DE D5

Save the contents of Register Pair DE to the top of the stack.

4466H
5801H

PUSH HL E5

Save the contents of Register Pair HL to the top of the stack.

4467H
5802H

LD A,(4709H) 3A 09 47

Load Track/Sector (High): Fetches a value from a memory location (4709H) into A. This value is likely the Track number or another high-byte component of the sector address.

446AH
5805H

LD D,A 57

Copies the value (Track/High-byte) into D.

446BH
5806H

LD E,00H 1E 00

Sets the low byte of the address, E, to 00H. DE is now loaded with a new Track/Sector address (e.g., Track X, Sector 0).

446DH
5808H

LD BC,4B00H 01 00 4B

Sets BC (the Buffer Pointer) back to 4B00H. The data will be read into the same buffer.

4470H
580BH

4473H
580EH

LD HL,45E8H 21 E8 45

Let Register Pair HL equal 45E8H to point to the message 0A + 'SOURCE DISK READ ERROR!' + 0DH.

4476H
5811H

If the NZ flag is set (signaling an error from 453CH), the routine jumps to 43A3H to display the error and HALT.

4479H
5814H

LD HL,49CEH 21 CE 49

Sets HL to point to the Stored Reference Disk ID (49CEH).

447CH
5817H

LD DE,4BCEH 11 CE 4B

Sets DE to point to the Disk ID in the new buffer (4B00H + an offset).

447FH
581AH

LD B,0AH 06 0A

Sets B as the counter for 10 bytes to compare.

4481H
581CH

LD A,(DE) 1A

Loads a byte from the new buffer into A..

4482H
581DH

CP (HL) BE

Compares the new byte in A against the reference byte in memory pointed to by HL

4483H
581EH

JR NZ,5828H 20 08

If the bytes are Not Zero (they don't match), it jumps to 5828H (448DH) to display the "WAKE UP!!! THAT'S NOT THE SAME SOURCE DISK!" error and restart the disk prompt process.

4485H
5820H

INC DE 13

INCrement pointer in Pair DE by 1.

4486H
5821H

INC HL 23

INCrement pointer in Register Pair HL by 1.

4487H
5822H

DJNZ 581CH 10 F8

LOOP back to 581CH to keep comparing the 10 bytes, 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.

If we are here, then all 10 bytes matched and we are at a successful exit.

4489H
5824H

POP HL E1

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

448AH
5825H

POP DE D1

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

448BH
5826H

POP BC C1

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

448CH
5827H

RET C9

RETurn to the caller.

448DH - Display "WAKE UP!!!" Error, Pop BC/DE/HL, Clear (44E8H), and JUMP to 4437H to Restart Source Disk Prompt Loop

448DH
5828H

LD HL,46A7H 21 A7 46

Let Register Pair HL equal 46A7H to point to the message 1D + 'WAKE UP!!! THAT'S NOT THE SAME SOURCE DISK!' + 0DH

4490H
582BH

GOSUB to 457BH to display the message pointed to by HL on the screen, and RETurn. HL is not affected..

4493H
582EH

POP HL E1

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

4494H
582FH

POP DE D1

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

4495H
5830H

POP BC C1

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

4496H
5831H

XOR A AF

Set Register A to ZERO and clear all Flags.

4497H
5832H

LD (44E8H),A 32 E8 44

Store a ZERO into memory location 44E8H which is the nn in a CP nn opcode in the 44E7H routine.

449AH
5835H

JR 4437H 18 9B

Jump back to 4437H (original address 57D2H) to restart the "Verify Source Disk" routine from the beginning. Since 44E8H was just cleared to 0 at 4497H, the smart prompt logic at 44E7H will NOT skip the prompt - the user will see "INSERT SOURCE DISK (ENTER)" again. This creates the retry loop: wrong disk → display error → clear flag → prompt again → verify Pack ID → repeat until correct disk inserted.

449CH - Turn on Bit 6 in A, prompt for the Destination Disk and make sure its not write protected, and RETurn

449CH
5837H

LD A,nnH3E 00

Load destination drive select code from self-modified operand at 449DH. Contains 01H, 02H, 04H, or 08H for drives 0-3.

