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pcjs/disks/pcx86/dos/compaq/1.11/README.md
Jeff Parsons 6d8090649f More boot sector README updates
2017-07-01 13:20:05 -07:00

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---
layout: page
title: COMPAQ MS-DOS 1.11
permalink: /disks/pcx86/dos/compaq/1.11/
machines:
- id: ibm5150-compaq111
type: pcx86
config: /devices/pcx86/machine/5150/mda/64kb/debugger/machine.xml
resume: 1
autoMount:
A:
name: COMPAQ MS-DOS 1.11
B:
name: None
autoType: $date\r$time\r
---
COMPAQ MS-DOS 1.11
------------------
Released in 1983 by COMPAQ Computer Corp, this version of MS-DOS reports itself as:
The COMPAQ Personal Computer DOS
Version 1.11
(C) Copyright COMPAQ Computer Corp. 1982
(C) Copyright Microsoft 1981, 82
To learn more about this double-sided 320Kb diskette, see the
[Directory Listing](#directory-of-compaq-ms-dos-111-diskette) and [Boot Sector](#compaq-ms-dos-111-boot-sector) below.
{% include machine.html id="ibm5150-compaq111" %}
### Directory of COMPAQ MS-DOS 1.11 Diskette
Volume in drive A has no label
Directory of A:\
IOSYS COM 1998 05-13-83 12:00p
MSDOS COM 6136 05-13-83 12:00p
COMMAND COM 4959 05-13-83 12:00p
LINK EXE 41856 05-13-83 12:00p
EDLIN COM 2432 05-13-83 12:00p
SYS COM 645 05-13-83 12:00p
DEBUG COM 6619 05-13-83 12:00p
BASIC COM 549 05-13-83 12:00p
BASICA COM 547 05-13-83 12:00p
FORMAT COM 3296 05-13-83 12:00p
EXE2BIN EXE 1280 05-13-83 12:00p
DISKCOPY COM 1812 05-13-83 12:00p
DISKCOMP COM 1291 05-13-83 12:00p
MODE COM 1800 05-13-83 12:00p
CHKDSK COM 1720 05-13-83 12:00p
COMP COM 1484 05-13-83 12:00p
WORDS 1242 05-13-83 12:00p
BASICA EXE 54272 05-13-83 12:00p
TEST EXE 18560 05-13-83 12:00p
DEMO BAT 14 05-13-83 12:00p
DEMO BAS 7808 05-13-83 12:00p
DEMO1 BAS 17920 05-13-83 12:00p
DEMO2 BAS 17792 05-13-83 12:00p
DEMO3 BAS 21120 05-13-83 12:00p
DEMO4 BAS 6528 05-13-83 12:00p
JUGGLER DAT 16128 05-13-83 12:00p
USA DAT 16128 05-13-83 12:00p
INTEREST BAS 384 05-13-83 12:00p
28 file(s) 256320 bytes
Total files listed:
28 file(s) 256320 bytes
53248 bytes free
One curiosity regarding this disk are the BASIC files. **BASIC.COM** and **BASICA.COM** are nothing more than
tiny programs to load **BASICA.EXE**, a stand-alone version of BASIC that doesn't require any BASIC ROMs. And
**BASICA.EXE** reports a version number that differs from the DOS version:
The COMPAQ Personal Computer BASIC
Version 1.13
(C) Copyright COMPAQ Computer Corp. 1983
### COMPAQ MS-DOS 1.11 Boot Sector
The boot sector of the COMPAQ MS-DOS 1.11 disk image contains the following bytes:
00000000 fa bc e7 01 b8 c0 07 8e d0 fb 8e d8 8e c0 33 c0 |..............3.|
00000010 cd 13 b8 01 02 bb 00 02 b9 04 00 33 d2 e8 8b 00 |...........3....|
00000020 eb 1c bb da 00 8a 07 3c 24 74 0c 53 b4 0e bb 07 |.......<$t.S....|
00000030 00 cd 10 5b 43 eb ee 33 c0 cd 16 eb c3 c3 fc 8b |...[C..3........|
00000040 f3 bf c4 00 b9 0b 00 f3 a6 75 d7 83 c6 15 b9 0b |.........u......|
