Building MS-DOS 2.11 today from reconstructed source and tools #VintageComputing

After having good luck with rebuilding core PC DOS 1.1 from source code, Michal Necasek decided to do the same with the DOS 2.11 source code released by the Computer History Museum (CHM). An extensive collection of notes were compiled while banging the released source code into shape. That turned out to be a lot harder than building DOS 1.1 for two reasons.

  • One is that the released DOS 2.11 source code is a lot more extensive and includes source code for numerous utilities (CHKDSK, DEBUG, EDLIN, SYS, etc.).
  • The other, bigger reason is that the CHM unfortunately created a bit of a mess when releasing the code and sorting out the pieces was not trivial.

The CHM placed all DOS 2.x related files in just two directories, ‘v20object’ and ‘v20source’. It is now clear that the files came from at least three distinct sources:

  • MS-DOS 2.00 OEM distribution disks
  • MS-DOS 2.11 source code of unknown provenance
  • Miscellaneous debris such as WordStar 3.20 overlay files

I determined that the entire DOS 2.11 source code can be successfully built with MASM 1.25 from 1983, about the same vintage as the source code. Whether that was actually the version used is anyone’s guess at this point, but it easily could have been.

The post is a treasure trove as to how DOS was created for both IBM and for third party providers (OEMs).

Read more in the detailed post on the OS/2 Museum Website.

Also see the previous post Microsoft releases MS-DOS v1.25 and v2.0 Source Code as Open Source on GitHub


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