Model 4 ROM Explained - XDROM - Diagnostic Part

GOSUB to 345EH display the messages pointed to by HL at the screen location of DE.

31D5H - MEMORY - Start of the memory test.

31D5

LD HL,3C16H

Let HL = 3C16H (an address left of middle on the first line of the screen).

31D8

LD BC,3C2FH

Let BC = 3C2FH (an address about 3/4 of the way to the right on the first line of the screen).

31DB

LD A,(BC)

Fetch the character from the screen pointed to by Register Pair BC and put it into Register A.

31DC

INC A

LET A = A + 1 to increment the ASCII value of that character by 1.

31DD

LD (BC),A

Replace the character on screen with the character value + 1.

31DE

GOSUB to 330CH, which does some SERIOUS byte manipulation on the character held in Register A to turn it into an ASCII HEX Digit and display it at (HL), and then BUMP HL.

31E1

LD IX,3C30H

LET IX = 3C30H (a screen location on the top line of the screen).

31E5

LD HL,8000H

LET HL = 8000H.

31E8

CALL 32B7H

GOSUB to 32B7H to write memory locations 8000H-FFFFH out and then read them back.

31EB

LD A,(IX+00H)

Fetch the value stored at IX+00H and put it into Register A.

31EE

OR A

Since a LD command does not affect the flags, an OR A is necessary to the flags based on the contents Register A.

31EF

JR NZ,3215H

If the value held at 3C30H is NOT 00H then JUMP to 3215H.

If we are here, then we are going to move the RAM test to 8100H and JUMP to it there.

31F1

LD IX,FC30H

Let IX = FC30H.

31F5

LD DE,8100H

Let DE = 8100H.

31F8

LD BC,0059H

Let BC = 59H (or 89 bytes).

31FB

LD HL,32B3H

Let HL = 32B3H.

31FE

LDIR

Move the byte from (HL) to (DE), then HL and DE are incremented, and BC is decremented until BC is zero. Interrupts CAN fire during this command.

3200

LD H,B

LET Register H = Register B (which is 0).

3201

LD L,C

LET Register L = Register C (which is 0).

3202

LD SP,FFFEH

Set the STACK POINTER to top of memory.

3205

CALL 8100H

GOSUB to 8100H (the relocated 32B3H routine). Note: Entering the memory test at 32B3 sets the RAM Bank Select at: Lower 32K Ram in Bank 0 / Upper 32K RAM in Bank 2.

3208

LD SP,37FEH

Set the STACK POINTER to 37FEH.

320B

LD IX,3C30H

Let IX = 3C30H.

320F

LD A,(IX+00H)

Fetch the contents of the memory location pointed to by IX+00H into Register A.

3212

OR A

Since a LD command does not affect the flags, an OR A is necessary to the flags based on the contents Register A.

3213

JR Z,31D5H

If the memory location pointed to by IX+00H into Register A was ZERO, JUMP to 31D5H to do another pass.

If the memory location pointed to by IX+00H into Register A was NOT zero then we have a problem, so we need to display the memory test failure messages and addresses.

3215

LD DE,3C80H

Let DE = 3C80H, the first character on the 3rd line of the video screen.

3218

LD HL,3278H

Let HL = 3278H to point to a message.

321B

GOSUB to 345EH display the messages pointed to by HL at the screen location of DE, in this case "Intr Errors= Hard Errors= Last Error @" + 00h.

321E

LD DE,3D00H

Set DE to be the 1st character on the 5th line of the screen.

3221

INC HL

Let HL = HL + 1 (to point to the next error message).

3222

GOSUB to 345EH display the messages pointed to by HL at the screen location of DE, in this case "Failing Rows:",00h.

3225

PUSH IX

Save register pair IX to the top of the STACK.

3227

POP BC

Move the value at the top of the STACK (which is the value stored in Register IX, i.e., 3C30H) to Register Pair BC.

3228

LD HL,3C8CH

Set HL to be the 13th character on the 3rd line of the screen. This would the screen location to put the first byte of the 2 byte number of "Intr Errors".

322B

LD A,(BC)

Load the value stored in the memory location pointed to by Register Pair BC (the first byte of the 2 byte number of "Intr Errors" to report) into Register A.

322C

INC BC

Bump BC to point to the second byte of the 2 byte number of "Intr Errors".

322D

GOSUB to 330CH, which does some SERIOUS byte manipulation on the character held in Register A to turn it into an ASCII HEX Digit and display it at (HL), and then BUMP HL.

3230

LD HL,3C9CH

Set HL to be the 29th character on the 3rd line of the screen. This would the screen location to put the first byte of the 2 byte number of "Hard Errors".

3233

LD A,(BC)

Load the value stored in the memory location pointed to by Register Pair BC (the first byte of the 2 byte number of "Hard Errors" to report) into Register A.

3234

INC BC

Bump BC to point to the screen location to put the second byte of the 2 byte number of "Hard Errors".

3235

GOSUB to 330CH, which does some SERIOUS byte manipulation on the character held in Register A to turn it into an ASCII HEX Digit and display it at (HL), and then BUMP HL.

3238

LD HL,3CADH

Set HL to be the 46th character on the 3rd line of the screen. This would the screen location to put the first byte of the 4 byte number of "Last Error @".

323B

LD A,(BC)

Load the value stored in the memory location pointed to by Register Pair BC into Register A.

323C

INC BC

Bump BC to the screen location to put the second byte of the 4 byte number of "Last Error @".

323D

GOSUB to 330CH, which does some SERIOUS byte manipulation on the character held in Register A to turn it into an ASCII HEX Digit and display it at (HL), and then BUMP HL.

3240

LD A,(BC)

Load the value stored in the memory location pointed to by Register Pair BC into Register A.

3241

INC BC

Bump BC to the screen location to put the third byte of the 4 byte number of "Last Error @".

3242

GOSUB to 330CH, which does some SERIOUS byte manipulation on the character held in Register A to turn it into an ASCII HEX Digit and display it at (HL), and then BUMP HL.

3245

LD HL,32A5H

Let HL = 32A5H to point to the message "Failing Rows:",00h.

3248

LD DE,3D00H

Set DE to be the 1st character on the 5th line of the screen.

324B

GOSUB to 345EH display the messages pointed to by HL at the screen location of DE, in this case "Failing Rows:",00h.

Now we will point to the 15th character on Row 5, right after the "Failing Rows:" message, to populate 8 single number rows (7 to 0). The byte at Register Pair BC will be the binary representation of the rows which failed.

324E

LD A,(BC)

Load the value stored in the memory location pointed to by Register Pair BC into Register A.

324F

LD B,08H

Let B = 08.

Start of a loop of 08 cycles.

3251

RLCA

Rotate the bits in A left (with Bit 7 going into both the CARRY and Bit 0).

3252

JR NC,325CH

If Bit 7 was a 0, meaning that that bit does not represent a FAILED ROW, move to the next cycle of the loop as we are not going to display anything (since it didn't fail).

3254

EX AF,AF'

Preserve the rotated Register A by EXchanging the values of AF and AF'.

3255

LD A,B

Let Register A = Register B (i.e., where we are in the loop of 8).

3256

ADD 2FH

ADD 2FH to Register A. 2F is one character below a 0 so this would turn a number into the ASCII equivalent of that number.

3258

LD (DE),A

Put the ASCII value of B onto the screen in the FAILING ROWS list.

3259

EX AF,AF'

Restore Register A by EXchanging (back) the values of AF and AF'.

325A

INC DE

Bump DE to the next space on the video display (so it doesn't overwrite the just displayed number).

325B

INC DE

Bump DE to the next space on the video display (so as to put a space between the displayed numbers).

325C

DJNZ 3251H

Loop back to 3251H and keep looping until Register B = 0 (i.e., all 8 bits of the byte holding the failing rows have been displayed or skipped).

325E

JP 344FH

JUMP to 344FH to disable the interrupts and HALT.

3261H - MEMORY - Message Storage Area.

3261

MESSAGE

64K Memory Test Pass=+00h

3278

MESSAGE

Intr Errors= Hard Errors= Last Error @ + 00h

32A5

MESSAGE

Failing Rows: + 00h

32B3H - MEMORY - An alternate way of entry to the memory test but with setting Lower 32K Ram in Bank 0 / Upper 32K RAM in Bank 2.

32B3

LD A,20H

Let A = Binary 0010 0000.

32B5

OUT (84H),A

OUTput Register A to Port 84H, which will have the effect of saying: Model III ROMs Enabled, Video/Keyboard are Model III, Video Mode 64x16, Lower 32K Ram in Bank 0 / Upper 32K RAM in Bank 2, and Video Page Select 0.
NOTE: Port 84H handles miscellaneous memory items. Output:

Bits 0-1: Identify the Model of the Computer
Bit 2: Video Display Mode [0=64x16, 1=80x24]
Bit 3: Reverse Video
Bit 4-6: RAM Bank Select
Bit 7: Video Page Select [0=Page 0, 1=Page 1]

32B7H - MEMORY - What the memory test does, at least on the initial passes is to load up memory with 00H-FF, test it, decrement each memory location 1, test it, etc. So, for example, a row of RAM might have 43H-52H, and on the next pass, that row will have 42H-51H, and then 41H-50H.

32B7

PUSH IX

Save register pair IX to the STACK. On FIRST call, this will put 3C30H at the top of the STACK.

32B9

POP BC

Move the value at the top of the STACK (which was IX) to Register Pair BC. On FIRST call, this will put 3C30H into BC.

32BA

LD D,05H

Let D = 05H.

32BC

XOR A

Zero register A and clear the flags.

32BD

LD (BC),A

(Beginning of a Loop). Put the contents of Register A into the memory location pointed to by Register Pair BC. On first call, this will put a 00H into 3C30H.

32BE

INC BC

Bump BC to point to the next location on the screen.

32BF

DEC D

Decrement the value held in Register D by 1 (for a count of 6).

32C0

JR NZ,32BDH

LOOP back to 32BDH to keep loading 6 characters, starting at 3C30H with 00H.

If we are here, the screen locations 3C30H-3C35H have had 00H put into them.

32C2

LD B,FFH

LET Register B = FFH.

32C4H - Write from 8000H and then read it back.

32C4

PUSH BC

(This is the start of a loop, with loopback at 32E3H) Save the contents of Register Pair BC to the top of the STACK.

32C5

PUSH HL

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

32C6

LD D,B

Save Register B into Register D.

32C7

LD BC,8000H

Let Register BC = 8000H.

32CA

PUSH BC

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

32CB

LD (HL),D

(Start of a loop until BC = 0). Put the contents of Register D into the memory buffer set up at 8000H.

32CC

INC D

Let D = D + 1 (on first run, 00H).

