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As a 60+ year old programmer who's pretty familiar with writing code for PDP-11s (down to device driver level) I'm interested in the link from PDPs to CP/M, whi

by zh3 5y ago

As a 60+ year old programmer who's pretty familiar with writing code for PDP-11s (down to device driver level) I'm interested in the link from PDPs to CP/M, which (as with MSDOS) I've also written for at a low level. While the link from CP/M to QDOS/MSDOS is clear to me, the jump from PDP11 trap instructions to CP/M's BDOS invocation is less clear. Is is the general idea of system calls/returns, or a deeper connection I've missed?

cmrdporcupine 5y ago
What I remember reading is that Gary Kildall was a big RT-11 and OS-8 user and so it's more that the shell/command line was patterned after some things he was familiar with there. I don't think anybody has made the claim that CP/M's internals were patterned off of PDP OS internals as much. More interface conventions.
- zh3 5y ago
  I didn't know that; having RT-11 at work and running CP/M at home, there was a 'sort of' similiarity there but not like the same kind of underlying structure between CP/M and DOS (hence the reason I was asking). Thanks!
WalterBright 5y ago
I don't see any conceptual difference between the methods used to interface with the OS. It's more one of detail.
zabzonk 5y ago
I'm also a 60+ year old programmer (let's make that 60++), but I'm not clear what you are asking. I was quite proud of a Forth-like language I wrote using CP/M (Wordstar, assembler) back round about 1980 - but once it was loaded into memory (via CP/M) it didn't use CP/M BDOS at all, system calls or trap instructions.
- zh3 5y ago
  That's interesting, how did you do I/O? Bypassing the OS was a lot easier back then :)
  - zabzonk 5y ago
    Direct memory access, BIOS (such as it was). I was actually toying with a version that would be fully CP/M hosted, so that it could read and write CP/M files but then I got a proper paying programming job - previously I was a microbiologist, moonlighting, and who could resist the delights of a DecSystem10? And then an IBM 4381?

by zh3 5y ago

cmrdporcupine 5y ago

What I remember reading is that Gary Kildall was a big RT-11 and OS-8 user and so it's more that the shell/command line was patterned after some things he was familiar with there. I don't think anybody has made the claim that CP/M's internals were patterned off of PDP OS internals as much. More interface conventions.

zh3 5y ago
I didn't know that; having RT-11 at work and running CP/M at home, there was a 'sort of' similiarity there but not like the same kind of underlying structure between CP/M and DOS (hence the reason I was asking). Thanks!

WalterBright 5y ago

I don't see any conceptual difference between the methods used to interface with the OS. It's more one of detail.

zabzonk 5y ago

I'm also a 60+ year old programmer (let's make that 60++), but I'm not clear what you are asking. I was quite proud of a Forth-like language I wrote using CP/M (Wordstar, assembler) back round about 1980 - but once it was loaded into memory (via CP/M) it didn't use CP/M BDOS at all, system calls or trap instructions.

zh3 5y ago
That's interesting, how did you do I/O? Bypassing the OS was a lot easier back then :)
- zabzonk 5y ago
  Direct memory access, BIOS (such as it was). I was actually toying with a version that would be fully CP/M hosted, so that it could read and write CP/M files but then I got a proper paying programming job - previously I was a microbiologist, moonlighting, and who could resist the delights of a DecSystem10? And then an IBM 4381?