449EH
5839H

OR 40H F6 40

Set bit 6 to mark this as a destination disk operation. This bit distinguishes dest from source in the smart prompt logic (44E7H). A now = drive_code OR 40H.

44A0H
583BH

PUSH HL E5

Save the contents of Register Pair HL to the top of the stack.

44A1H
583CH

LD HL,4683H 21 83 46

Let Register Pair HL equal 4683H to point to the message 1D + 'INSERT DESTINATION DISK (ENTER)' + 1DH + 03H.

44A4H
583FH

CALL 44E7H CD E7 44

GOSUB to 44E7H to displays the message pointed to by HL and wait for the user to press the ENTER key before returning.

44A7H
5842H

LD A,(37ECH) 3A EC 37

Fetch the FDC's status (which is mirrored to 37ECH) and store it into Register A.

7	6	5	4	3	2	1	0
Not Ready	Protected	Head Loaded	Seek Error	CRC Error	Track 00	Index	Busy

44AAH
5845H

LD HL,463EH 21 3E 46

Let Register Pair HL equal 463EH to point to the message 0A + 'DESTINATION DISK IS WRITE PROTECTED!' + 0DH.

44ADH
5848H

BIT 6,A CB 77

44AFH
584AH

If the NZ FLAG (Not Zero) has been set, then the disk is not Write Protected, JUMP to 43A3H to Display the message pointed to by HL and prompt for ENTER; keep polling the keyboard until ENTER, and then HALT.

44B2H
584DH

POP HL E1

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

44B3H
584EH

RET C9

RETurn to the caller.

44B4H - Swap to Destination Disk Routine

Ensures the correct destination disk is inserted. For two-drive backup, just returns. For single-drive backup, reads the GAT and verifies the 76H backup-in-progress marker. If wrong disk, forces re-prompt.

44B4H
584FH

LD A,(44E8H) 3A E8 44

Load the drive code from last prompt. 44E8H is self-modifying storage containing drive code with bit 6 set if destination.

44B7H
5852H

BIT 6,A CB 77

Test bit 6. If set, we already prompted for destination disk and it should still be inserted.

44B9H
5854H

JR NZ,449CH 20 E1

If NZ (bit 6 set), destination already inserted. Jump to 449CH to re-select drive without re-prompting user.

44BBH
5856H

CALL 449CH CD 9C 44

Call destination prompt routine: sets bit 6 flag, displays "INSERT DESTINATION DISK (ENTER)", waits for ENTER, selects drive, checks write protection.

44BEH
5859H

LD A,(44EFH) 3A EF 44

Load drive difference (source - dest). Zero = single-drive backup, non-zero = two different drives.

44C1H
585CH

OR A B7

Set flags based on drive difference. Z = single-drive (same drive for source and dest), NZ = two-drive backup.

44C2H
585DH

RET NZ C0

If NZ (two drives), no GAT verification needed - destination hasn't been swapped. Return immediately.

44C3H - Single-Drive Destination Verification

For single-drive backup only. Reads GAT from destination disk and verifies 76H backup-in-progress marker. If wrong disk inserted, clears prompt flag and loops back to retry.

44C3H
585EH

CALL 441AH CD 1A 44

Issue FDC RESTORE command to move head to Track 0 and wait for ready. Required to read GAT at Track 0, Sector 0.

44C6H
5861H

PUSH BC C5

Preserve caller's buffer pointer on stack.

44C7H
5862H

PUSH DE D5

Preserve caller's track/sector values.

44C8H
5863H

PUSH HL E5

Preserve HL.

44C9H
5864H

LD BC,4B00H 01 00 4B

Point BC to general purpose sector buffer. GAT will be read here.

44CCH
5867H

LD DE,0000H 11 00 00

D=0 (track), E=0 (sector) = GAT sector location.

44CFH
586AH

Call Disk I/O Executive to read Track 0, Sector 0 into buffer at 4B00H. Returns Z=success, NZ=error.

44D2H
586DH

POP HL E1

Restore caller's HL from stack.

