CP/M creator Gary Kildall's memoirs released as free download

"Glenn came to my tool shed computer room in 1975, so we could "adapt" CP/M to the IMSAI hardware. What this means is that I would rewrite the parts of CP/M that manage things like diskette controllers and CRTs.

Well. come on, I'd already done this so many times that the tips of my fingers were wearing thin, so I designed a general interface, which I called the BIOS (BASIC I/O System) that a good programmer could change on the spot for their hardware. This little BIOS arrangement was the secret to the success of CP/M.

With the BIOS in place, a programmer could make CP/M work with their specialized hardware. With all those hobbyists out there, believe me, there was no shortage of specialized hardware. Glenn and I built a BIOS that afternoon and stuck CP/M on an IMSAI. He demo'd it to Ed Faber and the IMSAI engineers, and they loved it."

Writing a BIOS for a new machine in a single afternoon. Those were the days...

In retrospect, MS-DOS was a rather trivial program. Sometimes I wonder why I and/or many others did not write an equivalent, even just for fun.