> 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 Res

> 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.