44D3H
586EH

POP DE D1

Restore caller's track/sector.

44D4H
586FH

POP BC C1

Restore caller's buffer pointer.

44D5H
5870H

LD HL,4601H 21 01 46

Point HL to "DESTINATION DISK READ ERROR!" message.

44D8H
5873H

If NZ (read failed), jump to error handler to display message and HALT. Unrecoverable.

44DBH
5876H

LD A,(4B00H) 3A 00 4B

Load first byte of GAT from buffer. Should be 76H (backup-in-progress marker).

44DEH
5879H

CP 76H FE 76

Compare with 76H (HALT opcode). We wrote this at initialization to mark backup-in-progress. Protects against accidentally using wrong disk.

44E0H
587BH

RET Z C8

If Z (76H found), this is the correct destination disk. Verification complete - return to caller.

44E1H
587CH

XOR A AF

Wrong disk inserted! Set A=0 to force re-prompting.

44E2H
587DH

LD (44E8H),A 32 E8 44

Store 0 to self-modifying location at 44E8H. This clears the "destination already prompted" state.

44E5H
5880H

JR 44B4H 18 CD

Loop back to 44B4H to restart destination verification. User will be prompted again for destination disk.

44E7H - Smart Disk Swap Prompt Logic

Determines whether to prompt user for disk swap. Uses self-modifying code at 44E8H to remember last prompted drive. If requested drive matches last prompted, skips the prompt entirely.

44E7H
5882H

CP nnH FE 00

Compare requested drive (A) with last prompted drive (nn at 44E8H). Self-modifying: nn is updated each time we prompt. Z flag set if disks match.

44E9H
5884H

JR Z,4527H 28 3C

If Z (correct disk already inserted), skip user prompt. Jump to 4527H to select drive and return. User isn't bothered with unnecessary prompt.

44EBH
5886H

LD (44E8H),A 32 E8 44

Store current drive code to 44E8H (the operand of CP instruction above). Next call will compare against this new value.

44EEH
5889H

LD A,nnH 3E 00

Load drive difference from self-modified operand at 44EFH. Value = (source_drive - dest_drive). Zero = single-drive backup, non-zero = two-drive backup.

44F0H
588BH

OR A B7

Since a LD command does not set any FLAGS, Set FLAGS based on the contents of Register A (and, in all events, clear the CARRY FLAG).

44F1H
588CH

JR NZ,58C2H 20 34

If the NZ FLAG (Not Zero) has been set, JUMP to 58C2H to poll the FDC Status; Select the Drive based on (44E8H), if the Drive wasn't Ready wait 1 Second, and RETURN

44F3H
588EH

PUSH BC C5

Save the contents of Register Pair BC to the top of the stack.

44F4H
588FH

PUSH DE D5

Save the contents of Register Pair DE to the top of the stack.

44F5H
5890H

PUSH HL E5

Save the contents of Register Pair HL to the top of the stack.

44F6H
5891H

LD A,0DH 3E 0D

Let Register A equal CARRIAGE RETURN.

44F8H
5893H

GOSUB to 0033H in the Model I ROM to display the character held in Register A at the current cursor position.

Top of a loop

44FBH
5896H

GOSUB to 457BH to display the message pointed to by HL on the screen, and RETurn. HL is not affected.

44FEH
5899H

LD A,(3840H) 3A 40 38

Poll the keyboard row with the ENTER key (i.e., 3840H) and put the results into Register A. NOTE: 3840H is the value held at Keyboard Row 7 - which is ENTER, CLEAR, BREAK, UP, DOWN, LEFT, RIGHT, SPACE.

4501H
589CH

RRCA 0F

Make it easier to test for ENTER by rotating the bits in A right (i.e., higher bit moves lower), with Bit 0 being copied to both the CARRY FLAG and Bit 7.

4502H
589DH

JR C,5899H 38 FA

If the C FLAG (Carry) has been set, the ENTER was hit, so JUMP to 5899H to poll the keyboard again.

4504H
589FH

LD BC,4000H 01 00 40

Let Register Pair BC equal 4000H / (Decimal: 16384).

