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Plexus P/20 Emulator

This is the source code for a an emulator which more-or-less faithfully simulates a Plexus P/20 UNIX server.

What's a Plexus P/20?

Plexus was a company manufacturing Unix servers, initially based on Z8000 CPUs but later switching to Motorola 680x0 CPUs. The P20 was a 'lower-cost' machine supporting up to 16 users. It was sold with up to 2MiB of RAM (although the hardware supports up to 8MiB) and two 68010 CPUs running at 10 or 12.5MHz. The machine this emulator is based on has an 87MiB MFM hard disk, a 5.25" floppy drive and a tape drive as well.

This emulator

This emulator emulates most aspects of a Plexus P/20 system: you can boot from a hard disk image, log into UNIX and play around. The things currently unsupported are the tape drive, floppy drive, and any Multibus cards.

The easiest way to get this working is by using the version compiled to WebAssembly. To use this, point your (recently-ish) browser the web version and enjoy.

If you want to do more specialized things, like changing the emulator code, or you want to run the emulator faster, you might want to clone the repo and compile it from scratch. The emulator has no dependencies aside from a C compiler and make: to compile simply run 'make'. (If that fails, try 'gmake'.) This will produce the 'emu' binary.

To actually run the emulator, you need the system roms (U15-MERGED.BIN and U17-MERGED.BIN) as well as a hard disk image. Both can be found at Adrian Blacks P20 repo.

Notes about the source code

This project incorporates Musashi rather than make it into a submodule because it needs some changes compared to the upstream version. Specifically, the Plexus software needs proper support for the stack frame generated by bus error exceptions; this has been mostly backported from the Musashi version in the MAME project. Additionally, for SystemV syscall tracing support, a callback for trap instructions is added.

Credits

The bulk of the code is written by Sprite_tm. However, this emulator would not have been possible without:

  • Adrian Black, for un-earthing a Plexus P/20 and documenting the process of getting it working,

  • Ewen McNeill, Peter Kooiman, rakslice and Paul Brook for sending in pull requests to add/fix various issues.

  • The people at the Plexus-P20 channel on Usagi Electrics discord for pulling apart the diag ROMs, schematics, and UNIX kernel to figure out how exactly the issue works. Also for providing mental support and general interest.

  • Karl Stenerud, the writer of Musashi, the MC680x0 emulator this project uses.