Gary Kildall 40 Years CP/M old source Code | HackaDay.com / ComputerHistory.org

'...CP/M pioneered the idea of a ROM based Basic Input Output/System (BIOS) for commonly used routines on a given computer. The use of BIOS made CP/M easy to port. Eventually it was ported to thousands of different machines and architectures, including the Altair, IMSAI 8080, C-64, and C-128 and Apple II systems.

Gary and his company Digital Research, were one of the top contenders for the operating system on IBM’s new personal computer. Ultimately, Microsoft got the job by purchasing 86-DOS from Seattle Computer Products. Somewhat ironically, 86-DOS itself was written based on the CP/M Application Programming interface (API)...'

Source: Gary Kildall: CP/M Source Code Released

which link to: ComputerHistory.org Early Digital Research CP/M Source Code