00000050 00 f3 a6 75 cd b8 01 02 bb 00 02 b9 02 00 33 d2 |...u..........3.|
00000060 e8 48 00 8a 27 80 fc fe be 26 01 74 0a 80 fc ff |.H..'....&.t....|
00000070 74 02 eb ae be 3c 01 b8 60 00 50 07 8b 04 33 db |t....<..`.P...3.|
00000080 8b 4c 02 8b 54 04 e8 22 00 8b 44 06 03 5c 08 8b |.L..T.."..D..\..|
00000090 4c 0a 8b 54 0c e8 13 00 8b 44 0e 03 5c 10 8b 4c |L..T.....D..\..L|
000000a0 12 8b 54 14 e8 04 00 ff 2e 52 01 bf 05 00 57 50 |..T......R....WP|
000000b0 cd 13 72 03 58 58 c3 33 c0 cd 13 58 5f 4f 75 ee |..r.XX.3...X_Ou.|
000000c0 58 e9 5e ff 49 4f 53 59 53 20 20 20 43 4f 4d 4d |X.^.IOSYS COMM|
000000d0 53 44 4f 53 20 20 20 43 4f 4d 0a 0d 4e 6f 6e 2d |SDOS COM..Non-|
000000e0 53 79 73 74 65 6d 20 64 69 73 6b 20 6f 72 20 64 |System disk or d|
000000f0 69 73 6b 20 65 72 72 6f 72 0a 0d 52 65 70 6c 61 |isk error..Repla|
00000100 63 65 20 61 6e 64 20 73 74 72 69 6b 65 20 61 6e |ce and strike an|
00000110 79 20 6b 65 79 20 77 68 65 6e 20 72 65 61 64 79 |y key when ready|
00000120 0a 0d 24 47 41 53 01 02 08 00 00 00 08 02 00 02 |..$GAS..........|
00000130 01 01 00 00 08 02 00 10 01 02 00 00 06 02 03 00 |................|
00000140 00 01 08 02 00 0c 01 01 00 00 03 02 00 10 01 01 |................|
00000150 00 01 00 00 60 00 28 43 29 43 6f 70 79 72 69 67 |....`.(C)Copyrig|
00000160 68 74 20 43 4f 4d 50 41 51 20 43 6f 6d 70 75 74 |ht COMPAQ Comput|
00000170 65 72 20 43 6f 72 70 6f 72 61 74 69 6f 6e 20 31 |er Corporation 1|
00000180 39 38 32 00 00 00 00 00 00 00 00 00 00 00 00 00 |982.............|
00000190 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
000001a0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
000001b0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
000001c0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
000001d0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
000001e0 00 00 00 00 00 00 00 e7 01 00 00 00 00 00 00 00 |................|
000001f0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
Unlike most DOS boot sectors, this boot sector doesn't start with an Intel x86 JMP instruction.
Let's take a look at the code, using the PCjs Debugger to stop at the first instruction:
>> bp 0:7c00
bp &0000:7C00 set
>> g
bp &0000:7C00 hit
stopped (5153504 opcodes, 42808074 cycles, 9014 ms, 4749065 hz)
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 FA CLI
>> u 7c00
&0000:7C00 FA CLI
&0000:7C01 BCE701 MOV SP,01E7
&0000:7C04 B8C007 MOV AX,07C0
&0000:7C07 8ED0 MOV SS,AX
&0000:7C09 FB STI
At this point, the stack has been set to an unused region INSIDE the boot sector, because
07C0:01E7 is the same as 0000:7C00+01E7 or 0000:7DE7. This puts a fairly low limit on stack
usage: as more stack is used, the COMPAQ copyright string will be trashed first, then boot
sector data, then boot sector code.
Curiously, the value 01E7 is also stored at 07C0:01E7. Also curious is the choice of an ODD
address for the stack (although, for the 8088, this presumably didn't affect performance).