32CD

INC HL

Let HL = HL + 1 (on first run, 8001H).

32CE

DEC BC

Decrement the value held in Register Pair BC by 1 (on first run, 7FFFH).

32CF

LD A,B
OR C

Since the Z-80 cannot test BC against Zero, it is a common trick to LD the MSB into A and then OR the LSB. Only if MSB and LSB are both 0 will Register A be 0.

32D1

JR NZ,32CBH

If BC is still not ZERO, JUMP BACK to 32CBH.

If we are here, 8000H and higher (HL) have been filled with numbers and BC (8000H and lower) hit zero.

32D3

POP BC

Move the value at the top of the STACK (which is 8000H) to Register Pair BC.

32D4

POP HL

Move the value at the top of the STACK (which is 8000H) to Register Pair HL.

32D5

PUSH HL

Save the contents of Register Pair HL (which is 8000H) to the STACK.

32D6

LD A,(HL)

(Start of a loop). Load the value stored in the memory location pointed to by Register Pair HL into Register A.

32D7

CP D

Compare Register A with what's in Register D. If Register A equals Register D, the Z FLAG is set; otherwise, the NZ flag will be set.

32D8

JR NZ,32EDH

If there is no match, JUMP to 32EDH.

32DA

DEC BC

Decrement the value held in Register Pair BC by 1 (on first run, 7FFFH).

32DB

INC HL

Let HL = HL + 1 (on first run, 8001H).

32DC

INC D

Let D = D + 1 (on first run, 00H).

32DD

LD A,B
OR C

Since the Z-80 cannot test BC against Zero, it is a common trick to LD the MSB into A and then OR the LSB. Only if MSB and LSB are both 0 will Register A be 0.

32DF

JR NZ,32D6H

If BC is still not ZERO, JUMP BACK to 32D6H.

If we are here, then BC hit 00H, as did HL.

32E1

POP HL

Move the value at the top of the STACK (i.e. 8000H) to Register Pair HL.

32E2

POP BC

Move the value at the top of the STACK to Register Pair BC.

32E3

DJNZ 32C4H

Loop back to 32C4H and keep looping until Register B = 0.

32E5

JR 32E9H

Skip the next 2 instructions and JUMP to 32E9H.

32E7H - MEMORY - Restore HL and BC from the STACK

32E7

POP HL

Move the value at the top of the STACK to Register Pair HL.

32E8

POP BC

Move the value at the top of the STACK to Register Pair BC.

32E9H - Clear A, reset the TRS-80, RETurn (to 31EBH)

32E9

XOR A

Zero register A and clear the flags.

32EA

OUT (84H),A

OUTput Register A to Port 84H, which will have the effect of saying: Model III ROMs Enabled, Video/Keyboard are Model III, Video Mode 64x16, Lower 32K RAM in Bank 0 and Upper 32K RAM in Bank 1, and Video Page Select 0.
NOTE: Port 84H handles miscellaneous memory items. Output:

Bits 0-1: Identify the Model of the Computer
Bit 2: Video Display Mode [0=64x16, 1=80x24]
Bit 3: Reverse Video
Bit 4-6: RAM Bank Select
Bit 7: Video Page Select [0=Page 0, 1=Page 1]

32EC

RET

Return(to 31EBH).

32EDH - MEMORY - This routine is JUMPed to from 32D8H if the 8000H readback failed. It puts gibberish onto the screen based on the value of the failed read.

32ED

XOR D

Bitwise XOR on A with D. .

32EE

OR (IX+04H)

IX should be 3C30H, so IX+04H = 3C34H or the 54th character on line 1 of the video screen. This instruction takes the byte that is already there and, OR's it with the XOR'd A.

32F1

LD (IX+04H),A

Put that manipulated byte back on the screen.

32F4

LD A,(IX+00H)

Put the character appearing on the screen at IX+00H into Register A.

32F7

INC A

LET A = A + 1.

32F8

JR Z,32E7H

If A hits FFH, JUMP to 32E7H to restore all Register Pairs from the STACK, reset the Model III, and RETURN.

32FA

LD (IX+00H),A

Put the value held in Register A onto the screen at screen location IX+00H.

32FD

LD A,(HL)

Fetch the Load the value stored in the memory location pointed to by Register Pair HL into Register A.

32FE

CP D

Compare Register A (the value from the memory location pointed to by HL) with the value held in Register D. If Register A equals the value held in Register D, the Z FLAG is set.

32FF

JR Z,3304H

If the value in Register A equals the value held in Register D, skip the next instruction (i.e., JUMP to 3304H).

3301

INC (IX+01H)

If the value in Register A does NOT equal the value held in Register D, BUMP the value stored on the screen at memory location IX+01H.

3304

LD (IX+00H),L

Put the value held in Register L onto the screen at screen location IX+00H.

3306

INC BC

LET BC = BC + 1.

3307

LD (IX+00H),H

Put the value held in Register H onto the screen at screen location IX+00H.

3309

LD (BC),A

Put the value stored in Register A into the memory location pointed to by Register Pair BC.

330A

JR 32DAH

JUMP BACK to 32DAH.

330CH - SHARED ROUTINE - This routine does 4 RRA's to the character held in Register A, and then JUMPs to 3317H to do some SERIOUS byte manipulation on the character held in Register A, puts the manipulated character into (HL), Bumps HL, and JUMPs to 3317H to do that again before it RETurns.

330C

LD D,A

Save the character held in Register A into Register D.

330D

CALL 3313H

GOSUB to 3313H to do that 4 RRA's, some byte manipulation, and put the manipulated character into (HL), Bump HL, and come back.

3310

LD A,D

Restore the character from Register D back into Register A.

3311

JR 3317H

JUMP to 3317H to do that to the same character but in the next (HL) and then RETurn.

3313H - SHARED ROUTINE - Do 4 RRA commands to move the bits around

3313

RRA

Rotate A right one bit, putting the old BIT 0 into the CARRY FLAG, and putting the old CARRY FLAG into BIT 7, which is the SIGN bit. So if A = 01 before the instruction, then A will be 00 and the C Flag set once run.

3314

RRA

Rotate A right one bit, putting the old BIT 0 into the CARRY FLAG, and putting the old CARRY FLAG into BIT 7, which is the SIGN bit. So If A = 00 before the instruction, then A will be 80 and the C Flag NOT set once run.

3315

RRA

Rotate A right one bit, putting the old BIT 0 into the CARRY FLAG, and putting the old CARRY FLAG into BIT 7, which is the SIGN bit. So If A = 80 before the instruction, then A will be 40 and the C Flag NOT set once run.

3316

RRA

Rotate A right one bit, putting the old BIT 0 into the CARRY FLAG, and putting the old CARRY FLAG into BIT 7, which is the SIGN bit. So If A = 40 before the instruction, then A will be 20 and the C Flag NOT set once run.

3317H - SHARED ROUTINE - Take Register A, do some manipulation to turn a hex value into an ASCII character representing that hex value, and put the manipulated character into (HL), Bump HL, and RETURN

3317

AND 0FH

Mask Register A against 0FH (Binary: 0000 1111) to turn off Bits 4-7. So if A was 20H, then A becomes 00H.

3319

ADD 90H

ADD 90H (Binary: 1001 0000)to Register A. If A was 00H, then A becomes 90H.

331B

DAA

Adjust A for BCD addition and subtraction operations.

331C

ADC 40H

ADD with CARRY 40H. So if A was 90H, A becomes D0H.

331E

DAA

Adjust A for BCD addition and subtraction operations. if A was D0H, it is now 30H, which is a 0.

331F

LD (HL),A

Put the adjusted character into the memory location pointed to by (HL).

3320

INC HL

Bump HL by one.

3321

RET

RETurn to caller.

3322H - DISKETTE - This is the DISK TEST routine entry point.

3322

LD HL,352DH

Set HL to point to 352DH.

3325

GOSUB to 345EH display the messages pointed to by HL at the screen location of DE, in this case "Cylinder Sector Drive:" + 00H.

3328

LD A,(3810H)

Put the character which has been pressed on Keyboard Row 4 ("01234567") into Register A.

332B

AND 0FH

Mask Register A against 0FH (Decimal: 0000 1111) to keep only bits 0-3 (meaning "0123").

332D

JR Z,3328H

If the Z flag is set, then a key other than 0-3 was pressed, so LOOP back to 3328H to scan the keyboard again.

332F

LD B,A

Store the "0-3" character which was pressed into Register B.

3330

XOR A

Zero register A and clear the flags.

3331

RR B

The RR B command rotates the contents of Register B rotated right one bit position, with the contents of bit 0 are copied to the carry flag and the previous contents of the carry flag are copied to bit 7. So ...

0 sets masked B as "0000 0001". After the RR B, B is "0000 0000" and CARRY is SET
1 sets masked B as "0000 0010". After the RR B, B is "0000 0001" and CARRY is 0
2 sets masked B as "0000 0011". After the RR B, B is "0000 0010" and CARRY is 0
3 sets masked B as "0000 0100". After the RR B, B is "0000 0100" and CARRY is 0

3333

JR C,3338H

If the CARRY flag is set (meaning a 0 was pressed), skip the next couple of instructions (which keeps bumping Register A until it matches the drive number) and JUMP to 3338H since Register A is already 0.

3335

INC A

LET A = A + 1.

3336

JR 3331H

LOOP back to JUMP to 3331H so that Register A will ultimately match the drive number.

3338H - DISKETTE - Continue with the DISKETTE Test. On Entry Register A is holding a number 0-3 matching the disk drive number selected for testing.

3338

LD (42FCH),A

42FCH appears to be a storage area for the disk drive selected. Put the drive number selected (held in Register A) into the memory location 42FCH.

333B

ADD 30H

ADDing 30H to a number turns it into its ASCII equivalent, turning, lets say, the INTEGER 0 into the STRING 0.

333D

LD (DE),A

Put the ASCII equivalent of the drive number selected onto the screen immediately following the "Drive:" message.

333E

CALL 3453H

GOSUB to 3453H to reset the FDC, set track number and density, get the FDC status, and error out if it is CRC ERROR, SEEK ERROR, or NOT READY. Otherwise, clear all flags and Register A, and RETURN.

3341

JP NZ,3439H

If NZ is set, meaning there is a CRC ERROR, SEEK ERROR, or NOT READY, then JUMP to 3439H to display the error message on screen and then HALT.

3344

LD IY,3440H

Let Register IY = 3440H.

3348

LD C,00H

Let Register C = 00H.

334A

LD B,00H

Let Register B = 00H.

334C

GOSUB to 3371H to update the screen with the Cylinder and Sector being tested. B points to the cylinder and C points to one HIGHER than the Sector.

