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A simple golang CP/M emulator

15 April 2024 21:00

A couple of years ago I wrote a simple text-based adventure game in Z80 assembly language, to amuse our child. The game was written for CP/M, because that is the operating system my single-board Z80-based computer runs upon.

Later I ported the game to the ZX Spectrum 48k.

Recently I went through a burst of enthusiasm and started to overhaul the code a little, adding word-wrapping and fixing a couple of bugs. That lead to a new release, and also a brief amount of (positive) feedback on hacker news.

After mulling it over I realized that the number of CP/M BIOS functions I was using was very minimal, almost only the minimum you'd expect:

  • Write a character to STDOUT.
  • Write a $-terminated string to STDOUT.
  • Read a character from STDIN.
  • Read a line from STDIN.

It crossed my mind that implementing those syscalls should be trivial, and if I bundled implementations with a Z80 emulator library I'd have a means of running the game without a real CP/M installation, and without using the ZX Spectrum port.

So I picked a golang-based Z80 emulator, and started hacking.

After a day I had a working system, and I added a few more syscalls:

  • Open File, Create File, Delete File, Close File.
  • Console I/O.
  • Read Record.

After that? I can now play Zork 1, Zork 2, Zork 3, and The Hitchhiker's guide to the Galaxy, from Infocom.

I suspect I'm "done" for now, though it might be nice to add WriteRecord and the other missing functions there's no obvious use for yet another CP/M, especially with a CCP.

Still CP/M. In Golang. For text-based adventures:

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