After the next two instructions, all of DS:0, ES:0, and SS:0 point to the beginning of the
boot sector (just like CS:7C00):
&0000:7C0A 8ED8 MOV DS,AX
&0000:7C0C 8EC0 MOV ES,AX
This resets the drive:
&0000:7C0E 33C0 XOR AX,AX ; perform a drive reset
&0000:7C10 CD13 INT 13
Next, read 1 sector (the 4th sector, which is the first sector of the directory) into memory
immediately above the boot sector (07C0:0200):
&0000:7C12 B80102 MOV AX,0201
&0000:7C15 BB0002 MOV BX,0200
&0000:7C18 B90400 MOV CX,0004
&0000:7C1B 33D2 XOR DX,DX
&0000:7C1D E88B00 CALL 7CAB ; read the disk
&0000:7C20 EB1C JMP 7C3E
Next, the error handler and restart sequence:
&0000:7C22 BBDA00 MOV BX,00DA
&0000:7C25 8A07 MOV AL,[BX]
&0000:7C27 3C24 CMP AL,24
&0000:7C29 740C JZ 7C37
&0000:7C2B 53 PUSH BX
&0000:7C2C B40E MOV AH,0E
&0000:7C2E BB0700 MOV BX,0007
&0000:7C31 CD10 INT 10
&0000:7C33 5B POP BX
&0000:7C34 43 INC BX
&0000:7C35 EBEE JMP 7C25
&0000:7C37 33C0 XOR AX,AX
&0000:7C39 CD16 INT 16
&0000:7C3B EBC3 JMP 7C00 ; start over
&0000:7C3D C3 RET ; unreachable RET?
Here's the error message (setting DS:BX to 07C0:00DA references the same memory as CS:7CDA):
>> db cs:7cda cs:7d22
&0000:7CDA 0A 0D 4E 6F 6E 2D 53 79-73 74 65 6D 20 64 69 73 ..Non-System dis
&0000:7CEA 6B 20 6F 72 20 64 69 73-6B 20 65 72 72 6F 72 0A k or disk error.
&0000:7CFA 0D 52 65 70 6C 61 63 65-20 61 6E 64 20 73 74 72 .Replace and str
&0000:7D0A 69 6B 65 20 61 6E 79 20-6B 65 79 20 77 68 65 6E ike any key when
&0000:7D1A 20 72 65 61 64 79 0A 0D-24 ready..$
Next, we have the code to validate the directory contents:
&0000:7C3E FC CLD
&0000:7C3F 8BF3 MOV SI,BX
&0000:7C41 BFC400 MOV DI,00C4 ; ES:DI -> 7C0:C4 (aka 0:7CC4)
&0000:7C44 B90B00 MOV CX,000B
&0000:7C47 F3 REPZ
&0000:7C48 A6 CMPSB
&0000:7C49 75D7 JNZ 7C22 ; if first DIRENTRY was not "IOSYS.COM", jump to error handler
&0000:7C4B 83C615 ADD SI,0015
&0000:7C4E B90B00 MOV CX,000B
&0000:7C51 F3 REPZ
&0000:7C52 A6 CMPSB
&0000:7C53 75CD JNZ 7C22 ; if second DIRENTRY was not "IOSYS.COM", jump to error handler
&0000:7C55 B80102 MOV AX,0201
&0000:7C58 BB0002 MOV BX,0200
&0000:7C5B B90200 MOV CX,0002
&0000:7C5E 33D2 XOR DX,DX
&0000:7C60 E84800 CALL 7CAB ; read 1st FAT sector
At this point, the directory sector is gone, and all this code did with it was verify the names of
the first two entries. It did not examine or record their starting cluster numbers. And in the FAT
sector just read, only one byte (the first byte) will be examined next: the media ID byte.
Further reading will rely on on two hard-coded tables inside the boot sector: one table at DS:126 (CS:7D26)
if the FAT media ID byte is 0xFE (160Kb diskette), or another table at DS:13C (CS:7D3C) if the FAT media ID
byte is 0xFF (320Kb diskette). If the FAT media ID byte is neither, off to the error handler we go.
Here's the table for the 160Kb diskette:
>> dw cs:7d26 l11.
&0000:7D26 0201 0008 0000 0208 0200 0101 0000 0208 ................
&0000:7D36 1000 0201 0000 ......
and here's the table for the 320Kb diskette:
>> dw cs:7d3c l11.
&0000:7D3C 0206 0003 0100 0208 0C00 0101 0000 0203 ................
&0000:7D4C 1000 0101 0100 ......