334F

INC B

Let B = B + 1 to advance the track number.

3350

LD A,B

Let A = Register B to point to the track number.

3351

CP 28H

Compare Register A with 28H (Decimal: 40 = 40 Tracks). If Register A equals track 40, the Z FLAG is set; otherwise NZ is set.

3353

JR NZ,334CH

If we are not at track 40, JUMP to 334CH to test the next track.

If we are here, we have hit track 40.

32E8

POP BC

Move the value at the top of the STACK to Register Pair BC.

3355

DEC B

Decrement the value held in Register B (the track number being tested) by 1 (to track 39).

3356H - This routine will do the track test in reverse.

3356

GOSUB to 3371H to update the screen with the Cylinder and Sector being tested. B points to the cylinder and C points to one HIGHER than the Sector.

3359

DEC B

Decrement the value held in Register B (the track number being tested) by 1.

335A

JR NZ,3356H

Loop back 2 instructions if we have not hit track 0 yet.

If we are here, we have hit track 0.

335C

LD HL,0027H

Let HL = 0027H. L is then Decimal 39 (for track 39).

Start of a loop between here and 336DH to do even more testing, this time with wild swings ... 00 then 39 then 01 then 38 ...

335F

LD B,H

Let B = H. On the first loop B will = 00H.

3360

PUSH HL

Save the contents of Register Pair HL to the STACK.

3361

GOSUB to 3371H to update the screen with the Cylinder and Sector being tested. B points to the cylinder and C points to one HIGHER than the Sector.

3364

POP HL

Restore the HL back from top of the STACK to Register Pair HL.

3365

LD B,L

Let B = L. On the first loop B will = 27H (Track 39).

3366

PUSH HL

Save the contents of Register Pair HL to the STACK.

3367

GOSUB to 3371H to update the screen with the Cylinder and Sector being tested. B points to the cylinder and C points to one HIGHER than the Sector.

336A

POP HL

Move the value at the top of the STACK to Register Pair HL.

336B

INC H

Now that 2 tests have been run on the value of HL, one to track H and one to track L, we now increase track H (the low end).

336C

DEC L

and reduce track L (the high end).

336D

JR NZ,335FH

Loop back to test track H and track L until Track L is zero.

336F

JR 334AH

We're done with this phase so JUMP to 334AH for more.

3371H - DISKETTE - Display the 2 bytes comprising BC onto the screen at location IY. In specific, it loads the screen responses for CYLINDER and SECTOR, overwriting them as they update.

3371

PUSH IY

Store the contents of Register IY to the top of the STACK. IY should be holding a screen position value.

3373

POP HL

Restore the contents at the top of the STACK (here, the value of Register IY) into Register Pair HL because the routine at 330CH requires that HL be a video location.

3374

LD A,B

Let Register A = Register B (for the first of the 2 bytes held in Register Pair BC) so that Register B can be converted into an ASCII character for display.

3375

GOSUB to 330CH, which does some SERIOUS byte manipulation on the character held in Register A to turn it into an ASCII HEX Digit and display it at (HL), and then BUMP HL.

3378

LD A,C

Let Register A = Register C (for the second of the 2 bytes held in Register Pair BC) so that Register B can be converted into an ASCII character for display.

3379

SET 3,L

Turn Bit 3 of Register L on, which has the same effect as adding 8 to Register L, pushing the video location 8 bytes.

337B

GOSUB to 330CH, which does some SERIOUS byte manipulation on the character held in Register A to turn it into an ASCII HEX Digit and display it at (HL), and then BUMP HL.

337E

PUSH BC

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

337F

LD E,B

Put the contents of Register B into Register E.

3380

CALL 3765H

GOSUB to 3765H to set Register A=1, HL=4300, B=0CH, and JUMP to 3546 to decrement A, reset the floppy controller and wait, set C to 01-08H based on the drive number selected before exiting to 355DH.

3383H is one of many interrupt jump points in this routine and is a VERY common one at that.

3383

JR Z,33C6H

JUMP to 33C6H if the Z (ZERO) Flag is set. 33C6H POPs the STACK to BC, Bump C, Set A as C - 12H and, if A = 0, set C = 0. Return out of this routine.

If we are here, there was an error, so we need to set up to display the error message on screen.

3385

LD HL,346AH

Let HL = 346AH. This is a base address to point to the error messages lookup table.

3388

ADD A,A

Let Register A = entry Register A * 2. Example: For an ID NOT FOUND, the error code is 0BH, so after this instruction, A = 16H.

3389

LD D,00H

Let Register D = 0 because the math requires use of the Register Pair DE and not just Register E.

338B

LD E,A

Let Register E = Register A. At this point DE has been set to 00(entry A*2)H. In our example, Register E = is now 16H and DE = 0016H.

338C

ADD HL,DE

LET HL = HL + DE to advance HL by 2 * entry A bytes. In our example, HL is now 3480H. HL is in the middle of a lookup table and is set at E0H (next instruction 34H). .

338D

LD A,(HL)

Load the value stored in the memory location pointed to by Register Pair HL into Register A. In our example, HL is now 3480H so Register A will contain the memory contents of 3480, which is E0H.

338E

INC HL

Let HL = HL + 1. In our example, HL is now 3481H.

338F

LD H,(HL)

Load the value stored in the memory location pointed to by Register Pair HL into Register H. In our example, H is now 34H.

3390

LD L,A

Let Register L = Register A. Now HL has been re-set to be what was the memory contents of HL. In our example, L is now E0H and HL is 34E0H, which points to the error message ID NOT FOUND.

3391

EX DE,HL

Save HL by EXchanging the value held in Register Pair DE and the value held in Register Pair HL. In our example, DE is 34E0H (pointing to the error message) and HL is 0016H.

3392

LD HL,0010H

Let HL = 10H.

3395

PUSH IY

Store the screen position value of Register IY to the stop of the STACK.

3397

POP BC

Move the value at the top of the STACK (which is the screen position value of Register IY) to Register Pair BC.

3398

ADD HL,BC

LET HL = HL + BC. This is the same adding HL and IY, so it is pushing the screen location held in IY out by 10H characters (to 3C50H).

3399

EX DE,HL

Exchange the value held in Register Pair DE (the pointer to the error message) and the value held in Register Pair HL (the screen location).

339A

XOR A

Zero register A and clear the flags.

339B

GOSUB to 345EH display the messages pointed to by HL at the screen location of DE.

339E

CALL 3453H

GOSUB to 3453H to reset the FDC, set track number and density, get the FDC status, and error out if it is CRC ERROR, SEEK ERROR, or NOT READY. Otherwise, clear all flags and Register A, and RETURN.

33A1

XOR A

Zero register A and clear the flags.

33A2

LD HL,3F80H

Let HL = 3F80H.

33A5

PUSH IY

Store the screen location held in Register IY to the top of the STACK.

33A7

POP DE

Restore the screen position held in Register IY into Register Pair DE.

33A8

SBC HL,DE

Subtract with CARRY DE from HL. On the first run, HL = 3F80H, DE = 3C40H so the SBC leaves HL = 0340H.

33AA

JR Z,33B3H

If the SBC results in 0 (meaning HL and DE were the same), JUMP to the next routine at 33B3H to scroll the display.

33AC

LD DE,0040H

If HL and DE were not the same, Let DE = 0040H.

33AF

ADD IY,DE

Push out the screen position held in Register IY by 40H bytes (= 1 video line).

33B1

JR 33C6H

JUMP to 3BC6H to restore the STACK to BC, Bump C, Set A as C - 12H and, if A = 0, set C = 0. Return.

33B3H - DISKETTE - This routine will scroll the display by moving everything on the 3rd video line and later to the 2nd video line and later.

33B3

LD HL,3C80H

Let HL = 3C80H which is the first character on the 3rdline of the video display.

33B6

LD DE,3C40H

Let DE = 3C30H which is the first character on the 2nd line of the video display.

33B9

LD BC,0341H

Let BC = 341H (or 833 bytes).

33BC

LDIR

Moves the byte from (HL) to (DE), then HL and DE are incremented, and BC is decremented until BC is zero. Interrupts CAN fire during this command.

33BE

LD A,20H

Put a SPACE into Register A.

33C0

LD C,37H

Let Register C = 37H.

33C2

EX DE,HL

Exchange the value held in Register Pair DE (i.e., 3F81H) and the value held in Register Pair HL (i.e., 3FC1H).

33C3

CALL 33E2H

GOSUB to 33E2H to do a little more screen cleanup and then fall through.

33C6H - DISKETTE - This is the exit routine for a 3371H call. STACK to BC, Bump C, Set A as C - 12H and, if A = 0, set C = 0. Return

33C6

POP BC

Move the value at the top of the STACK to Register Pair BC.

33C7

INC C

Let Register C = Register C + 1.

33C8

LD A,C

Let Register A = Register C.

33C9

SUB 12H

Subtract 12H from Register A.

33CB

RET NZ

If the NZ flag is set because Register A was not 12H, RETURN.

33CC

LD C,A

IF A is zero, then Let Register C = 0.

33CD

RET

RETurn to caller.

33CEH - BOOT - This routine is CALLed from 3426H as an alternate entry to 33D0H but with the character in Register A being a SPACE.

33CE

LD A,20H

Put a SPACE into Register A and then pass through.

33D0H - MULTIPLE USE ROUTINE - This routine fills Video Page 1 with the character held in Register A, swaps to Video Page 0, and fills that with the character held in Register A.

33D0

EX AF,AF'

EXchange the values of AF and AF' (since EXX does not include Register Pair AF). Since we flip around, let me point out this PRESERVES Register A.

33D1

LD A,80H

Let A = 1000 0000 Binary. This value. when sent to port 84H, will change the VIDEO PAGE SELECT to PAGE 1.

33D3

CALL 33D9H

GOSUB to 33D9H to set the VIDEO PAGE SELECT 1 to PAGE 1, fill that video page with the character held in Register A, and return here.

33D6

EX AF,AF'

EXchange the values of AF and AF'.

33D7

LD A,00H

Let A = 0000 0000 Binary. This value. when sent to port 84H, will, among other things, change the VIDEO PAGE SELECT to PAGE 0.

This code sets the Model III MISC controls (including the VIDEO PAGE SELECT), and fills the screen with the character held in Register A.

33D9

OUT (84H),A

OUTput Register A to Port 84H.
NOTE: Port 84H handles miscellaneous memory items. Output:

Bits 0-1: Identify the Model of the Computer
Bit 2: Video Display Mode [0=64x16, 1=80x24]
Bit 3: Reverse Video
Bit 4-6: RAM Bank Select
Bit 7: Video Page Select [0=Page 0, 1=Page 1]

33DB

EX AF,AF'

EXchange the values of AF and AF'. This RESTORES Register A.

