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More discussion on the PC-DOS 1.10 boot sector and how it compares to the COMPAQ 1.11 boot sector

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Jeff Parsons 2017-07-01 12:21:06 -07:00 • committed by Jeff Parsons
commit 6d16f582c7
7 changed files with 474 additions and 307 deletions
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151
disks/pcx86/dos/ibm/1.10/README.md
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@ -5,7 +5,7 @@ permalink: /disks/pcx86/dos/ibm/1.10/
machines:
- id: ibm5150-pcdos110
type: pcx86
config: /devices/pcx86/machine/5150/mda/64kb/machine.xml
config: /devices/pcx86/machine/5150/mda/64kb/debugger/machine.xml
resume: 1
autoMount:
A:
@ -109,6 +109,155 @@ The boot sector of the PC-DOS 1.11 disk image contains the following bytes:
000001e0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
000001f0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
This is also the boot sector as it appears on any other single-sided 160Kb diskette formatted by PC-DOS 1.10.
However, PC-DOS 1.10 added supported for a second format: double-sided 320Kb diskettes. And on those larger diskettes,
the boot sector is slightly different:
1c1
< 00000000 eb 27 90 08 00 14 00 00 00 00 00 00 00 00 00 00 |.'..............|
---
> 00000000 eb 27 90 03 01 14 00 00 00 00 00 00 00 00 00 00 |.'..............|
The bytes at offset 0x0003 (0x7C03 when the boot sector is loaded into memory) contain the initial sector
and head numbers of IBMBIO.COM. On a 160Kb diskette, the first data sector is at sector #8 and head #0, but since
a 320Kb diskette is allocated a larger root directory (7 sectors instead of 4), that pushes the first data sector
out to sector #3 and head #1. The total number of sectors to read (0x14) remains the same as PC-DOS 1.00, but that
value is now stored at offset 0x0005 (0x7C05) instead of 0x0002 (0x7C02).
Using the PCjs Debugger, we can examine the boot sector in its native environment:
bp &0000:7C00 hit
stopped
AX=0000 BX=7C00 CX=0004 DX=0000 SP=0100 BP=E4B7 SI=0000 DI=0044
SS=0030 DS=0040 ES=0000 PS=F296 V0 D0 I1 T0 S1 Z0 A1 P1 C0
&0000:7C00 EB27 JMP 7C29
>> tr
AX=0000 BX=7C00 CX=0004 DX=0000 SP=0100 BP=E4B7 SI=0000 DI=0044
SS=0030 DS=0040 ES=0000 PS=F296 V0 D0 I1 T0 S1 Z0 A1 P1 C0
&0000:7C29 FA CLI ;cycles=15
>> u 7c2a 7d1a
&0000:7C2A 8CC8 MOV AX,CS
&0000:7C2C 8ED8 MOV DS,AX
&0000:7C2E 33D2 XOR DX,DX
&0000:7C30 8ED2 MOV SS,DX
&0000:7C32 BC007C MOV SP,7C00
&0000:7C35 FB STI
&0000:7C36 B86000 MOV AX,0060
&0000:7C39 8ED8 MOV DS,AX
&0000:7C3B 8EC0 MOV ES,AX
&0000:7C3D 33D2 XOR DX,DX
&0000:7C3F 8BC2 MOV AX,DX
&0000:7C41 CD13 INT 13
&0000:7C43 7269 JC 7CAE
&0000:7C45 E88500 CALL 7CCD
&0000:7C48 72DD JC 7C27
&0000:7C4A 2E CS:
&0000:7C4B 833E037C08 CMP [7C03],0008
&0000:7C50 7406 JZ 7C58
&0000:7C52 2E CS:
