cpmulator - A CP/M emulator written in golang

This repository contains a CP/M emulator, with integrated CCP, which is designed to run CP/M binaries:

• The project was initially created to run my text-based adventure game, which I wrote a few years ago, to amuse my child.
  • That was written in Z80 assembly language and initially targeted CP/M, although it was later ported to the ZX Spectrum.

Over time this project has become more complete, and I've now implemented enough functionity to run many of the well-known CP/M programs:

• The Aztec C-Compiler.
• Borland's Turbo Pascal
• Many early Infocom games:
  • Zork 1, 2, & 3.
  • Planetfall.
  • etc.
• Microsoft BASIC
• Wordstar

The biggest caveat is that I've not implemented any notion of disk-based access. This means that, for example, opening, reading/writing, and closing files is absolutely fine, but any API call that refers to tracks, sectors, or disks will fail (with an "unimplemented syscall" error).

A companion repository contains a collection of vintage CP/M software you can use with this, or any other, CP/M emulator:

• https://github.com/skx/cpm-dist

Installation & Versioning

This emulator is written using golang, so if you have a working golang toolchain you can install in the standard way:

go install github.com/skx/cpmulator@latest

If you were to clone this repository to your local system you could build, and install it, by running:

go build .
go install .

If neither of these options are suitable you may download a binary from our release page.

Releases will be made as/when features seem to justify it, but it should be noted that I consider the CLI tool, and the emulator itself, the "product". That means that the internal APIs will change around as/when necessary - so far changes have been minor, but I'm not averse to changing parameters to internal packages, or adding/renaming/removing methods as necessary without any regard for external users.

Quick Start

• Build/Install this application.
• Clone the associated repository of binaries:
  • git clone https://github.com/skx/cpm-dist.git /tmp/cpm-dist
• Launch the emulator, pointing at the binaries:
  • cpmulator -cd /tmp/cpm-dist -directories
• Start something:
  • "B:", then "MBASIC" - to run BASIC. (Type "SYSTEM" to exit.)
  • "G:", then "ZORK1" - to play zork1.
  • "P:", then "TURBO" - to run turbo pascal.
  • "E:", then "WS" - to run wordstar.

Portability

The CP/M input handlers need to disable echoing when reading (single) characters from STDIN. There isn't a simple and portable solution for this in golang, although the appropriate primitives exist so building such support isn't impossible.

Usage of is demonstrated in the standard library:

• x/term package
  • ReadPassword - ReadPassword reads a line of input from a terminal without local echo.

Unfortunately there is no code there for reading only a single character, rather than a complete line. In the interest of expediency I resort to executing the stty binary, rather than attempting to use the x/term package to manage echo/noecho state myself, and this means the code in this repository isn't 100% portable; it will work on Linux and MacOS hosts, but not Windows.

I've got an open bug about fixing the console (input), #65.

Usage

If you launch cpmulator with no arguments then one of the integrated CCPs ("console command processor") will be launched, dropping you into a familiar shell:

$ cpmulator
A>dir

A: LICENSE .    | README  .MD  | CPMULATO.    | GO      .MOD
A: GO      .SUM | MAIN    .GO  | RET     .COM

A>TYPE LICENSE
The MIT License (MIT)
..
A>

You can terminate the CCP by typing EXIT. The following built-in commands are available:

Show the built-in commands of the default CCP:

• CLS
  • Clear the screen.
• DIR
  • Try "DIR *.COM" if you want to see only executables, for example.
• EXIT / HALT / QUIT
  • Terminate the CCP.
• ERA
  • Erase the named files, wildcards are permitted.
• TYPE
  • View the contents of the named file - wildcards are not permitted.
• REN
  • Rename files, so "REN NEW=OLD" - again note that wildcards are not permitted, nor is cross-drive renaming.

There are currently a pair of CCP implementations included within the emulator, and they can be selected via the -ccp command-line flag:

• "ccp"
  • This is the default, but you can choose it explicitly via cpmulator -ccp=ccp ...
  • The original/default one, from Digital Research.
• "ccpz"
  • Launch this via cpmulate -ccp=ccpz ..
  • An enhanced one with extra built-in commands.
  • Notably "GET 0100 FOO.COM" will load a binary into RAM, at address 0x100. Then "JMP 0100" will launch it.
  • The prompt changes to show user-number, for example if you run "USER 3".
  • If a command isn't found in the current drive A: will be searched instead, which is handy.