33DC

LD HL,3C00H

Let HL = 3C00H, which is the first character location on the video display.

33DF

LD BC,03FFH

Let BC = 3FFH, which is the number of bytes on the video display.

33E2H - This routine is a pass through as well as being called. It is called as a subroutine from the DISK scan after scrolling the screen and DE will be 3FC1 and HL will be 3F81.

33E2

LD (HL),A

Fetch the contents of Register A on the screen location pointed to by Register Pair HL.

33E3

LD D,H

Copy HL to DE, which needs to be in 2 steps. First Let D = H.

33E4

LD E,L

and then Let E = L.

33E5

INC DE

Bump DE by 1 (which is then HL + 1).

33E6

LDIR

Copy each character on the screen to the next location on the screen. This is accomplished by LDIR which moves the byte from (HL) to (DE) (which in this case is simply HL + 1), then HL and DE are incremented, and BC is decremented until BC is zero. Interrupts CAN fire during this command.

33E8

RET

RETurn to caller.

33E9H - MEMORY - This routine is CALLed from 319DH to program the CRT Controller Registers, but to set HL at 3401H.

33E9

LD HL,3401H

LET HL = 3401H.

33ECH - This routine is CALLed from 3197H to program the CRT Controller Registers.

33EC

LD A,09H

Let Register A = 09H.

33EE

LD C,89H

Let Register C = 89H, which is the CRT CONTROLLER DATA REGISTER.

Top of a loop

33F0

OUT (88H),A

Send the value held in Register A to Port 88H (the CRT Controller Control Register).

33F2

OUTD

This command takes the byte held in the memory location pointed to by HL and writes it to Port C (the the CRT CONTROLLER DATA REGISTER). HL and B are then both decremented. When called from 3197H, HL starts at 31C0H.

33F4

DEC A

Decrement the value held in Register A by 1.

33F5

RET M

If the SIGN FLAG is set, RETURN.

33F6

JR 33F0H

If the SIGN FLAG is NOT set, LOOP BACK to 3BF0H.

33F8H - MEMORY - This table is moved to the CRT Controller Control Registers in the 33E9H if called from the MEMORY TEST page.

33F8

33F9

33FA

33FB

33FC

33FD

33FE

33FF

3400

3401

3402

3403

3404

3405

3406

3407

3408H - PRINTER - Main Menu Jump Point

3408

LD A,20H

Put a SPACE into Register A.

Start of a loop.

340A

CALL 3419H

GOSUB to 3419H to keep polling the Printer Port (0F8H) until No Printer Fault and Device Select are returned and then send the character held in Register A out to the printer. Register A is preserved.

340D

INC A

Increment the character held in Register A by 1 (these characters get printed).

340E

CP 80H

Compare Register A with Decimal 128, which is one higher than the highest character to print. Results:

If A < that value, it is safe to print another character and the CARRY FLAG will be set
if A >= 128, it is NOT safe to print that character, and the NO CARRY FLAG will be set.

3410

JR C,340AH

If the CARRY flag is set (i.e., the character is less than 128 and therefore can be printed) LOOP back to 340AH to keep testing and printing.

If we are here, then Register A is now set to a character too high to print.

3412

LD A,0DH

Let Register A = a CARRIAGE RETURN.

3414

CALL 3419H

3417

JR 3408H

LOOP back to to 3408H and keep printing until the end of time.

3419H - PRINTER - Keep polling the Printer Port (0F8H) until No Printer Fault and Device Select are returned. Register A is preserved.

3419

PUSH AF

Save the contents of Register Pair AF to the STACK so we can use Register A in the loop that follows.

Start of a loop.

341A

IN A,(F8H)

Fetch the Printer Status Byte into Register A.
NOTE: F8H is the printer port. If Bit 7 is set, the printer is not busy. If Bit 6 is set the printer is not out of paper. If bit 5 is set, the device is selected. If Bit 4 is set, no printer fault.

341C

AND F0H

AND A against F0H (Binary: 1111 0000) to strip off BITS 3-0, leaving BITS 7-4 intact.

341E

CP 30H

Compare Register A with 30H (Decimal: 0011 0000). If Register A equals that 0011 0000, then the printer port is responding No Printer Fault and Device Select, meaning all good! If so, Z is set; otherwise NZ is set.

3420

JR NZ,341AH

If NZ is set, meaning there is something wrong, LOOP back to 341AH to keep polling and testing.

3422

POP AF

If we are here, then we got the OK, so restore Register Pair AF from the top of the STACK.

3423

OUT (FAH),A

Sent the contents of Register A to the PRINTER OUTPUT port of 0FAH.

3425

RET

RETurn.

3426H - BOOT - Main Menu Jump Point

3426

GOSUB to 33CEH to clear the screen and reset the Model 4.

3429

CALL 3453H

GOSUB to 3453H to reset the FDC, set track number and density, get the FDC status, and error out if it is CRC ERROR, SEEK ERROR, or NOT READY. Otherwise, clear all flags and Register A, and RETURN.

342C

JR NZ,3439H

If there was an error, JUMP to 3439H to display the error message on screen and then HALT.

342E

LD C,01H

If we are here, then there was no error from the FDC. Let Register C = 01H.

3430

LD DE,0000H

Let Register Pair DE = 0000H.

3433

CALL 3767H

GOSUB to 3767H to set HL=4300, B=0CH, and JUMP to 3546H to decrement A, reset the floppy controller and waits, sets C to 01-08H based on the drive number selected before exiting to 355DH.

3436

JP Z,4300H

If there were no errors, JUMP to 4300H to run DOS.

3439H - SHARED ROUTINE (BOOT and DISKETTE). This is the jump point if there is a CRC ERROR, SEEK ERROR, or NOT READY from the FDC.

3439

PUSH AF

Save the contents of Register Pair AF to the STACK. This will preserve the FDC error code (held in Register A).

343A

GOSUB to 33CEH to clear the screen and reset the Model 4.

343D

POP AF

Restore the contents of Register Pair AF from the STACK.

343E

ADD A,A

Let A = 2 x the FDC ERROR CODE as a basis for offset to the proper message.

343F

LD HL,346AH

Let HL = 346AH. This is a base address to point to the error messages lookup table.

3442

LD D,00H

Let Register D = 00H since any operation will require use of the Register Pair DE and not just Register E.

3444

LD E,A

Put the ERROR CODE * 2 into Register E.

3445

ADD HL,DE

Advance Register Pair HL by DE bytes.

3446

LD A,(HL)

Load the value stored in the memory location pointed to by Register Pair HL (which is the LSB of the message) into Register A.

3447

INC HL

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

3448

LD H,(HL)

Load the value stored in the memory location pointed to by Register Pair HL (which is the MSB of the message) into Register H.

3449

LD L,A

LET Register L (which is the LSB of the message) = Register A. Now HL points to the error message.

344A

XOR A

Zero register A and clear the flags.

344B

LD E,A

Let E = 00H.

344C

GOSUB to 345EH display the messages pointed to by HL at the screen location of DE, in this case, an error message, and then fall through to the below HALT routine.

344FH - MEMORY - This routine is the jump point when a final, hard error, has been found, but with DISABLING the interrupts.

344F

Disable Interrupts so they don't interrupt this routine. NOTE: This turns off the clock.

3450

HALT

Freeze the system.

3451

JR 3450H

JUMP to the HALT.

3453H - DISKETTE - This is the entry point for a routine to ultimately reset the FDC, set track number and density, get the FDC status, and error out if it is CRC ERROR, SEEK ERROR, or NOT READY. Otherwise, clear all flags and Register A, and RETURN.

3453

LD A,01H

Let Register A = 01H.

3455

LD B,04H

Let Register B = 04H.

3457

CALL 3546H

GOSUB to 3546H to decrement A, reset the floppy controller and waits, sets C to 01-08H based on the drive number selected before exiting to 355DH.

345A

LD (42FFH),A

Put the value held in Register A into the memory location at 42FFH

345D

RET

RETurn to caller.

345EH - SHARED ROUTINE - This routine is called to display one of the messages. When called from the DISKETTE test routine, for example, it displays "Cylinder Sector Drive:" + 00H. On entry from the DISKETTE test routine, BC and DE are 00, HL is 352DH, and A is 10H.

345E

LD A,D

LET Register A = Register D.

345F

OR 3CH

OR Register A against 3CH (the MSB of a screen location).

3461

LD D,A

Put the masked Register A back into Register D.

3462

LD A,(HL)

Load the value stored in the memory location pointed to by Register Pair HL into Register A. On the first run from the DISKETTE TEST routine, HL is 352DH which is the message pointer to "Cylinder Sector Drive:" + 00H.

3463

OR A

Since a LD command does not affect the flags, an OR A is necessary to the flags based on the contents Register A. On the first run from the DISKETTE TEST routine, A is 43H (which is the "C" in Cylinder).

3464

RET Z

If the Z flag is set because we hit the end of that message, RETURN.

3465

LD (DE),A

Put the character in Register A into the memory location pointed to by Register Pair DE. On the first run from the DISKETTE TEST routine, DE is 3C00H which is the first spot on the screen.

3466

INC HL

BUMP Register Pair HL to point to the next character in the message message.

3467

INC DE

BUMP Register Pair DE to point to the next space on the screen.

3468

JR 3462H

Loop back to 3462H to keep displaying characters until this routine is exited in 3464H by hitting the 00H delimeter.

346AH - DISKETTE - This is a lookup table for Disk errors in 3482H

346A

A0H

346B

34H

346C

A0H

346D

34H

346E

A0H

346F

34H

3470

A0H

3471

34H

3472

A0H

3473

34H

3474

A0H

3475

34H

3476

F3H

3477

34H

3478

B0H

3479

34H

347A

BFH

347B

34H

347C

A0H

347D

34H

347E

D0H

347F

34H

3480

E0H

3481

34H

3482H - DISKETTE - Error Message Storage Area

3482

MESSAGE

Host Not Ready+00H

3491

MESSAGE

Host Is Ready+00H

34A0

MESSAGE

Drive Not Ready+00H

34B0

MESSAGE

Data CRC Error+00H

34BF

MESSAGE

Track Seek Error+00H

34D0

MESSAGE

Lost Data Error+00H

34E0

MESSAGE

ID Not Found Error+00H

34F3

MESSAGE

Drive Door Open+00H

3503

MESSAGE

Drive Not Available+00H

3517

MESSAGE

Loading...+00H

3522

MESSAGE

Load Error+00H

352D

MESSAGE

Cylinder Sector Drive:+00H

3546H - DISKETTE - This routine decrements A, resets the floppy controller and waits, sets C to 01-08H based on the drive number selected before exiting to 355DH.