These tables are then followed by a DWORD jump address:
>> dw cs:7d52 l2
&0000:7D52 0000 0060 ..`.
which means that the "JMP FAR [0152]" instruction will jump to 60:0 after the reads are complete.
&0000:7C63 8A27 MOV AH,[BX]
&0000:7C65 80FCFE CMP AH,FE
&0000:7C68 BE2601 MOV SI,0126
&0000:7C6B 740A JZ 7C77
&0000:7C6D 80FCFF CMP AH,FF
&0000:7C70 7402 JZ 7C74
&0000:7C72 EBAE JMP 7C22
&0000:7C74 BE3C01 MOV SI,013C
&0000:7C77 B86000 MOV AX,0060
&0000:7C7A 50 PUSH AX
&0000:7C7B 07 POP ES
&0000:7C7C 8B04 MOV AX,[SI]
&0000:7C7E 33DB XOR BX,BX
&0000:7C80 8B4C02 MOV CX,[SI+02]
&0000:7C83 8B5404 MOV DX,[SI+04]
&0000:7C86 E82200 CALL 7CAB
&0000:7C89 8B4406 MOV AX,[SI+06]
&0000:7C8C 035C08 ADD BX,[SI+08]
&0000:7C8F 8B4C0A MOV CX,[SI+0A]
&0000:7C92 8B540C MOV DX,[SI+0C]
&0000:7C95 E81300 CALL 7CAB
&0000:7C98 8B440E MOV AX,[SI+0E]
&0000:7C9B 035C10 ADD BX,[SI+10]
&0000:7C9E 8B4C12 MOV CX,[SI+12]
&0000:7CA1 8B5414 MOV DX,[SI+14]
&0000:7CA4 E80400 CALL 7CAB
&0000:7CA7 FF2E5201 JMP FAR [0152]
The final chunk of code is the disk read interface, which reads AL sector(s) identified by the CHS values in CX:DX
into ES:BX, using DI as a retry counter:
&0000:7CAB BF0500 MOV DI,0005
&0000:7CAE 57 PUSH DI
&0000:7CAF 50 PUSH AX
&0000:7CB0 CD13 INT 13
&0000:7CB2 7203 JC 7CB7
&0000:7CB4 58 POP AX
&0000:7CB5 58 POP AX
&0000:7CB6 C3 RET
&0000:7CB7 33C0 XOR AX,AX
&0000:7CB9 CD13 INT 13
&0000:7CBB 58 POP AX
&0000:7CBC 5F POP DI
&0000:7CBD 4F DEC DI
&0000:7CBE 75EE JNZ 7CAE
&0000:7CC0 58 POP AX ; retry count exhausted
&0000:7CC1 E95EFF JMP 7C22 ; jump to PRINT function
And then we have the assorted strings and tables:
>> db 7cc4 l13c
&7CC4 49 4F 53 59 53 20 20 20-43 4F 4D 4D 53 44 4F 53 IOSYS COMMSDOS
&7CD4 20 20 20 43 4F 4D 0A 0D-4E 6F 6E 2D 53 79 73 74 COM..Non-Syst
&7CE4 65 6D 20 64 69 73 6B 20-6F 72 20 64 69 73 6B 20 em disk or disk
&7CF4 65 72 72 6F 72 0A 0D 52-65 70 6C 61 63 65 20 61 error..Replace a
&7D04 6E 64 20 73 74 72 69 6B-65 20 61 6E 79 20 6B 65 nd strike any ke
&7D14 79 20 77 68 65 6E 20 72-65 61 64 79 0A 0D 24 47 y when ready..$G
&7D24 41 53 01 02 08 00 00 00-08 02 00 02 01 01 00 00 AS..............
&7D34 08 02 00 10 01 02 00 00-06 02 03 00 00 01 08 02 ................
&7D44 00 0C 01 01 00 00 03 02-00 10 01 01 00 01 00 00 ................
&7D54 60 00 28 43 29 43 6F 70-79 72 69 67 68 74 20 43 `.(C)Copyright C
&7D64 4F 4D 50 41 51 20 43 6F-6D 70 75 74 65 72 20 43 OMPAQ Computer C
&7D74 6F 72 70 6F 72 61 74 69-6F 6E 20 31 39 38 32 00 orporation 1982.
