There was also Concurrent CP/M (possibly only a /86 variant).
On Sat, May 4, 2024 at 2:02 AM Bill Degnan via cctalk <cctalk(a)classiccmp.org>
wrote:
  Herb Johnson has some good info on the history of cp/m
here
 
https://www.retrotechnology.com/#dri
 Bill
 On Fri, May 3, 2024, 8:23 PM Murray McCullough via cctalk <
 cctalk(a)classiccmp.org> wrote:
  I came across an article that said CP/M came out
in April 1974. I
 
 remember
  using this OS in the microcomputer world in the
late 70’s; early 80’s. It
 came from PL/M, (Programming Language for Microcomputers) later renamed
 CP/M(Control Program for Microcomputers). I’m not sure what its legacy is
 though as far as I can recall it was wrapped up in litigation for quite
 some time. It was used in the 8-bit world but not sure what it's role was
 in the early PC world!
 Happy computing,
 Murray 🙂