3546

DEC A

LET A = A - 1.

3547

JP Z,354AH

JUMP to the next instruction (354AH) if the Z (ZERO) Flag is set.

354A

CALL 363AH

GOSUB to 363AH.

Note: 363AH resets the Floppy Drive Controller, Push BC, Set Register B as 00H, and JUMP to 3E46H to LOOP for FF, Restores BC From the STACK (so B is unchanged from entry value), and RETURNs.

354D

LD A,C

Let Register A = Register C.

354E

OUT (F2H),A

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

3550

LD A,(42FCH)

Fetch the disk drive from 42FCH and put it into Register A.

3553

LD C,01H

Let Register C = 01H.

3555

OR A

Since a LD command does not affect the flags, an OR A is necessary to the flags based on the contents Register A.

The next loop sets C=01 when the Disk Drive Selected (held in A) is 0, C=02 when the drive 1, C=04 when the drive is drive 2, and C=08 when the drive is drive 3 and then exits to 355DH.

3556

JR Z,355DH

If the Drive Drive was 0, then exit the loop by JUMPing to the next routine (3D5DH).

3558

RL C

The RL C command rotates the contents of C rotated left one bit position with the contents of bit 7 are copied to the carry flag and bit 0.

355A

DEC A

LET A = A - 1.

355B

JR 3556H

LOOP back to 3556H to keep Decrementing A and Rotating C.

355DH - DISKETTE - We're here when Z is set from the loop in the above routine and C is now 01, 02, 04, or 08 based on the disk drive selected.

355D

LD A,C

Let Register A = Register C. A will hold 1 for Drive 0, 2 for Drive 1, 4 for Drive 2, and 8 for Drive 3.

355E

OR 80H

OR Register A against 80H (Decimal: 1000 0000) to turn Bit 7 on (which will force a DOUBLE DENSITY selection onto byte holding the drive letter).

3560

LD (42FDH),A

Put the masked Register A into the memory location 42FDH.

3563

OUT (F4H),A

OUTput the value held in Register A to Port F4H.
NOTE: Port F4H is the Disk Drive, Side, and Disk Density Select. Bits 0-3 are the drive select of 0-3, Bit 4 is the Side Select, Bit 5 is Writer Precom (on/off), Bit 6 is Wait State Generation (on/off) and Bit 7 is 0 for single density and 1 for double density.

3565

LD A,B

Put the contents of Register B into Register A.

3566

CP 05H

Compare Register A (which was Register B) with 05H. Results:

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

3568

JR NC,3588H

If A >= 05H (so NC is set), JUMP to 3588H.

356A

LD A,0CH

Let Register A = 0CH (Decimal: 0000 1100). This command will be sent to the FDC in the next instruction, so let me explain here that "0000 wxyz" is the FDC instruction for RESTORE. w=1 says to load head at beginning, x=1 says verify, y=z=0 says 6ms stepping).

356C

CALL 364AH

GOSUB to 364AH to send the command held in Register A to the FDC (Port F0H) and LOOP until anything other than "NOT READY" is returned by the FDC. Register A holds the response from the FDC on exit.

356F

LD B,A

Let Register B = Register A (the result from the FDC).

3570

AND DFH

Mask Register A against 1101 1111 to turn off bit 5 and keep the rest.

3572

CP 81H

Compare Register A with 81H (Decimal: 1000 0001). If Register A equals 81H (meaning the FDC responded only BUSY and NOT READY), the Z FLAG is set; otherwise the NZ flag is set.

3574

JR NZ,35C7H

NZ will be set if the floppy drive responded anything other than BUSY and NOT READY, and in that case, JUMP to 35C7H to test for SEEK ERROR, CRC ERROR, and NOT READY and jump out if they are found; return with A=0 and clear flags if OK.

3576

LD A,C

Restore the old contents of Register A (1 for Drive 0, 2 for Drive 1, 4 for Drive 2, and 8 for Drive 3) from Register C.

3577

AND 9EH

Mask Register A by ANDing it against 1001 1110 to turn off bits 0, 5, and 6.

3579

LD D,05H

Let Register D = 05H.

357B

JR Z,3585H

If the Z FLAG is set (based on the AND 9EH, above), then JUMP to 3585H to put the contents of Register D into Register A (in this case an 05H), set the flags, and RETURN.

357D

LD D,03H

Let Register D = 03H

357F

CP 06H

Compare Register A with 06H. If Register A equals 06H, the Z FLAG is set; otherwise the NZ FLAG is set.

3581

JR Z,3585H

If the Z FLAG is set (based on the CP 06H, above), then JUMP to 3585H to put the contents of Register D into Register A (in this case an 03H), set the flags, and RETURN.

3583

LD D,06H

Let Register D = 06H.

3585H - DISKETTE - This routine is called when we need to fail out of the DISKETTE test routine. A specified value is put into Register A, the flags are set, and then we RETurn out of the call.

3585

LD A,D

Copy the contents of Register D (which seems to have been an specifically set value before calling this routine) into Register A.

3586

OR A

Since a LD command does not affect the flags, an OR A is necessary to the flags based on the contents Register A.

3587

RET

RETurn to caller.

3588H - DISKETTE - Routine is jumped to from 3568H if Register A (which was Register B) is >= 05H. I have no idea when this occurs; I couldn't trigger it. FWIW, both D and E point to the track number to test.

3588

CP 32H

Compare Register A with 32H. If Register A equals 32H, the Z FLAG is set; otherwise the NZ Flag is set.

358A

JR NZ,35AFH

If Register A is not 32H then JUMP to 3DAFH, with B continuing to be 0CH.

If we are here, then Register A was 32H.

358C

LD B,04H

Let Register B = 04H.

358E

LD D,01H

Let Register D = 01H.

This is the start of a DJNZ loop for a count of 4.

3590

CALL 363AH

3593

LD A,D

Let Register A = Register D. I do not see that the prior call changed Register D at all.

3594

OUT (F4H),A

OUTput the value held in Register A to Port F4H, and if that is 0000 0001, then the FDC is being instructed to select Drive 0..
NOTE: Port F4H is the Disk Drive, Side, and Disk Density Select. Bits 0-3 are the drive select of 0-3, Bit 4 is the Side Select, Bit 5 is Writer Precom (on/off), Bit 6 is Wait State Generation (on/off) and Bit 7 is 0 for single density and 1 for double density.

3596

RLCA

Rotate the bits in A left (with Bit 7 going into both the CARRY and Bit 0). By doing this, A is multiplied by 2.

3597

LD D,A

Preserve the rotated (doubled) Register A into Register D.

3598

IN A,(F0H)

INput from Port F0H and put the result into Register A.
NOTE: Port F0H is the FDC Status port on input. Input Results:

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

359A

AND 20H

Mask Register A by ANDing 0010 0000, to remove all bits except for Bit 5.

359C

JR Z,35ACH

If Bit 5 was LOW (no error), JUMP to 3DACH.

359E

XOR A

Zero register A and clear the flags.

359F

OUT (F0H),A

OUTput the value held in Register A (which is a ZERO) to Port F0H.
NOTE: Port F0H is the FDC Status Register. Outputting 0000 0000 is issuing the command RESTORE (000), SINGLE RECORD (0), SIDE 0 (0), NO 15MS DELAY (0), DISABLE SIDE COMPARE (0).

35A1

CALL 3643H

Delay for a loop of 13H cycles to allow the FDC to process the command via a GOSUB to 3643H.

Begin a loop.

35A4

IN A,(F0H)

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

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

35A6

XOR 01H

XOR 01, meaning If Bit 0 of Register A is a 1 (meaning the FDC returned BUSY) then make it a 0 and vice versa. Leave all the other bits alone.

35A8

AND 05H

MASK that result by ANDing it against 0000 0101 to keep only bits 0 and 2. If Bit 2 is low then we are not yet at "TRACK 0" from the FDC, and If Bit 0 is low then we received a "BUSY" from the FDC.

35AA

JR Z,35A4H

If the result of the Masked/XOR'd A was Bit 0 and 2 low, LOOP back to 35A4H and keep polling the FDC until we get TRACK 0 and NOT BUSY.

35AC

DJNZ 3590H

Loop back to 3590H and keep looping until Register B = 0.

35AE

RET

RETurn to caller.

35AFH - DISKETTE - This routine is JUMPED to by 358AH if Register A did not equal 32H. On Entry E holds the track number to test.

35AF

IN A,(F1H)

Fetch the value currently held by the FDC Track Register (Port F1H) into Register A.

35B1

CP E

Check to see if we are actually at the track number we want to test by comparing Register A (which is the track that the FDC reports as current) with the contents of Register E (the track where we want to be). If Register A equals the contents of Register E, the Z FLAG is set; otherwise the NZ FLAG is set

35B2

JR Z,35C1H

If we are at the track we want to test then Register A will equal Register E, so JUMP down to 35C1H.

35B4

LD A,E

Since Register A did not equal Register E, we are NOT at the track we need, so load up Register A with the track we need to test (held in Register E).

35B5

OUT (F3H),A

OUTput the byte stored in Register A to the FDC Data Register (Port F3H).

35B7

LD A,1CH

Let A = 0001 1 1 00. When OUTput to the FDC (Port F0H) this sends the command: Seek (0001), Load head at beginning (1), Verify (1) 6ms stepping rate (00).

35B9

CALL 364AH

GOSUB to 364AH to send the command held in Register A to the FDC (Port F0H) and LOOP until anything other than "NOT READY" is returned by the FDC. Register A holds the response from the FDC on exit.

35BC

LD D,A

Preserve the contents of Register A (the FDC response code) into Register D.

35BD

AND 98H

MASK Register A against 98H (Binary: 1001 1000) to keep only Bits 3, 4, and 7 (which are CRC ERROR, SEEK ERROR, and NOT READY).

35BF

JR NZ,35C6H

If any of those error bits are high JUMP to 35C6H to Set B = D (the FDC response value).

If we are here, then EITHER the FDC track register equalled E (and 35B2 jumped here) OR we told the FDC to go to track E and didn't get any errors, so we are where we need to be on the diskette.

35C1

LD A,B

Let Register A = Register B

35C2

CP 0CH

Compare Register A with 0CH. If Register A equals 0CH, the Z FLAG is set; otherwise the NZ FLAG is set.

35C4

JR Z,35E5H

If Register A is still holding onto the 0CH value, JUMP to 35E5H.

35C6H - DISKETTE - Alternate entry point to the disk test continuation routine EXCEPT that the contents of Register D (the response from the FDC that spat out an error) are preserved into Register B.

35C6

LD B,D

Preserve Register D (the FDC response code) in Register B and THEN continue the diskette test with the next instruction.