&7D84 00 00 00 00 00 00 00 00-00 00 00 00 00 00 00 00 ................
&7D94 00 00 00 00 00 00 00 00-00 00 00 00 00 00 00 00 ................
&7DA4 00 00 00 00 00 00 00 00-00 00 00 00 00 00 00 00 ................
&7DB4 00 00 00 00 00 00 00 00-00 00 00 00 00 00 00 00 ................
&7DC4 00 00 00 00 00 00 00 00-00 00 00 00 00 00 00 00 ................
&7DD4 00 00 00 00 00 00 00 00-00 00 00 00 00 00 00 00 ................
&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 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.
Finally, notice the letters `GAS` in the boot sector at offset 0x7D23. Presumably, they're the initials of
author, and somewhat less conspicuous and more modest than the `Robert O'Rear` signature in the
[PC-DOS 1.00 Boot Sector](/disks/pcx86/dos/ibm/1.00/#pc-dos-100-boot-sector).
### Mounting the COMPAQ MS-DOS 1.11 Diskette
Note that the disk cannot be mounted by a modern operating system (e.g., macOS), because it lacks a proper
BPB at offset 0x000B.
To resolve the mounting problem, one solution is to use the [DiskDump](/modules/diskdump/) utility's *--forceBPB*
option, which creates a mountable disk image by making the following boot sector modifications:
00000000 eb fe 90 50 43 4a 53 2e 4f 52 47 00 02 02 01 00 |...PCJS.ORG.....|
00000010 02 70 00 80 02 ff 01 00 08 00 02 00 00 00 00 00 |.p..............|
00000020 00 00 00 00 00 8a 07 3c 24 74 0c 53 b4 0e bb 07 |.......<$t.S....|
The following (hex) byte ranges are modified:
- 0000-0002: 2-byte Intel x86 JMP instruction followed by 1-byte x86 NOP
- 0003-000A: 8-byte OEM signature string (we use the fake "PCJS.ORG" OEM signature)
- 000B-001D: 19-byte BPB describing a 320Kb diskette image with 8 sectors/track and 2 heads
Ordinarily, only the 19-byte BPB would be required, but it turns out that macOS won't mount the disk image
unless the boot sector *also* begins with an Intel x86 JMP instruction. So the *--forceBPB* option updates all
30 bytes at the beginning of the boot sector, making the disk image mountable, but also rendering it unbootable --
which is OK if all you want to do is mount the image and copy files from it.
One common misconception is that the last word of a mountable boot sector must also contain 0xAA55 (ie, the low
byte at offset 0x1FE must be 0x55 and the high byte at offset 0x1FF must be 0xAA). This claim is often backed up
by pointing out that some (newer) ROMs will not boot from a disk if that signature is missing.
However, that evidence is misleading, because those ROMs check only *hard disk* boot sectors for the 0xAA55 signature,
not diskettes. Which makes sense, because hard disk support didn't exist in DOS until version 2.0 -- the same version
of DOS that also introduced BPBs and the 0xAA55 boot sector signature.
Here's how you can use [DiskDump](/modules/diskdump/) to a mountable disk. Note that *two* DiskDump commands are
required, because BPB modification only happens when converting an IMG file to a JSON file; the second DiskDump command
converts the modified JSON back into an IMG file.
diskdump --disk=COMPAQ-DOS111.img --format=json --forceBPB --output=COMPAQ-DOS111-BPB.json
warning: BPB has been updated
327680-byte disk image saved to COMPAQ-DOS111-BPB.json
diskdump --disk=COMPAQ-DOS111-BPB.json --format=img --output=COMPAQ-DOS111-BPB.img
327680-byte disk image saved to COMPAQ-DOS111-BPB.img
The next series of commands make the image read-only (otherwise, macOS may create hidden files inside the image after
mounting it), mount the image as `/Volumes/Untitled`, and then copy the contents of the image to a folder named `Disk`:
chmod -w COMPAQ-DOS111-BPB.img
open COMPAQ-DOS111-BPB.img
cp -pr /Volumes/Untitled Disk
[Return to [COMPAQ MS-DOS Disks](/disks/pcx86/dos/compaq/)]
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