You can also launch a binary directly by specifying it's path upon the command-line, followed by any optional arguments that the binary accepts or requires:

$ cpmulator /path/to/binary [optional-args]

Command Line Flags

There are several command-line options which are shown in the output of cpmulator -help, but in brief:

• -cd /path/to/directory
  • Change to the given directory before running.
• -directories
  • Use directories on the host for drive-contents, discussed later in this document.
• -log-path /path/to/file
  • Output debug-logs to the given file, creating it if necessary.
• -prn-path /path/to/file
  • All output which CP/M sends to the "printer" will be written to the given file.
• -quiet
  • Enable quiet-mode, which cuts down on output.
• -syscalls
  • Dump the list of implemented BDOS and BIOS syscalls.
• -version
  • Show the version number of the emulator, and exit.

Note that some of these options can be changed at runtime.

Startup Processing

When the CCP is launched for interactive execution, we allow commands to be executed at startup:

• If SUBMIT.COM and AUTOEXEC.SUB exist on A:
• Then the contents of AUTOEXEC.SUB will be executed.
  • We secretly run "SUBMIT AUTOEXEC" to achieve this.

This allows you to customize the emulator, or perform other "one-time" setup via the options described in the next section.

Runtime Behaviour Changes

There are a small number of extensions added to the BIOS functionality we provide, and these extensions allow changing the behaviour of the emulator at runtime.

The behaviour changing is achieved by having a small number of .COM files invoke the extension functions, and these binaries are embedded within our emulator to improve ease of use, via the static/ directory in our source-tree - This means no matter what you'll always find some binaries installed on A:, despite not being present in reality.

CCP Handling

We default to loading the Digital Research CCP, but allow the CCPZ to be selected via the -ccp command-line flag. The binary A:CCP.COM lets you change CCP at runtime.

Ctrl-C Handling

Traditionally pressing Ctrl-C would reload the CCP, via a soft boot. I think that combination is likely to be entered by accident, so in cpmulator we default to requiring you to press Ctrl-C twice in a row to reboot the CCP.

The binary A:CTRLC.COM which lets you change this at runtime. Run A:CTRLC 0 to disable the Ctrl-C behaviour, or A:CTRLC N to require N consecutive Ctrl-C keystrokes to trigger the restart-behaviour (max: 9).

Console Output

We default to pretending our output device is an ADM-3A terminal, this can be changed via the -console command-line flag at startup. Additionally it can be changed at runtime via A:CONSOLE.COM.

Run A:CONSOLE ansi to disable the output emulation, or A:CONSOLE adm-3a to restore it.

You'll see that the cpm-dist repository contains a version of Wordstar, and that behaves differently depending on the selected output handler. Changing the handler at run-time is a neat bit of behaviour.

Quiet Mode

When CCP is soft/warm-booted it prints a banner showing the currently active CCP, and the console-output device which is in-use.

Running A:QUIET 1 will silence this output, essentially enabling "quiet mode", running with no arguments will show the current state, and running A:QUIET 0 will restore the default behaviour.

Sample Binaries

I've placed a copy of my own lighthouse of doom game within the dist/ directory, to make it easier for you to get started:

$ cd dist/
$ ../cpmulator LIHOUSE.COM
..
Written by Steve Kemp in 2021, version unreleased-git.

  https://github.com/skx/lighthouse-of-doom

Any references to the Paw Patrol are entirely deliberate.

Press any key to start.

A companion repository contains a larger collection of vintage CP/M software you can use with this emulator:

• https://github.com/skx/cpm-dist

Drives vs. Directories

By default when you launch cpmulator with no arguments you'll be presented with the CCP interface, with A: as the current drive. In this mode A:, B:, C:, and all other drives, will refer to the current-working directory where you launched the emulator from (i.e. they have the same view of files). This is perhaps the most practical way to get started, but it means that files are repeated across drives:

• i.e. "A:FOO" is the same as "B:FOO", and if you delete "C:FOO" you'll find it has vanished from all drives.
  • In short "FOO" will exist on drives A: all the way through to P:.

