Building MS-DOS 4.00 under OS/2 2.x

Building & Booting DOS on OS/2!

Now that we’ve moved beyond the initial shockwave of the MS-DOS 4.00 source code dump, I thought it was time to try to pull off the ultimate trick of the time, building under OS/2 and using the exciting feature of the time “DOS from Drive A:” Long before VMware / Virtual PC for the PC OS/2 took Intel’s 80386’s hardware virtualization mode, “v86 mode” to the logical conclusion allowing you to boot native MS-DOS under OS/2. Sadly, the old 1989-1991 OS/2 betas do not include this feature. Although I have to wonder if it did exist and just wasn’t publicly available.

Many of the programs used to build MS-DOS are off the shelf, the MASM assembler, Microsoft C 5.1, and its associated tools are just retail versions. To change things up, I did use the 386MASM assembler just to see if it maintained MASM 5.1 compatibility. And it does. The only gotcha is that all the tools are *NOT* marked Presentation Manager compatible, so launching them from a window opens a full screen session. Very annoying!


I’m guessing the fix is in a toolkit? Either way, in Microsoft C 6.0, the utility exehdr lets us modify an OS/2 executable so it’ll now be WINDOWCOMPAT. So at least it ‘feels’ better now.

One thing is for sure, building DOS under OS/2 is a lot more enjoyable than doing a native build as you can minimize the build task, although the MS-DOS only programs do pop up when it generates text indexes & tables. But you do retain some control of your machine during the build, which is great! Although E is a terrible editor for source code, and the one in 6.78 has a nasty bug where it’ll truncate large files. Were people really using text mode editors for everything back then? I guess i like the fonts of the GUI, despite having used machines of the era.

Otherwise, the end result is the same, you get a build of DOS 4.

I went ahead and tried to build using 6.78 and no doubt compiling DOS is an absolute torture test. So far, the DOS Box has locked OS/2 once, and PM Shell has crashed once as well.

I altered the Makefiles to use ‘rm’ instead of the built in ‘del’ command, because if you try to delete a file that doesn’t exist, del returns an error, which then triggers an end to the NMAKE process. Very annoying! However, the ‘rm’ included in Microsoft C 5.1 doesn’t suffer the same defect. Using 86Box with an 83Mhz Pentium OverDrive it took about 18 minutes to build DOS-4.

Building DOS 4 on OS/2 6.78

I did capture the video and converted it to a GIF so you can quickly see the reboot & the UI crashing. FUN!

Compiled under OS/2

And it even boots!

For anyone interested I’ve put zips on that can be extracted under OS/2. I also made a pkzip disk set incase loading a 6MB zip file is an issue.

Building MS-DOS 4.00 under OS/2 2.x : neozeede : Free Download, Borrow, and Streaming : Internet Archive

Leave a Reply