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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
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12
disks/pcx86/dos/compaq/1.11/README.md
View file

@ -302,9 +302,15 @@ look at the code, using the PCjs Debugger to stop at the first instruction:
&7DE4 00 00 00 E7 01 00 00 00-00 00 00 00 00 00 00 00 ................
&7DF4 00 00 00 00 00 00 00 00-00 00 00 00
In terms of raw code (90 instructions), it's shorter than the PC-DOS 1.00 boot sector (99 instructions),
even though it supports twice as many disk formats. This is because it uses tables to describe both the
different disk formats *and* the discrete read operations, relieving the code from dealing with track boundaries.
In terms of raw code (90 instructions), it's shorter than both the
[PC-DOS 1.00 Boot Sector](/disks/pcx86/dos/ibm/1.00/#pc-dos-100-boot-sector) (99 instructions) and
[PC-DOS 1.10 Boot Sector](/disks/pcx86/dos/ibm/1.10/#pc-dos-110-boot-sector) (116 instructions),
because it uses tables to describe both the different disk formats *and* the discrete read operations,
relieving the code from dealing with head and track size calculations.
It's arguably an improvement over the [PC-DOS 1.10 Boot Sector](/disks/pcx86/dos/ibm/1.10/#pc-dos-110-boot-sector),
because it didn't need to be altered to support both 160Kb and 320Kb diskettes. On the other hand, that flexibility
also makes it slightly slower to boot, because it must read the first FAT sector to determine the diskette format.
### Mounting the COMPAQ MS-DOS 1.11 Diskette

151
disks/pcx86/dos/ibm/1.10/README.md
View file

@ -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