4507H
58A2H

LD A,(3840H) 3A 40 38

450AH
58A5H

RRCA 0F

Make it easier to test for ENTER by rotating the bits in A right (i.e., higher bit moves lower), with Bit 0 being copied to both the CARRY FLAG and Bit 7.

450BH
58A6H

JR C,58BAH 38 12

If the C FLAG (Carry) has been set, JUMP to 58BAH to Restore HL, BC, and DE from the stack; Poll the FDC Status; Select the Drive based on (44E8H), if the Drive wasn't Ready wait 1 Second, and RETURN. Since this was a JUMP, that RETurn will return out of this routine

450DH
58A8H

DEC BC 0B

DECrement the value stored in Register Pair BC by 1.

58A9H
450EH
58A9H

LD A,B
OR C78

4510H
58ABH

JR NZ,58A2H 20 F5

If BC isn't ZERO yet, LOOP BACK to 58A2H to poll the keyboard again looking for an ENTER.

4512H
58ADH

LD A,1EH 3E 1E

Let Register A equal 1EH (Decimal: 30).

4514H
58AFH

GOSUB to 0033H in the Model I ROM to display the character held in Register A at the current cursor position.

4517H
58B2H

LD BC,3333H 01 33 33

Let Register Pair BC equal 3333H (Decimal: 13107).

451AH
58B5H

CALL 0060H CD 60 00

GOSUB to 0060H in the Model I ROM Routine to delay for the length of BC * 14.66 microseconds (for a delay of around 1/5 of a second).

451DH
58B8H

JR 5896H 18 DC

LOOP BACK to 5896H to redisplay the message pointed to by HL and wait for an ENTER.

451FH - Restore HL, BC, and DE from the stack; Poll the FDC Status; Select the Drive based on (44E8H), if the Drive wasn't Ready wait 1 Second, and RETURN.

451FH
58BAH

LD A,1EH 3E 1E

Let Register A equal 1EH (Decimal: 30), which will be displayed on screen.

4521H
58BCH

GOSUB to 0033H in the Model I ROM to display the character held in Register A at the current cursor position.

4524H
58BFH

POP HL E1

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

4525H
58C0H

POP DE D1

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

4526H
58C1H

POP BC C1

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

4527H
58C2H

LD A,(37ECH) 3A EC 37

Fetch the FDC's status (which is mirrored to 37ECH) and store it into Register A.

7	6	5	4	3	2	1	0
Not Ready	Protected	Head Loaded	Seek Error	CRC Error	Track 00	Index	Busy

452AH
58C5H

PUSH AF F5

Save the FDC status to the top of the stack so we can use Register A for the next instruction.

452BH
58C6H

LD A,(44E8H) 3A E8 44

Load the current drive select code from self-modifying storage at 44E8H. This contains the drive code (01H, 02H, 04H, or 08H) with bit 6 set if destination disk.

452EH
58C9H

LD (37E1H),A 32 E1 37

Store the value held in Register A into the Disk Drive Select Latch Address of 37E1H. 01H=Drive 0.

4531H
58CCH

POP AF F1

Restore the FDC status back into Register A.

4532H
58CDH

RLCA 07

Rotate the bits of Register left (i.e., lower bit moves higher) one bit position. The contents of bit 7 are copied to the CARRY FLAG and to bit 0, so a NOT READY status (held in Bit 7) will be reflected in the CARRY FLAG.

4533H
58CEH

PUSH BC C5

Put BC back in the stack so we can use it for a moment.

4534H
58CFH

LD BC,0000H 01 00 00

Let Register Pair BC equal 0000H for the longest possible delay.

4537H
58D2H

CALL C,0060H DC 60 00

If the C FLAG (Carry) has been set, then the drive signalled NOT READY, so GOSUB to 0060H in the Model I ROM Routine to delay for the length of BC * 14.66 microseconds (or about 1 second).

453AH
58D5H

POP BC C1

Restore BC back from the stack.

453BH
58D6H

RET C9

RETurn to the caller.

