About Archive Tags RSS Feed


Entries tagged cpm-dist

The CP/M emulator development continues

4 May 2024 12:00

In my previous post I introduced a toy CP/M Emulator I'd been working on.

At the time it was capable of running the Infocom text-based adventure games, so I thought it was done. Of course I also wanted to run Microsoft's original BASIC and it turned out that was a challenge because the coding of their interpreter didn't use the standard CP/M entry-point for making syscalls (call 0x0005).

Instead of calling 0x0005 to invoke the BDOS/BIOS functions the BASIC interpreter used the single-byte CALL instructions which are available on the Z80 processor. There are a bunch of these instructions:

  • RST 00
  • RST 08
  • RST 10
  • RST 18
  • RST 20
  • RST 28
  • RST 30
  • RST 38

Each of those instructions is equivalent to a call instruction with a fixed offset, "call 0x0010", "call 0x0020", etc. I had to rework the emulator to cope with this approach, which causes repetition but nothing too surprising. The end result is that now my emulator can run Microsoft Basic, Tasty Basic, and some more programs.

Things work but a couple of the syscalls are of the form "Return true if there is a pending keystroke", or "wait until there is keyboard input present and return the first character". I have some busy-loops which peg the CPU, which sucks but works. On the downside running the code on a MacOS machine has some weird issues with repeated keys and similar. So I need to look into fixing that for my own sense of peace.

I put together a little repository of binaries for playing with though, and that's been helpful. My emulator has a special flag which treats sub-directories as "Drives". So A: points to A/, B: points to B/, etc. That makes distributing and working with things easy!

| No comments


The CP/M emulator is working well

25 May 2024 12:00

In my recent posts I've talked about implementing BDOS and BIOS syscalls for my cp/m emulator. I've now implemented enough of the calls that I can run many of the standard binaries:

  • The Aztech C Compiler
  • Microsoft BASIC
  • Turbo Pascal
  • Wordstar
  • etc

Of course I've not implemented all the syscalls, so the emulation isn't 100% perfect and many binaries won't run. But I sent myself on a detour by implementing extra syscalls, custom syscalls.

Traditionally CP/M systems are "rebooted" by pressing Ctrl-C at the CCP prompt. I thought that was something I'd press by accident so I implemented the restart behaviour only when the user pressed Ctrl-C twice in a row. But then I added a custom syscall that lets you change hte value:

The Ctrl-C count is currently set to 2
A>ctrlc 1
The Ctrl-C count is currently set to 1

So you can now change the value at runtime. Similarly there is support for switching CCP at runtime, and even changing the default output-device from ADM-3A to ANSI, or vice-versa. It's kinda neat to make these kind of extensions, and happily the traditional BIOS has two syscalls reserved for custom use so I just used one of those.

I've added support for testing whether a binary is running under my emulator, or not, using a custom syscall. So I can run:

This binary is running under cpmulator:

cpmulator unreleased

On another emulator I see this:

Illegal BIOS call 31
No, this binary is not running under cpmulator.

Anyway I'm happy with the current state of things, and I fixed a couple of bugs which means I now have support for SUBMIT.COM which is a real time-saver.

| No comments


The CP/M emulator is good enough, I think.

9 July 2024 20:00

My previous post mentioned that I'd added some custom syscalls to my CP/M emulator and that lead to some more updates, embedding a bunch of binaries within the emulator so that the settings can be tweaked at run-time, for example running:

!CCP ccpz
!CONSOLE adm-3a

Those embedded binaries show up on A: even if they're not in the pwd when you launch the emulator.

Other than the custom syscalls I've updated the real BDOS/BIOS syscalls a bit more, so that now I can run things like the Small C compiler, BBC BASIC, and more. (BBCBasic.com used to launch just fine, but it turned out that the SAVE/LOAD functions didn't work. Ooops!)

I think I've now reached a point where all the binaries I care about run, and barring issues I will slow down/stop development. I can run Turbo Pascal, WordStar, various BASIC interpreters, and I have a significantly improved understanding of how CP/M works - a key milestone in that understanding was getting SUBMIT.COM to execute, and understanding the split between the BDOS and the BIOS.

I'd kinda like to port CP/M to a new (Z80-based) system - but I don't have such a thing to hand, and I guess there's no real need for it. Perhaps I can say I'm "done" with retro stuff, and go back to playing Super Mario Bros (1985) with my boy!

| No comments