The IBM guys had gone to Seattle expecting to license CP/M. Their "benchmark" (in business terms) was the Apple II with Microsoft's Z80 SoftCard running CP/M. As far as they could tell, Microsoft owned CP/M. Bill Gates told them otherwise, phoned Gary Kildall but had to be vague and then sent the IBM guys on a plane to California. I can only guess they were pretty upset even before they ever got to DRI.
People's memories of what happened vary. One of the IBM guys claimed no meeting happened and they went back to Seattle even more upset (which is the most popular version). Gary claims that after some back and forth between Dorothy and her lawyer she did sign the NDA and that he arrived towards the end of the meeting and had a nice chat about the technical side with the IBM people.
The PC was launched with a choice of three operating systems: PC-DOS (the licensed MS-DOS), CP/M86 and UCSD Pascal. But the first cost less than a third of the other two.
As I recall, PC-DOS sold for $40. CP/M86 sold for $180. People overwhelmingly bought PC-DOS because it was cheaper and nobody was able to explain why CP/M86 was worth the much higher price.
The company I worked for bought both, and I tried CP/M86. There was no reason to prefer it over PC-DOS, and that was that.
PC-DOS's predecessor, 86-DOS, was allegedly reverse engineered from CP/M. The subsequent lower development costs probably account for at least some of the price difference between the two products.
CP/M was (mostly) written in the PL/M language and ran on the 8-bit 8080/Z80 processors. 86-DOS was written in pure assembly language and ran on the 16-bit 8086 processor. Tim Paterson, its author, wrote an 8086 assembler and an 8080-to-8086 source code translator that ran on a CP/M machine. He used these tools to bootstrap 86-DOS.
86-DOS was intentionally designed to mimic the CP/M APIs to make it easy to port CP/M applications to 86-DOS through mechanical translation of the source code.
(And, surprise surprise, much of the business software that was available for IBM PC-DOS in the first couple of years were direct ports of existing CP/M applications: SuperCalc, WordStar, dBase II, etc.)
> CP/M was (mostly) written in the PL/M language and ran on the 8-bit 8080/Z80 processors. 86-DOS was written in pure assembly language and ran on the 16-bit 8086 processor.
In CP/M-80 1.x, most of the OS was written in PL/M. By the time we get to CP/M-80 2.x, the core of the OS (BDOS) has been rewritten in assembler for improved speed, while utilities (such as PIP) remain in PL/M.
CP/M-68K was written in C (although possibly with an earlier version in Pascal???). CP/M-8000 was written in C, probably ported from CP/M-68K.
CP/M-86 was mostly written in assembler, possibly with some bits (especially utilities) in PL/M and/or C as well.
And CP/M copied from DEC's operating systems, most especially RT/11.
I haven't examined the code myself, but the source code to the original PC-DOS is available and can be compared with CP/M. I've been told they were not copies.
Tim Paterson, of Seattle Computer Products and later Microsoft, gave a good talk at VCF West 2019 where he describes the history of 86-DOS (licensed to become PC-DOS and MS-DOS). There was an intention for significant API compatibility with CP/M, but no code was taken from CP/M.
People's memories of what happened vary. One of the IBM guys claimed no meeting happened and they went back to Seattle even more upset (which is the most popular version). Gary claims that after some back and forth between Dorothy and her lawyer she did sign the NDA and that he arrived towards the end of the meeting and had a nice chat about the technical side with the IBM people.
The PC was launched with a choice of three operating systems: PC-DOS (the licensed MS-DOS), CP/M86 and UCSD Pascal. But the first cost less than a third of the other two.