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Updated COMPAQ MS-DOS READMEs, along with a few PCx86 debugger improvements

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This commit is contained in:
Jeff Parsons 2017-06-30 12:03:35 -07:00 • committed by Jeff Parsons
commit 3abd13e8ca
11 changed files with 1803 additions and 1656 deletions
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97
disks/pcx86/dos/compaq/1.11/README.md
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@ -133,17 +133,45 @@ look at the code, using the PCjs Debugger to stop at the first instruction:
&0000:7C01 BCE701 MOV SP,01E7
&0000:7C04 B8C007 MOV AX,07C0
&0000:7C07 8ED0 MOV SS,AX
&0000:7C09 FB STI
&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).
;
&0000:7C0A 8ED8 MOV DS,AX
&0000:7C0C 8EC0 MOV ES,AX
&0000:7C0E 33C0 XOR AX,AX
;
; All of DS:0, ES:0, and SS:0 now point to the beginning of the boot sector, as does CS:7C00.
;
&0000:7C0E 33C0 XOR AX,AX ; perform a disk reset
&0000:7C10 CD13 INT 13
;
; 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
&0000:7C1D E88B00 CALL 7CAB ; call DISK_IO
&0000:7C20 EB1C JMP 7C3E
;
; ERROR: print error message and start over.
;
; 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..$
;
&0000:7C22 BBDA00 MOV BX,00DA
&0000:7C25 8A07 MOV AL,[BX]
&0000:7C27 3C24 CMP AL,24
@ -157,25 +185,54 @@ look at the code, using the PCjs Debugger to stop at the first instruction:
&0000:7C35 EBEE JMP 7C25
&0000:7C37 33C0 XOR AX,AX
&0000:7C39 CD16 INT 16
&0000:7C3B EBC3 JMP 7C00
&0000:7C3D C3 RET
&0000:7C3B EBC3 JMP 7C00 ; start over
&0000:7C3D C3 RET ; unreachable RET?
&0000:7C3E FC CLD
&0000:7C3F 8BF3 MOV SI,BX
&0000:7C41 BFC400 MOV DI,00C4
&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
&0000:7C49 75D7 JNZ 7C22 ; if first DIRENTRY was not "IOSYS.COM", jump to ERROR
&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
&0000:7C53 75CD JNZ 7C22 ; if second DIRENTRY was not "IOSYS.COM", jump to ERROR
&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
&0000:7C60 E84800 CALL 7CAB ; call DISK_IO to 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 ERROR 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
@ -203,6 +260,9 @@ look at the code, using the PCjs Debugger to stop at the first instruction:
&0000:7CA1 8B5414 MOV DX,[SI+14]
&0000:7CA4 E80400 CALL 7CAB
&0000:7CA7 FF2E5201 JMP FAR [0152]
;
; DISK_IO: read 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
@ -216,9 +276,10 @@ look at the code, using the PCjs Debugger to stop at the first instruction:
&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
&0000:7CC1 E95EFF JMP 7C22
&0000:7CBE 75EE JNZ 7CAE
&0000:7CC0 58 POP AX ; retry count exhausted
&0000:7CC1 E95EFF JMP 7C22 ; jump to PRINT function
>> 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
@ -241,6 +302,10 @@ 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.
### 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
@ -279,11 +344,15 @@ 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
mkdir COMPAQ-DOS111
cp -pr /Volumes/Untitled/ COMPAQ-DOS111
cp -pr /Volumes/Untitled Disk
[Return to [COMPAQ MS-DOS Disks](/disks/pcx86/dos/compaq/)]

2
disks/pcx86/dos/ibm/1.00/README.md
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@ -138,7 +138,7 @@ because it includes lots of commentary, allowing the boot sector to more or less
pop ax ; remaining sectors
call add_si_sectors ; bx += si*512
sub ax, si ; remaining -= read
jz done ; none left
jz done ; none left
inc ch ; next track
mov cl, 1 ; start at sector 1
mov si, 8 ; read up to 8 sectors