453CH - Disk I/O Executive (Read Sector Routine)

The core disk read function. Entry: BC=buffer, D=track, E=sector, 44E8H=drive code. Exit: Z=success (buffer filled), NZ=error. This routine selects the drive, seeks to the track, issues a READ SECTOR command, and transfers 256 bytes to RAM.

453CH
58D7H

PUSH BC C5

Push buffer address to stack. Will be popped into HL at 4540H after the setup call returns.

453DH
58D8H

CALL 4544H CD 44 45

Call drive select/seek routine at 4544H. Note: 4544H is the NEXT instruction after the return point, so this pushes a return address and continues. When 4544H's CALL 4527H returns, we continue at 4540H.

4540H
58DBH

POP HL E1

Pop buffer address (pushed at 453CH) into HL. Temporary holding - will move to BC after error check.

4541H
58DCH

RET Z C8

If Z flag set from setup, return immediately. Caller sees Z flag (error condition).

4542H
58DDH

LD B,H 44

Copy H to B. Moving buffer address from HL back to BC.

4543H
58DEH

LD C,L 4D

Copy L to C. Now BC = buffer address for the transfer loop.

4544H - Drive Select and Seek Setup

Selects the drive, loads track/sector to FDC registers, and issues SEEK command to position the head.

4544H
58DFH

CALL 4527H CD 27 45

Call drive select routine: reads FDC status, loads drive code from 44E8H, writes to drive select latch (37E1H), waits 1 second if drive not ready.

4547H
58E2H

LD (37EEH),DE ED 53 EE 37

Write track (D) to FDC Track Register and sector (E) to FDC Sector Register. The FDC will seek to this track and read this sector.

454BH
58E6H

PUSH DE D5

Save DE to stack. Will be restored at 4576H after transfer. DE will be reused for data register pointer.

454CH
58E7H

LD HL,37ECH 21 EC 37

Point HL to FDC command/status register (37ECH). Used for all subsequent FDC status reads and command writes.

454FH
58EAH

LD (HL),1BH 36 1B

Write 1BH to FDC: SEEK command (0001) with head load (h=1), no verify (V=0), 15ms step rate (r1r0=11). Head moves to track specified in Track Register.

4551H-4554H
58ECH-58EFH

PUSH AF / POP AF / PUSH AF / POP AF F5 F1 F5 F1

~42 T-states ≈ 24µs at 1.77MHz. Gives FDC time to begin processing SEEK before we poll status.

4555H - Wait for Seek Complete

Polls FDC status waiting for BUSY flag to clear, indicating seek is complete.

4555H
58F0H

LD A,(HL) 7E

Read FDC status register into A. Bit 0 = BUSY (1 = seek in progress).

4556H
58F1H

RRCA 0F

Rotate A right, Bit 0 (BUSY) moves to Carry flag.

4557H
58F2H

JR C,4555H 38 FC

If Carry set (BUSY=1), seek still in progress. Loop back to poll again. Exits when seek completes (BUSY=0).

4559H - Issue Read Command and Prepare Transfer

Seek complete. Issue READ SECTOR command and set up registers for the data transfer loop.

4559H
58F4H

LD (HL),88H 36 88

Write 88H to FDC: READ SECTOR command (100), single record (m=0), IBM format (S=1), no settle delay (E=0). FDC begins locating and reading the sector.

455BH
58F6H

LD DE,37EFH 11 EF 37

Point DE to FDC Data Register (37EFH). Bytes will be read from here via LD A,(DE) in the transfer loop.

455EH-4561H
58F9H-58FCH

PUSH BC / POP BC / PUSH BC / POP BC C5 C1 C5 C1

~42 T-states delay. Gives FDC time to locate sector and prepare first data byte before we enter the transfer loop.

4562H
58FDH

DI F3

Begin critical section. Must not allow interrupts during data transfer or we'll miss DRQ signals, causing Lost Data errors.

4563H
58FEH

JR 4568H 18 03

Jump to 4568H to start polling for DRQ. Skips the BUSY check at 4565H on first entry since we just issued the command and know FDC is busy reading.