35C7H - DISKETTE - If the selected floppy drive is not returning BUSY and NOT READY, we wind up here to continue the test. Set D to 08H and then JUMP to 35D3H to test for SEEK ERROR, CRC ERROR, and NOT READY and jump out if they are found; return with A=0 and clear flags if OK.

35C7

LD D,08H

Let D = 08H.

35C9

JR 35D3H

JUMP to 35D3H to test for SEEK ERROR, CRC ERROR, and NOT READY and jump out if they are found; return with A=0 and clear flags if OK.

35CBH - DISKETTE - This routine is jumped to from 3636H. It tests Bit 2 of the FDC Status and proceeds with D=0AH if that was high (and JUMP to 3585H) or 0BH if it was low (and PASS THROUGH to 35D3H).

35CB

LD D,0AH

Let Register D = 0000 1010

35CD

BIT 2,B

Test Bit 2 of Register B. Register B is very likely to contain the FDC Status from Port F0H. If so, Bit 2 indicates either TRACK 0 (if this was from a RESTORE, SEEK, or STEP command) or LOST DATA (if this was from a SECTOR READ or SECTOR WRITE command).

35CF

If Bit 2 of Register B is 1, JUMP to 3585H to put the contents of Register D into Register A (in this case an 0AH), set the flags, and RETURN.

35D1

LD D,0BH

If Bit 2 of Register B is 0, then Let D = 0BH and continue

35D3H - DISKETTE - This routine tests for SEEK ERROR, CRC ERROR, and NOT READY. If any are found, the error routine at 3585 is jumped to. Otherwise, Register A and the flags are reset, and we RETURN. On Entry Register B contains the response from the FDC and D has an error code.

35D3

BIT 4,B

Test Bit 4 of Register B and set the flags accordingly. Bit 4 from the FDC (Port F0H) is "SEEK ERROR".

35D5

If the Z FLAG is set (based on the BIT 4, B, above), then JUMP to 3585H to put the contents of Register D into Register A, set the flags, and RETURN.

35D7

LD D,07H

Let Register D = 07H.

35D9

BIT 3,B

Test Bit 3 of Register B and set the flags accordingly. Bit 3 from the FDC (Port F0H) is "CRC ERROR".

35DB

If the NZ flag is set (based on the BIT 3, B, above), then JUMP to 3585H to put the contents of Register D (in this case an 07H) into Register A, set the flags, and RETURN.

35DD

LD D,03H

Let Register D = 03H.

35DF

BIT 7,B

Test Bit 7 of Register B and set the flags accordingly. Bit 7 from the FDC (Port F0H) is "NOT READY".

35E1

If the Z FLAG is set (based on the BIT 7, B, above), then JUMP to 3585H to put the contents of Register D (in this case, an 03H) into Register A, set the flags, and RETURN.

35E3

XOR A

Zero register A and clear the flags.

35E4

RET

RETurn to caller.

35E5H - DISKETTE - This routine is jumped to from 35C54 if we are on the track that we want to test AND Register A is still holding 0CH.

35E5

XOR A

Zero register A and clear the flags.

35E6

LD B,A

Let B = 0

35E7

LD A,(42FDH)

Fetch the contents of memory location 42FDH into Register A. 42FDH stores a byte representing the Drive Number and Double Density, and was put there at 3560H

35EA

LD D,A

Let Register D = Register A.

35EB

LD C,82H

Let Register C = 1000 0010. Register C will ultimately be sent to the FDC Command/Status port (Port F0H), although it may change from the current value. At the current value, this sends the command: READ SECTOR (100), SINGLE RECORD (0), SIDE 0 (0), No 15ms Delay (0), Side Compare Flag Enabled (10).

35ED

IN A,(F2H)

Fetch the current sector from the FDC Sector Register (Port F2H) and put it into Register A.

35EF

CP 12H

Compare Register A with 12H (Decimal 18). Results:

If Register A equals sector 18, the Z FLAG is set.
If A < sector 18, the CARRY FLAG will be set
if A >= sector 18, the NO CARRY FLAG will be set.

35F1

JR C,35F9H

If we have not yet arrived at sector 18 on the track, JUMP to 35F9H to keep reading the track.

If we are here, then we have reached Sector 18, so we need to check SIDE 1 of the disk ...

35F3

SUB 12H

Reduce Register A (the sector number) by 18.

I'm not convinced this is best practices. This assumes nothing went wrong and the FDC never reported above 12H. Perhaps a LD A,00H would have been safer.

35F5

SET 4,D

Turn on Bit 4 of Register D. Bit 4 of the FDC Drive Select/Options value (Port F4) toggles the SIDE SELECT, so this command selects SIDE 1. Register D is holding the Drive Number in Bits 0-2, a flag for Side 0 (bit 4) and a flag for Double Density (bit 7).

35F7

SET 3,C

Turn on Bit 3 of Register C. Bit 3 of the FDC Command/Status Port (F0H), when applied to a READ SECTOR command, is the SIDE COMPARE FLAG, so this will flip that bit to COMPARE FOR SIDE 1.

35F9H - DISKETTE - This routine is JUMPed to from 35F1H if we have not yet hit Sector 18 on the track we are testing.

35F9

ADD A,B

Add the value in register B to the value in register A.

35FA

OUT (F2H),A

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

35FC

LD A,D

Let Register A = Register D (the FDC Drive Select/Options [Port F4H] command).

35FD

OUT (F4H),A

The variable shuffling that is about to happen is so that the INI command in 361EH is properly set. INI requires that the port be in Register C, the destination memory location in Register Pair HL, and the bytes to read in Register B.

35FF

LD B,00H

Let Register B = 00H.

3601

LD E,H

Let Register E = Register H. Register Pair HL should be 4300H.

3602

LD A,C

Let Register A = Register C. Register C is holding the FDC Command/Status Port (F0H) command.

3603

LD C,F3H

Let Register C = F3H. Port F3H is the FDC Data Register (the data byte to be READ or WRITTEN to disk).

The only register being modified from this point to the end of the routine is Register A. Everything else is now set up for the INI command in 361EH.

3605

OUT (F0H),A

OUTput the value held in Register A (formerly held in Register C) to Port F0H.
NOTE: Port F0H is the FDC Command Register.

3607

LD A,80H

Let Register A = 1000 0000

3609

OUT (E4H),A

OUTput the value held in Register A to Port E4H.
NOTE: Port E4H is the non-maskable interrupt latch. Sending an 80H to Port E4 sets DISABLE DRQ and ENABLE INTRQ. This is done because the routine at 361EH has no exit, and will exit only on INTRQ

360B

CALL 3643H

Delay for a loop of 13H cycles to allow the FDC to process the command via a GOSUB to 3643H.

360E

LD A,D

Let Register A = Register D (the FDC Drive Select/Options [Port F4H] command).

360F

OUT (F4H),A

3611

IN A,(F0H)

Fetch the FDC Status from Port F0H and put it into Register A.
NOTE: Port F0H is the FDC Status port on input. Input Results:

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

3613

BIT 1,A

Test Bit 1 of the FDC Status. Bit 1 is the Index/DRQ flag.

3615

JP NZ,361EH

If the Index/DRQ flag is set to 1, JUMP to 361EH to continue the test, as we NOW have the right track, sector, side and INDEX mark.

3618

RLCA

Test to see if the FDC reports BUSY (Bit 7) by rotating the bits in A left (with Bit 7 going into both the CARRY and Bit 0).

3619

JP NC,3611H

If NC is set, then the drive did NOT report BUSY, so LOOP back to 3611H until we hit the index.

361C

JR 362DH

If we are here then we have found the INDEX and the FDC isn't BUSY, so JUMP to 3E2DH.

361EH - DISKETTE - This routine is JUMPed to from 3615H if we have found the INDEX mark after moving to the right track, sector, and side to test. It loads the buffer at 4300H with the sector values but is exited only by an interrupt.

361E

INI

Read a byte from Port C (set at 3603H to be Port F3H) into the memory location pointed to by HL (on entry 4300H), then DECrement HL and INCrement B. Z is set if B = 0; otherwise it is reset.

3620

LD A,D

Let Register A = Register D (the FDC Drive Select/Options [Port F4H] command).

3621

OR 40H

Mask Register A via OR against 0100 0000 to turn on Bit 6 of Register A. Bit 6 of a command going to the FDC Drive Select port (F4H) turns on WAIT STATE GENERATION.

Start of a loop of reading bytes into memory location HL, exited only via an interrupt.

3623

OUT (F4H),A

3625

INI

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

3627

JR 3623H

LOOP back to 3623H.

3629H - DISKETTE - Put the value at the top of the STACK into Register Pair BC, Clear Register A and all flags, Disable the DRQ and INTRQ interrupts and then continue to 362DH

3629

POP BC

Move the value at the top of the STACK to Register Pair BC.

362A

XOR A

Zero register A and clear the flags.

362B

OUT (E4H),A

OUTput the value held in Register A (a 0) to Port E4H to deal with interrupts and then fall through to the common jump point of 362DH.
NOTE: Port E4H is the non-maskable interrupt latch. Sending a 0 disable the DRQ and INTRQ interrupts.

362DH - DISKETTE - If we are here, we are on the right drive, side, track, sector, and the INDEX just triggered!

362D

IN A,(F0H)

Fetch the FDC Status from Port F0H and put it into Register A.
NOTE: Port F0H is the FDC Status port on input. Input Results:

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

362F

LD B,A

Preserve the FDC Status into Register B

3630

CALL 3638H

GOSUB to 3638H to POP the STACK into the Program Counter.

3633H is one of many interrupt jump points in this routine.

3633

LD A,H

Let Register A = Register H. Register H seems to be either 43H or 44H as a pointed to a memory buffer holding sector contents.

3634

SUB E

3635

LD E,A

Put the difference back into Register E.

3636

JR 35CBH

Continue on by JUMPing to 3DCBH.

3638H - DISKETTE - Execute a RETN

3638

RETN

RETN is used at the end of a non-maskable interrupt service routine to POP the top of the STACK entry into PC. The value of IFF2 is copied to IFF1 so that maskable interrupts are allowed to continue as before. NMIs are not enabled on the TI.

363AH - Reset the Floppy Drive Controller, Push BC, Set Register B as 00H, and JUMP to 3646H. B is unchanged from entry value.

363A

LD A,D0H

Let A = D0H (Decimal: 1101 0000).

363C

OUT (F0H),A

OUTput the value held in Register A to Port F0H. When D0H is sent, it resets the FDC and puts FDC in mode 1 (INTRQ; ?000? terminate command without interrupt)
NOTE: Port F0H is the FDC Status Register. Outputting 1000 0000 is issuing the command READ SECTOR (100), SINGLE RECORD (0), SIDE 0 (0), NO 15MS DELAY (0), DISABLE SIDE COMPARE (0).