If you prefer you may configure drives to be distinct, each drive referring to a distinct sub-directory upon the host system (i.e. the machine you're running on):

$ mkdir A/  ; touch A/LS.COM ; touch A/FOO.COM
$ mkdir B/  ; touch B/DU.COM ; touch B/BAR.COM
$ mkdir G/  ; touch G/ME.COM ; touch G/BAZ.COM

Now if you launch the emulator you'll see only the files which should be visible on the appropriate drive:

$ cpmulator -directories
A>DIR A:
A: FOO     .COM | LS      .COM

A>DIR B:
B: BAR     .COM | DU      .COM

A>DIR G:
G: BAZ     .COM | ME      .COM

A>DIR E:
No file

A companion repository contains a larger collection of vintage CP/M software you can use with this emulator:

• https://github.com/skx/cpm-dist

This is arranged into subdirectories, on the assumption you'll run with the -directories flag, and the drives are thus used as a means of organization. For example you might want to look at games, on the G: drive, or the BASIC interpreters on the B: drive:

frodo ~/Repos/github.com/skx/cpm-dist $ cpmulator  -directories
A>g:
G>dir *.com
G: HITCH   .COM | LEATHER .COM | LIHOUSE .COM | PLANET  .COM
G: ZORK1   .COM | ZORK2   .COM | ZORK3   .COM

G>dir b:*.com
B: MBASIC  .COM | OBASIC  .COM | TBASIC  .COM

Note that it isn't currently possibly to point different drives to arbitrary paths on your computer, but that might be considered if you have a use-case for it.

Implemented Syscalls

You can see the list of implemented syscalls, along with a mention of how complete their implementation is, by running:

$ cpmulator -syscalls
BDOS
	00 P_TERMCPM
	01 C_READ
	02 C_WRITE
	03 A_READ
..snip..
BIOS
	00  BOOT
	01  WBOOT
..snip..

Items marked "FAKE" return "appropriate" values, rather than real values. Or are otherwise incomplete.

The only functions with significantly different behaviour are those which should send a single character to the printer (BDOS "L_WRITE" / BIOS "LIST"), they actually send their output to the file print.log in the current-directory, creating it if necessary. (The path may be altered via the -prn-path command-line argument.)

The implementation of the syscalls is the core of our emulator, and they can be found here:

• cpm/cpm_bdos.go - BDOS functions.
  • https://www.seasip.info/Cpm/bdos.html
• cpm/cpm_bios.go - BIOS functions.
  • https://www.seasip.info/Cpm/bios.html

Debugging Failures & Tweaking Behaviour

When an unimplemented BIOS call is attempted the program it will abort with a fatal error, for example:

$ ./cpmulator FOO.COM
{"time":"2024-04-14T15:39:34.560609302+03:00",
  "level":"ERROR",
  "msg":"Unimplemented syscall",
  "syscall":255,
  "syscallHex":"0xFF"}
Error running FOO.COM: UNIMPLEMENTED

If things are mostly working, but something is not quite producing the correct result then we have some notes on debugging:

• DEBUGGING.md

For reference the memory map of our CP/M looks like this:

• 0x0000 - Start of RAM
• 0xDE00 - The CCP
• 0xF000 - The BDOS (fake)
• 0xFE00 - The BIOS (fake)

Credits and References

• Much of the functionality of this repository comes from the excellent Z80 emulator library it is using, written by @koron-go.
• The default CCP comes from my fork of the original cpm-fat
  • However this is largely unchanged from the original CCP from Digital Research, although I did add the CLS, EXIT, HALT & QUIT built-in commands.
• Reference Documentation
  • CP/M BDOS function reference.
  • CP/M BIOS function reference.
• Other emulators which were useful resources when some functionality was unclear:
  • https://github.com/ivanizag/iz-cpm
    • Portable CP/M emulation to run CP/M 2.2 binaries for Z80.
    • Has a handy "download" script to fetch some CP/M binaries, including BASIC, Turbo Pascal, and WordStar.
    • Written in Rust.
  • https://github.com/jhallen/cpm
    • Run CP/M commands in Linux/Cygwin with this Z80 / BDOS / ADM-3A emulator.
    • Written in C.

Release Checklist

The testing that I should do before a release:

• Play lighthouse of doom to completion, either victory or death.
• Compile a program with ASM & LOAD. Confirm it runs.
• Compile HELLO.C and ECHO.C with Aztec C Compiler.
  • Confirm the generated binaries run.
• Run BBC Basic, and play a game.
  • Test "SAVE" and "LOAD" commands.
  • Test saving tokenized AND raw versions. (i.e SAVE "FOO", and SAVE "FOO", A.)
• Compile a program with Turbo Pascal.
  • Confirm the generated binary runs.
• Play Zork1 for a few turns.
  • Test SAVE and RESTORE commands, and confirm they work.
• Test BE.COM
• Test STAT.COM
• Test some built-in shell-commands; ERA, TYPE, and EXIT.

Bugs?

Let me know by filing an issue. If your program is "real" then it is highly likely it will try to invoke an unimplemented BIOS function.

Steve