4565H - Read Transfer Loop - BUSY Check

Part of the read data transfer loop. Checks if FDC has finished the read operation. Jumped here from 456BH when DRQ was not set.

4565H
5900H

RRCA 0F

Rotate status right, moving Bit 0 (BUSY flag) into Carry. If BUSY=1, FDC still processing. If BUSY=0, read complete.

4566H
5901H

JR NC,4573H 30 0B

If NC (BUSY=0), the 256-byte read is complete. Jump to 4573H to check FDC status for errors and return.

This midsection code is jumped to repeatedly poll the FDC, wait until the Data Request (DRQ) flag is set, transfer a byte from the FDC data register (DE) to the memory buffer (BC), and increment the buffer pointer (BC). It continues indefinitely until the FDC stops asserting the DRQ signal.

4568H
5903H

LD A,(HL) 7E

Read FDC Status. Fetches the FDC Status/Command Register value into A. HL is set to point to this register (37ECH) from the prior routine..

4569H
5904H

BIT 1,A CB 4F

Check for Data Request (DRQ). Tests Bit 1 of the FDC status. This bit is typically the Data Request (DRQ) flag.

456BH
5906H

JR Z,5900H 28 F8

Wait for Data Ready. If the Z flag is set (Bit 1 is 0), the FDC is not ready to transfer data yet, so the routine jumps back to the polling entry point at 5900H (4565H) to re-read the FDC status.

456DH
5908H

LD A,(DE) 1A

Fetch Data. If the Z flag is clear (Bit 1 is 1), the FDC is ready. This instruction reads one byte of data from the FDC Data Register (pointed to by DE which was set to 37EFH in the prior routine) into A.

456EH
5909H

LD (BC),A 02

Store to Buffer. The byte in A is stored into the memory buffer pointed to by BC. BC acts as the destination memory pointer.

456FH
590AH

INC BC 03

Advance Buffer Pointer. Increments BC to point to the next byte location in the memory buffer.

4570H
590BH

JP 4568H C3 68 45

Continue Loop. Jumps back to 4568H to read the FDC status again and continue transferring the next byte of the sector.

4573H - FDC Error Handler Analysis. Check the FDC for an Error, Restore track/sector to DE if not, otherwise Force Interrupt and return.

4573H
590EH

LD A,(HL)

Get the disk controller status and put it into Register A.

7	6	5	4	3	2	1	0
Not Ready	Write Protected	Write Fault	Record Not Found	CRC Error	Lost Data	Data Request	Busy

4574H
590FH

AND 5CH
AND 01011100

AND the controller status against 5CH (Binary: 01011100) to isolate the error flags. This represents Write Protected (Bit 6), Record Not Found (Bit 4), CRC Error (Bit 3), and Lost Data (Bit 2)

4576H
5911H

POP DE

Restore the track/sector address from the stack into Register Pair DE.

4577H
5912H

RET Z

If there was no error, then RETURN.

4578H
5913H

LD (HL),0D0H

If there WAS an error, then put a 0D0H (Decimal: 1101 0000) into the FDC command/status register to stop the controller with a force interrupt.

Bit Number								Function
7	6	5	4	3	2	1	0
1	1	0	1	I3	I2	I1	I0	Command=Force Interrupt Bit 7-4: Command Code (1101) I3: 1=Interrupt Immediately I2: 1=Interrupt on the next Index Pulse I1: 1=Interrupt the next time Ready goes to Not Ready I0: 1=Interrupt the next time Not Ready goes to Ready

457AH
5915H

RET

RETURN.

457BH - Display the message pointed to by HL on the screen.

457BH
5916H

PUSH HL E5

Save the pointer to the message being displayed to the stack.

457CH
5917H

LD A,(HL) 7E

Fetch the character of the message being displayed being pointed to at the moment into Register A.

457DH
5918H

CP 03H FE 03

Compare the character to display to a 03H, which is a standard delimeter to mean END OF MESSAGE. If it is 03H, the Z FLAG will be set

457FH
591AH

JR Z,5924H 28 08

If the character was an 03H, and the Z FLAG (Zero) had been set, JUMP to 5924H commence exiting this routine.

4581H
591CH