363E

PUSH BC

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

363F

LD B,00H

Let Register B = 00H in preparation for the next instruction.

3641

JR 3646H

JUMP to 3646H to Delay FFH loops, Restore BC from the top of the STACK, and RETurn.

3643H - DISKETTE - Delay (13H loops) to let the FDC process a command and RETurn

3643

PUSH BC

Save the contents of Register Pair BC to the STACK.

3644

LD B,13H

Let B = 13H. This will be a delay loop long enough to allow a FDC command to process.

3646H - Delay REGISTER B loops, Restore BC from the top of the STACK, and RETurn

3646

DJNZ 3646H

Loop back to 3646H and keep looping until Register B = 0.

3648

POP BC

Restore Register Pair BC from the top of the STACK.

3649

RET

RETurn to caller.

364AH - DISKETTE - Send the command held in Register A to the FDC and LOOP until anything other than "NOT READY" is returned by the FDC. Register A holds the response from the FDC on exit.

364A

OUT (F0H),A

OUTput the value held in Register A to Port F0H.
NOTE: Port F0H is the FDC Status Register. Outputting 1000 0000 is issuing the command READ SECTOR (100), SINGLE RECORD (0), SIDE 0 (0), NO 15MS DELAY (0), DISABLE SIDE COMPARE (0).

364C

CALL 3643H

Delay for a loop of 13H cycles to allow the FDC to process the command via a GOSUB to 3643H.

364F

LD C,00H

Let Register C = 00H.

3651

IN A,(F0H)

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

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

3653

BIT 0,A

Check BIT 0 (the bit for "BUSY" status on the FDC) of Register A.

3655

RET Z

If the FDC replied "BUSY", RETURN.

3656

BIT 7,A

Check BIT 0 (the bit for "NOT READY" status on the FDC) of Register A.

3658

RET NZ

If the FDC replied anything other than "NOT READY", RETURN.

3659

LD C,A

Let C = Register A.

365A

JR 3651H

LOOP back to 3651H.

365CH - BOOT FROM RS-232 - Routine Entry Point.

365C

LD A,04H

Let Register A = 04H. I'm not at ALL clear why this is here, as 33CEH resets Register A.

365E

GOSUB to 33CEH to clear the screen and reset the Model 4.

3661

LD A,64H

Let Register A = 0110 0100. When applied to port EAH, this is DTR On, RTS On, Send Break Signal, Parity Enabled, 0 Stop Bits, Word Length 8, Parity Odd.

3663

OUT (EAH),A

OUTput the value held in Register A to Port EAH.
NOTE: Port EAH is the RS-232 UART Control Register/Status Register. For output:

Bit 0: Data Terminal Ready
Bit 1: Request to End
Bit 2: Break
Bit 3: Parity Enable
Bit 4: Stop Bits
Bit 5: Select
Bit 6: Word Length
Bit 7: Parity (0=Odd, 1=Even)

3665

LD HL,3482H

Let Register Pair HL = 3482H to point to the message "Host Not Ready"

3668

LD DE,0000H

Let Register Pair DE = 0000H so that, in part, D=00H so that the message at 3482H will be placed on the screen at 3C00H + 00H

366B

GOSUB to 345EH display the messages pointed to by HL at the screen location of DE, in this case "Host Not Ready".

Do a loop polling Port E8H, Masking against 00100000 to leave only Bit 5 on, and LOOPing back until Bit 5 is NOT ZERO.

366E

IN A,(E8H)

Fetch the RS-232 Status Register (Port E8H) and put the result into A.
NOTE: Port E8H is the RS-232 Status Register & Master Reset. Input:

Bit 4: Ring Indicator
Bit 5: Carrier Detect
Bit 6: Data Set Ready
Bit 7: Clear to Send

3670

AND 20H

AND Register A against 0010 0000 to preserve only Bit 5 (= Carrier Detect).

3672

JR NZ,366EH

If there is no carrier detect, LOOP back to 3E6EH.

At this point, we have a carrier detect.

3674

LD DE,0000H

Let Register Pair DE = 0000H so that, in part, D=00H so that the message at 3491H will be placed on the screen at 3C00H + 00H

3677

LD HL,3491H

Let Register Pair HL = 3491H to point to the message "Host Is Ready"

367A

GOSUB to 345EH display the messages pointed to by HL at the screen location of DE, in this case "Host Is Ready".

367D

LD HL,42FEH

Let HL = 42FEH as an initial value for the loop at 3680H.

3680H - This location is LOOPED back to from either 3699H, 36A0H or 36A4H.

3680

LD A,FFH

Let Register A = FFH as an initial value for the loop at 3682H. It will be reduced by 11H each large loop pass, but then ultimately jumped back here on a failure to start the large loop again.

3682H - This location is LOOPED back to from 3697H

3682

LD C,A

Preserve Register A in Register C

3683

OUT (E9H),A

OUTput the value held in Register A to Port E9H.
NOTE: Port E9H is the RS-232 Baud Rate Selects and Sense Switches. Outputting to Port E9H will set the Baud Rate as follows:

Bits 0-3 - Select the Receive Rate
Bits 4-7 - Select the Transmit Rate

3685

OUT (E8H),A

OUTput the value held in Register A to Port E8H.
NOTE: Port E8H is the RS-232 Status Register & Master Reset. Outputting ANYTHING to Port E8H resets the RS-232.

3687

LD B,0AH

Let Register B = 0AH. This will form a counter for the DJNZ statement in 3692H

3689

GOSUB to 36F1H to wait for DATA READY from the RS-232 and return either NZ if the RS-232 threw an error (with Register A holding the 3 RS-232 error bits), or Z with A holding a byte from the RS-232. Only Register A is affected.

368C

JR NZ,3692H

if 36F1H returns with NZ set, then the RS-232 threw an error, so JUMP forward to 3692H.

If we are here, then we fetched a byte from the RS-232 into Register A.

368E

CP 55H

Compare Register A with 55H. If Register A equals 55H, the Z FLAG is set; otherwise the NZ FLAG is set.
NOTE:A 55H is a standard header byte of an executable file

3690

JR Z,369BH

If the byte read from the RS-232 is a 55H then JUMP to 369BH to continue, otherwise pass through to retry the RS-232. This is the loop exit.

3692H - This routine is passed through if the byte received from the RS-232 was not 55H -or- was jumped to from 368CH if the RS-232 threw an error.

3692

DJNZ 3689H

Loop back to 3689H and keep looping until Register B = 0.

If we are here, then the RS-232 spat out errors 10H times.

3694

LD A,C

Restore Register A from Register C (preserved at 3682H).

3695

SUB 11H

Let A = A - 11H.
NOTE: If a borrow results from the subtraction, the C FLAG will be set. Since A was set to FFH at the beginning of this routine (and preserved in Register C), this is actually a counter of 16. FF to EE to DD etc.

3697

JR NC,3682H

If, as a result of the subtraction, there was no borrow (i.e., a loop of 16), JUMP back to 3682H to keep polling.

If we are here, then the RS-232 spat out errors 10H x 16 times.

3699

JR 3680H

JUMP to 3680H to restart the process, with A as FFH.

369BH - BOOT FROM RS-232 - If we are here, then we received a 55H byte from the RS-232 at 3690H.

369B

LD B,0AH

Let Register B = 0AH for a DJNZ loop of 10.

Start of a loop which exits if we get 55H from the RS-232 10 times.

369D

36A0

JR NZ,3680H

If the RS-232 threw an error, JUMP to 3680H to start the RS-232 polling/reading process all over again.

36A2

CP 55H

Compare Register A with 55H. If Register A equals 55H, the Z FLAG is set; otherwise the NZ FLAG will be set.

36A4

JR NZ,3680H

If the RS-232 did not provide a 55H, JUMP to 3680H to start the RS-232 polling/reading process all over again.

If we are here then the RS-232 gave us a 55H with no errors.

36A6

DJNZ 369DH

Loop back to 369DH and keep looping until Register B = 0.

If we are here then the RS-232 gave us TEN 55H's with no errors, so proceed ...

36A8

LD DE,0184H

Let Register Pair DE = 0184H to point to a location on the screen..

36AB

LD HL,3710H

Let Register Pair HL = 3710H to point to the Found Baud Rate + 00H message

36AE

CALL 36FDH

GOSUB to 36FDH to display a message to both the screen AND the RS-232, and RETurn once sent. Only Registers A and HL are affected. In this case, the message is Found Baud Rate + 00H.

From what I can tell, if we are here, Register C is still holding FF as it was set in 3682H and DE is pointing the to screen location after the message. I can't test because I cannot figure out how to get TRS80GP to fake RS-232 responses.

36B1

LD A,C

Let Register A = Register C.

36B2

AND 0FH

Mask Register A by ANDing it againt 00001111 to keep only bits 0-3

36B4

ADD 41H

Let A = A + 41H

36B6

LD (DE), A

Put the contents of Register A into the memory location pointed to by Register Pair DE.

36B7

IN A,(EBH)

Fetch a byte from the RS-232 (Port EBH) and put it into Register A.
NOTE: Port EBH is the RS-232C Data Register. It contains the data received from the RS-232C.

Start of a loop to keep getting bytes from the RS-232 until a FFH is received.

36B9

36BC

JR NZ,36D1H

If the RS-232 threw an error, JUMP DOWN to 36D1H to show a LOAD ERROR message and restart the BOOT FROM RS-232 routine.

36BE

CP FFH

Compare the byte received from the RS-232 (held in Register A) with FFH. If Register A equals FFH, the Z FLAG is set; otherwise the NZ FLAG is set.

36C0

JR NZ,36B9H

If the byte received from the RS-232 (held in Register A) is not FFH, LOOP back to 36B9H<.

If we are here, we received a FFH from the RS-232, so display LOADING and continue on.

36C2

LD DE,0244H

Let Register Pair DE = 0244H to point to a screen location.

36C5

LD HL,3517H

Let Register Pair HL = 3517H to point to the message Loading... + 00H.

36C8

CALL 36FDH

GOSUB to 36FDH to display a message to both the screen AND the RS-232, and RETurn once sent. Only Registers A and HL are affected. In this case, the message is Loading... + 00H.

36CB

CALL 3721H

GOSUB to 3721H to load the boot routine from the RS-232 and set HL with the transfer address. If there is any error, NZ is set.

36CE

JR NZ,36D1H

If there was an error loading the boot routine via 3721H, the NZ FLAG will be set. If so, JUMP to the next routine at 36D1H to display an error and restart the BOOT FROM RS-232 routine.

