A number of years ago, the Computer History Museum together with Microsoft released the source code for MS-DOS 1.25 (very close to PC DOS 1.1) and MS-DOS 2.11. I never did anything with it beyond glancing at the code, in no small part because the release was rather poorly organized.[…]
The obvious gaping hole is the lack of any source code for IBMBIO.COM. I do not know exactly what arrangement IBM and Microsoft had at the time, but in the days of DOS 1.x and 2.x OEMs did not get the source code for IBMBIO.COM/IO.SYS suitable for PC compatibles.
I toyed with the idea of writing my own IBMBIO.COM replacement, but eventually gave up because it’s not a totally trivial piece of code and I had no real documentation to work with (until much later). The MSDOS.ASM source code obviously uses the IBMBIO interface, but makes no attempt to document it. The provided IO.ASM source is quite useful, but SCP’s hardware was different enough from the IBM PC that it is of limited utility.
So, disassembler it was, and I produced reconstructed source code for PC DOS 1.1 IBMBIO.COM. Actually assembling it turned out to be a bit of an adventure; more on that below.
More early DOS shenanigans to brighten your day.