&0000:7C53 C606647D02 MOV [7D64],02
&0000:7C58 BB0000 MOV BX,0000
&0000:7C5B 2E CS:
&0000:7C5C 8B0E037C MOV CX,[7C03]
&0000:7C60 51 PUSH CX
&0000:7C61 B009 MOV AL,09
&0000:7C63 2AC1 SUB AL,CL
&0000:7C65 B400 MOV AH,00
&0000:7C67 8BF0 MOV SI,AX
&0000:7C69 56 PUSH SI
&0000:7C6A 33D2 XOR DX,DX
&0000:7C6C 33C0 XOR AX,AX
&0000:7C6E 8AC5 MOV AL,CH
&0000:7C70 2E CS:
&0000:7C71 F636647D DIV BYTE [7D64]
&0000:7C75 8AE8 MOV CH,AL
&0000:7C77 8AF4 MOV DH,AH
&0000:7C79 8BC6 MOV AX,SI
&0000:7C7B B402 MOV AH,02
&0000:7C7D CD13 INT 13
&0000:7C7F 722D JC 7CAE
&0000:7C81 5E POP SI
&0000:7C82 59 POP CX
&0000:7C83 2E CS:
&0000:7C84 2936057C SUB [7C05],SI
&0000:7C88 741F JZ 7CA9
&0000:7C8A 8BC6 MOV AX,SI
&0000:7C8C 2E CS:
&0000:7C8D F726657D MUL WORD [7D65]
&0000:7C91 03D8 ADD BX,AX
&0000:7C93 FEC5 INC CH
&0000:7C95 B101 MOV CL,01
&0000:7C97 51 PUSH CX
&0000:7C98 BE0800 MOV SI,0008
&0000:7C9B 2E CS:
&0000:7C9C 3B36057C CMP SI,[7C05]
&0000:7CA0 7C05 JL 7CA7
&0000:7CA2 2E CS:
&0000:7CA3 8B36057C MOV SI,[7C05]
&0000:7CA7 EBC0 JMP 7C69
&0000:7CA9 EA00006000 JMP &0060:0000
&0000:7CAE BE677D MOV SI,7D67
&0000:7CB1 E80200 CALL 7CB6
&0000:7CB4 EBFE JMP 7CB4
The next chunk of code is similar to `disk_error` in the [PC-DOS 1.00 Boot Sector](/disks/pcx86/dos/ibm/1.00/#pc-dos-100-boot-sector):
&0000:7CB6 32FF XOR BH,BH
&0000:7CB8 2E CS:
&0000:7CB9 AC LODSB
&0000:7CBA 247F AND AL,7F
&0000:7CBC 740B JZ 7CC9
&0000:7CBE 56 PUSH SI
&0000:7CBF B40E MOV AH,0E
&0000:7CC1 BB0700 MOV BX,0007
&0000:7CC4 CD10 INT 10
&0000:7CC6 5E POP SI
&0000:7CC7 EBEF JMP 7CB8
&0000:7CC9 C3 RET
&0000:7CCA E933FF JMP 7C00
The next chunk of code is identical to `check_sys_files` in the [PC-DOS 1.00 Boot Sector](/disks/pcx86/dos/ibm/1.00/#pc-dos-100-boot-sector):
&0000:7CCD BB0000 MOV BX,0000
&0000:7CD0 B90400 MOV CX,0004
&0000:7CD3 B80102 MOV AX,0201
&0000:7CD6 CD13 INT 13
&0000:7CD8 1E PUSH DS
&0000:7CD9 7233 JC 7D0E
&0000:7CDB 8CC8 MOV AX,CS
&0000:7CDD 8ED8 MOV DS,AX
&0000:7CDF BF0000 MOV DI,0000
&0000:7CE2 B90B00 MOV CX,000B
&0000:7CE5 26 ES:
&0000:7CE6 800D20 OR [DI],20
&0000:7CE9 26 ES:
&0000:7CEA 804D2020 OR [DI+20],20
&0000:7CEE 47 INC DI
&0000:7CEF E2F4 LOOP 7CE5
&0000:7CF1 BF0000 MOV DI,0000
&0000:7CF4 BE8B7D MOV SI,7D8B
&0000:7CF7 B90B00 MOV CX,000B
&0000:7CFA FC CLD
&0000:7CFB F3 REPZ
&0000:7CFC A6 CMPSB
&0000:7CFD 750F JNZ 7D0E
&0000:7CFF BF2000 MOV DI,0020
&0000:7D02 BE977D MOV SI,7D97
&0000:7D05 B90B00 MOV CX,000B
&0000:7D08 F3 REPZ
&0000:7D09 A6 CMPSB
&0000:7D0A 7502 JNZ 7D0E
&0000:7D0C 1F POP DS
&0000:7D0D C3 RET
&0000:7D0E BE1B7D MOV SI,7D1B
&0000:7D11 E8A2FF CALL 7CB6
&0000:7D14 B400 MOV AH,00
&0000:7D16 CD16 INT 16
&0000:7D18 1F POP DS
&0000:7D19 F9 STC
&0000:7D1A C3 RET
### Additional Information From [PC DOS Retro](https://sites.google.com/site/pcdosretro/doshist)
- distributed on 1 160KB floppy disk