If we are here, then the load from RS-232 completed without error and HL contains the transfer address.

36D0

JP (HL)

Jump to the address in HL to run the code transferred from the RS-232.

36D1H - BOOT FROM RS-232 - This routine puts a LOAD ERROR message along with a modified version of the error code received on the screen, and then restarts the BOOT FROM RS-232 routine again. This routine is JUMPed to from either 36BCH (if got the 55H's ok, but were looking for FFH's and got an error instead) or 36CEH (if we were already loading from the RS-232 but then got an error). When 36F1H exits with an error, A holds only the error bits returned by the RS-232 Controller (bits 3, 4, and 5).

36D1

RRCA

Rotate A right one bit, with the contents of BIT 0 being put into BOTH the CARRY FLAG and BIT 7. RS-232 error bits 5, 4, and 3 are moved to Bits 4, 3, and 2.

36D2

RRCA

Rotate A right one bit, with the contents of BIT 0 being put into BOTH the CARRY FLAG and BIT 7. RS-232 error bits 5, 4, and 3 are moved to Bits 3, 2, and 1.

36D3

RRCA

Rotate A right one bit, with the contents of BIT 0 being put into BOTH the CARRY FLAG and BIT 7. RS-232 error bits 5, 4, and 3 are moved to Bits 2, 1, and 0.

36D4

PUSH AF

Save the rotated RS-232 error bits held in Register A to the STACK.

36D5

GOSUB to 33CEH to clear the screen and reset the Model 4.

36D8

LD DE,03C4H

Let Register Pair DE = 03C4H to point to a location on the screen..

36DB

LD HL,3522H

Let Register Pair HL = 3522H to point to the Load Error + 00H message

36DE

CALL 36FDH

GOSUB to 36FDH to display a message to both the screen AND the RS-232, and RETurn once sent. Only Registers A and HL are affected. In this case, the message is Load Error + 00H.

36E1

POP AF

Restore Register A (i.e., the rotated RS-232 error bits) from the STACK.

36E2

ADD 41H

Let A = A + 41H so that A holds 41 + an offset of the value of the error bits.

36E4

INC DE

Bump Register Pair DE by 1 to point to the screen location after the displayed Load Error message.

36E5

LD (DE), A

Put the contents of Register A (an offsetted value of the error bits) into the memory location pointed to by Register Pair DE (a screen address).

36E6

LD BC,0000H

Let Register Pair BC = 0000H to set up a delay loop of FFFFH.

36E9

DJNZ 36E9H

Loop back to THIS INSTRUCTION and keep looping until Register B = 0.

36EB

DEC C

Decrement the value held in Register C by 1.

36EC

JR NZ,36E9H

Loop back to 36E9H until C has hit 0.

36EE

JP 3661H

JUMP to 3661H to put start the whole process of trying to boot from the RS-232 again.

36F1H - BOOT FROM RS-232 - This SUBROUTINE waits for DATA READY, and then either returns NZ with an error or fetches a byte from the RS-232 into Register A with the Z Flag set.

36F1

IN A,(EAH)

Fetch the status of the RS-232 Controller (Port EAH) into Register A.
NOTE: Port EAH is the RS-232 UART Control Register/Status Register. Input:

Bit 3: Parity error (1=True)
Bit 4: Framing Error (1=True)
Bit 5: Overrun (1=True)
Bit 6: Data Sent (1=True)
Bit 7: Data Ready (1=True)

36F3

BIT 7,A

Test Bit 7 of A, which is the bit for a "Data Ready".

36F5

JR Z,36F1

If the DATA READY is not indicated (because Bit 7 is 0), LOOP back to the top of this routine (36F1H) and poll again.

If we are here, then DATA READY was set and Register A holds the RS-232 Controller Status byte.

36F7

AND 38H

Mask Register A by ANDing it against 0011 1000 to leave only the error bits (bits 3, 4, and 5) live.

36F9

RET NZ

If NZ is set, there there was some error from the RS-232 Controller Status (we don't care which it was), so RETURN.

If we are here, then DATA READY was set and we have no errors from the RS-232 Controller.

36FA

IN A,(EBH)

Fetch a byte from the RS-232 (Port EBH) and put it into Register A.
NOTE: Port EBH is the RS-232C Data Register. It contains the data received from the RS-232C.

36FC

RET

RETurn to caller.

36FDH - BOOT FROM RS-232 - This SUBROUTINE is called from a number of locations to display a message to both the screen AND the RS-232, and RETurn once sent. Only Registers A and HL are affected. Before entry HL and DE are specifically set for a message display:

If called from 36AE, the message will be Found Baud Rate + 00H
If called from 36C8, the message will be Loading... + 00H
If called from 36DE, the message will be Load Error + 00H

36FD

PUSH HL

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

36FE

GOSUB to 345EH display the messages pointed to by HL at the screen location of DE.

3701

POP HL

Restore the value at the top of the STACK to Register Pair HL.

This is the beginning of a loop. On Entry HL points to the first letter of the message which was displayed.

3702

IN A,(EAH)

Fetch the status of the RS-232 Controller (Port EAH) into Register A.
NOTE: Port EAH is the RS-232 UART Control Register/Status Register. Input:

Bit 3: Parity error (1=True)
Bit 4: Framing Error (1=True)
Bit 5: Overrun (1=True)
Bit 6: Data Sent (1=True)
Bit 7: Data Ready (1=True)

3704

AND 40H

MASK Register A against 0100 0000 to leave only Bit 6 live. Bit 6 would be the "Data Sent" bit and will be NZ if TRUE and Z if FALSE.

3706

JR Z,3702H

If the RS-232 reports DATA NOT SENT (Because Bit 6 is 0), JUMP back 2 instructions (to 3702H) to read the status again.

If we are here, then the RS-232 reported DATA SENT. For some reason, the next instructions take the message which was displayed at the beginning of the routine, and push them out the RS-232 port, RETurning once it is completely sent.

3708

LD A,(HL)

Fetch the value stored in the memory location pointed to by Register Pair HL into Register A.

3709

OR A

Since a LD command does not affect the flags, an OR A is necessary to the flags based on the contents Register A.

370A

RET Z

If the Z flag is set because the memory contents pointed to by Register HL were zero (meaning, the end of the message delimiter was found), then RETURN.

370B

OUT (EBH),A

OUTput the value held in Register A (which was the memory contents pointed to by Register HL) to Port EBH.
NOTE: Port EBH is the RS-232C Data Register. It contains the data to be sent to the RS-232C.

370D

Bump Register Pair HL by 1.

370E

JR 3702H

LOOP back to 3702H until the entire message has been pushed out the RS-232 port.

3710H - BOOT FROM RS-232 - Message Storage Area for RS-232 Test.

3710

MESSAGE

"Found Baud Rate ",00h

3721H - BOOT FROM RS-232 - This SUBROUTINE loads the boot routine from the RS-232 and sets HL with the transfer address. It is called from 36CBH. If an error occurs, NZ is set and Register A will hold the 3 error bits)

3721

LD E,FFH

Let Register E = FFH. This value is set for the main loop at 3723H starts.

Start of a loop which encompasses this whole subroutine.

3723

3726

RET NZ

If the RS-232 threw an error, RETURN with NZ set and with Register A holding the 3 RS-232 error bits.

3727

DEC A

If we are here then we got a byte from the RS-232 and it is held in Register A. Decrement that value by 1 to make testing for ZERO easier.

3728

JR Z,373BH

If the byte we received was an 01H, JUMP to 373BH.
NOTE: An 01H, when applied to an executable program, indicates a LOAD BLOCK, so we need to process those bytes.

372A

DEC A

Decrement the value held in Register A by 1 again.

372B

JR NZ,3750H

If the byte we received was something other than a 02H, JUMP to 3750H.

If we are here, the RS-232 gave us 02H. An 02H in an executable program signals that the TRANSFER ADDRESS is coming. It then has 3 bytes afterwards -- the length of the block, then the LSB of the transfer address, and then the MSB of the transfer address.

372D

3730

RET NZ

If the RS-232 threw an error, RETURN with NZ set and with Register A holding the 3 RS-232 error bits.

3731 - This is both a pass through (if an 02H was received) and a a JUMP point from 3742H (if an 01H was received). In either case, the RS-232 is about to provide a load address in LSB, MSB order, so we need to capture that into Register HL.

3731

3734

RET NZ

If the RS-232 threw an error, RETURN with NZ set and with Register A holding the 3 RS-232 error bits.

If we are here, the RS-232 gave us 2 non-errors after giving us a 02H, and Register A holds the 2nd byte the RS-232 sent after the 02H or an 01H (if called from there), which is the LSB of the transfer address.

3735

LD L,A

LET Register L = Register A (the 2nd byte the RS-232 sent after the 02H or 01H).

3736

3739

LD H,A

LET Register H = Register A (the 3rd byte the RS-232 sent after the 02H or 01H), which is the MSB of the transfer address.

373A

RET

RETurn to caller.

373BH - BOOT FROM RS-232 - Process the LOAD BLOCK. This routine is jumped to if we received a 01H byte from the RS-232. An 01H, when applied to a executable program, indicates a LOAD BLOCK.

373B

373E

RET NZ

If the RS-232 threw an error, RETURN with NZ set and with Register A holding the 3 RS-232 error bits.

373F

DEC A

The first byte after an 01H indicates how many bytes the block is. It is either 00, 01, or 02 to indicate whether the block is 254, 255, or 256 bytes. Whatever that byte is, DECremenet it by 1.

3740

DEC A

and Decrement it again.

3741

LD B,A

Put the block code - 2 into Register B. This sets up a loop to read the appropriate number of bytes.

3742

CALL 3731H

GOSUB to 3731H to get the next 2 bytes from the RS-232 and put them into HL.

3745

RET NZ

The address to get the load address from the RS-232 provided an error, RETURN.

If we're here we have no error from the RS-232 and HL has the LOAD address. This is the start of a loop of B passes.

3746

3749

RET NZ

If the RS-232 threw an error, RETURN with NZ set and with Register A holding the 3 RS-232 error bits.

374A

LD (HL),A

Put the bytes received from the RS-232 into the memory location pointed to by HL.

374B

Bump Register Pair HL by 1.

374C

DJNZ 3746H

LOOP back to 3746H and keep looping until Register B = 0 (meaning the number of bytes indicated by the first byte after the 01H has been read).

374E

JR 3723H

LOOP back to 3723H for the next block.

3750H - This routine is jumped to if we received anything other than a 01H or 02H from the RS-232

3750

3753

LD B,A

Put Register A into Register B to set up a DJNZ loop for that many iterations.

Start of a DJNZ loop of that number of bytes.

3754