> 22 system calls ... that had the same function (and function number) as the CP/M code. That probably shows evidence of reverse engineering, not code copying, similar to the clean-room reimplementation of IBM's BIOS chips by Compaq. > the types of actions that send/receive text from a printer, phone, hard disk, etc. I know they are giving examples for a modern audience, but I was amused by the inclusion of phones in…
No reverse engineering necessary. Those CP/M system calls were all documented, and it never was a secret that QDOS was heavily modelled after CP/M.
Indeed. In fact, when IBM first built the PC, they'd tried to reach a deal with Digital Research to get an official version of CP/M as the PC's flagship OS. The deal fell through for some reason I can't recall at the moment, so IBM went to MicroSoft and explicitly asked for a CP/M work-alike and a BASIC interpreter (which was considered MS's forte back then). This was really a no-brainer, given how supremely popular CP/M and BASIC were on microcomputers at the time.
After Digital Research heard about the QDOS/PC-DOS deal, they threatened to sue IBM, alleging even then that it infringed their intellectual property. But they gave IBM an option: also sell CPM/86 alongside PC-DOS, and IBM gladly obliged. Problem was: PC-DOS cost something like $50 whereas CPM/86 cost something like $250, and consumers voted with their wallets.
(I recently read somewhere that IBM didn't tell DRI about their planned price tag for PC-DOS, and Gary Kildall was right-miffed about the situation).
Besides those two, UCSD Pascal was also available to run on the bare metal of PC, essentially as an OS. I've never used it myself, so I don't know what its OS-y features were like, or if it had any at all.