PC-DOS 1.1 Hacking - Playing with PC-DOS 1.10 bios/dos and command.com

Sources

The experiment started from the vintage MS-DOS code opened by Microsoft:

https://github.com/microsoft/MS-DOS/blob/master/v1.25/source/COMMAND.ASM

ibmbio.com sources are retrieved from https://www.os2museum.com/wp/pc-dos-1-1-from-scratch

MIT License (In line with Source License):

First Contact

There is a first failure on EQU symbol redefined. This prevents compilation.

156c156
< ZERO    EQU     $
---
> ZERO    =       $

To get to the official PC-DOS binary production:

A triple NOP replaces the MS instruction MOV [COMFCB],AL. This change fixes reload from command.com on drive A: by not overriding the first FCB byte with 0 (Default drive).

469a474,479
>         IF IBMVER
>         NOP
>         NOP
>         NOP
>         ENDIF
>         IF MSVER
470a481
>         ENDIF
2166d2176
<

The 2 previous changes allow recreation of the PC-DOS exact command.com, when MSVER is set to FALSE and IBMVER is set to TRUE

A third small change has been added in MSVER mode, before opening the command.com file. To be checked if this is really necessary ...

>         IF MSVER
>         MOV     AL, 0
>         MOV     [COMFCB], AL
>         ENDIF

Those variations beetween IBM and Microsoft seem to be linked to the command.com file being searched on the default drive (Microsoft) or on drive A: (IBM).

Configs

msdosenh: slightly enhanced PC-DOS 1.1 command.com marked as 1.25A/1.17A
- VER command
- Addition of command.com search on A: after default drive search failed
msdosorg: MS-DOS 1.25 command 1.17 vanilla command.com rebuilt
- Addition of FCB Drive 0'ing from Zenith ZDOS before opening command.com
pcdosenh: slightly enhanced PC-DOS 1.1 command.com marked as 1.10A
- VER command
- CLS command (BIOS)
- Check for command in A: if not found
- External MEM command
pcdosorg: PC-DOS 1.1 command.com rebuilt
- Checked against PC-DOS 1.1 distribution
- https://github.com/microsoft/MS-DOS/blob/master/v1.25/bin/COMMAND.COM
- SHA256(pcdosorg.com)= 84034261608f7b9d38a6b81b6896bb5cc45dc5ebfae0b0927081f86f354aa571

TODO: Other little changes

pcdosenh:
- Search path on A: if default drive seach fails
- Investigate further other MS-DOS 1.25 variations
bios and dos
format.com has been patch to format disk that can work with all emulators including qemu. the signature 55AA is added at the end of the 512 byte boot sector.

Build/Test Dependencies

Linux/macOS

pce emulator (test)
emu2 (for build from macosx/linux)
pc-dos 1.1 floppy image (for test if available)
exe2bin.exe from https://github.com/microsoft/MS-DOS/blob/master/v1.25/bin/EXE2BIN.EXE
ibm link.exe 1.10 from https://github.com/microsoft/MS-DOS/blob/master/v1.25/bin/LINK.EXE
microsoft masm.exe 1.10 (patched, cf below) https://github.com/microsoft/MS-DOS/blob/master/v2.0/bin/MASM.EXE

DOS

exe2bin.exe from https://github.com/microsoft/MS-DOS/blob/master/v1.25/bin/EXE2BIN.EXE
ibm link.exe from https://github.com/microsoft/MS-DOS/blob/master/v1.25/bin/LINK.EXE
microsoft masm.exe 1.10 (patched, cf below) https://github.com/microsoft/MS-DOS/blob/master/v2.0/bin/MASM.EXE

Build

Use Makefile on Linux, macOS
Use Makefile.dos on Win32, DOS (builds only PC-DOS configs)

MASM Fix

The 1.10 version of MASM part of the MS-DOS 2.00 distribution hangs on emulators, including dosbox and emu2.

There is a detailed explanation at: https://slions.net/threads/debugging-ibm-macro-assembler-version-1-00.33/

The execution of the Faulty code looks like this:

11AD:0000 B83B0F           MOV     AX,0F3B
11AD:0003 8ED8             MOV     DS,AX
11AD:0005 8C062400         MOV     [0024],ES
11AD:0009 FA               CLI
11AD:000A 8ED0             MOV     SS,AX
11AD:000C 268B1E0200       MOV     BX,ES:[0002]
11AD:0011 2BD8             SUB     BX,AX
11AD:0013 81FB0010         CMP     BX,1000
11AD:0017 7E03             JLE     001C

The fix is just about changing the JLE into JBE (7E03 -> 7603)

11AD:0000 B83B0F           MOV     AX,0F3B
11AD:0003 8ED8             MOV     DS,AX
11AD:0005 8C062400         MOV     [0024],ES
11AD:0009 FA               CLI
11AD:000A 8ED0             MOV     SS,AX
11AD:000C 268B1E0200       MOV     BX,ES:[0002]
11AD:0011 2BD8             SUB     BX,AX
11AD:0013 81FB0010         CMP     BX,1000
11AD:0017 7603             JBE     001C

The masm binary checked in git is fixed.

tsupplis / pcdos11-hacking