multicomp-cpm

An emulator for Grant Searle's Multicomp — the Z80 build of his multi-CPU FPGA computer — running CP/M 2.2 off an SD card image. Written from the RTL, in about 500 lines of C.

Loading CP/M...
CP/M BIOS 2.0 by G. Searle 2013
CP/M 2.2 (c) 1979 by Digital Research
A>DIR
A: DOWNLOAD COM
A>

There was no software emulator for this machine: virtual-multicomp only runs .hex files (no SD card, no CP/M), and the MiSTer ports are FPGA cores. So this one models the hardware straight from the VHDL.

Quick start

Linux (tested on Ubuntu 22.04, also fine under WSL):

git clone https://github.com/ajfa/multicomp-cpm
cd multicomp-cpm
./run.sh

The first run downloads the ROM and CP/M utilities, builds the emulator, creates a 128MB SD card image, formats its sixteen 8MB drives, installs CP/M and DOWNLOAD.COM, and verifies the result. It then drops you at A>. Takes a few seconds.

Keys:ctrl-] quits, ctrl-\ is the board's cold reset button. ctrl-C is not an exit — it goes through to CP/M as a warm boot.

Command	What it does
./run.sh	build if needed, prepare the disk, boot CP/M
./run.sh --check	headless self-test, prints PASS or FAIL
./run.sh --rebuild	rebuild the emulator and the disk from scratch
./run.sh --monitor	stop at the boot ROM monitor (SPACE, then X and Y)
./run.sh --mc2g	boot the mc-2g-1024 machine instead (see below)

What is emulated

Modelled on MicrocomputerZ80CPM.vhd, bufferedUART.vhd and Components/SDCARD/sd_controller.vhd from the MiSTer port of the design:

CPU	Z80 (superzazu's core, MIT)
ROM	8K at 0000-1FFF, paged out by OUT (38h). While the ROM is selected the internal RAM underneath is deselected, so writes there are discarded — same as the FPGA
RAM	64K internal
Console	6850-compatible UART at $80 (status/control) and $81 (data)
Second port	UART at $82/$83
Disk	SD controller at $88-$8F: 0 SDDATA, 1 SDSTATUS/SDCONTROL, 2-4 SDLBA0-2, 512-byte blocks by LBA

The card is 128MB = 16 CP/M drives of 8MB (A: to P:).

mc-2g-1024

-m mc2g emulates Rienk Koolstra's Multicomp derivative: 2K monitor ROM, OUT $39 switches it back on, 1MB of RAM behind the MMU4 ($F8 selects the page, $FD sets its 16K frame), a 50Hz interrupt, baud rate generators at $7B/$7E and a video terminal at $82/$83. Its multiboot monitor runs; note that an SD image whose volumes have empty system tracks (a RomWBW data disk, for instance) has nothing to boot, and Snnn will just reset the machine.

How the disk is built

Not by forging sectors from outside. run.sh runs Grant's own utilities on the emulated machine, feeding the Intel-HEX files through the serial port exactly as you would paste them into a terminal:

1. FORM128.HEX → G5000 — formats the sixteen drives.
2. CPM22.HEX + CBIOS128.HEX + PUTSYS.HEX → G5000 — writes the system to the boot track.
3. Boot CP/M, press cold reset (RAM survives, as on the real board), load DOWNLOAD2.HEX, run the stub at $FFE8 which copies 4100→0100 and restarts CP/M, then SAVE 2 DOWNLOAD.COM.

Importing software from an mc-2g / RomWBW image

A non-system drive in Searle's CBIOS (DSM=2047, DRM=511, 4096-byte blocks, no reserved tracks) has exactly the same geometry as the data area of an mc-2g / RomWBW 8MB volume, so those file systems transfer 1:1:

SRCDISK=disks/yourimage.dat ./run.sh --rebuild # imports volumes 0-4 into B:-F:
./import-slices.sh 5:6 6:7 # or pick volume:drive pairs
./tryvols.sh disks/yourimage.dat 001 002 # try to boot volumes of an image

Source volumes are 16640 sectors apart with 256 reserved; target drives are 16384 sectors apart, drive A: staying with CP/M.

Emulator options

-m NAME machine: 'searle' (default) or 'mc2g'
-r FILE boot ROM, Intel HEX or raw binary
-d FILE SD card image
-f FILE feed FILE into the console as if typed (repeatable);
a 0x1C byte presses cold reset, 0x1E waits for the machine to go quiet
-e exit once the fed input is consumed and the machine goes quiet
-l FILE log all console output
-T SECS stop after SECS
-R open the SD image read-only
-b batch mode, never read the real keyboard

Every SD write is buffered until the 512-byte block is complete, then written and flushed, so killing the emulator cannot tear a sector. Files still open in CP/M when you kill it are lost, exactly like pulling the card out of the real board.

License and credits

The emulator (src/multicomp.c, the scripts) is under the GNU General Public License v3.0 — see LICENSE. Parts of this repository are other people's work and are not covered by it — see NOTICE.md. Contributions require the agreement in CLA.md; see CONTRIBUTING.md. The Z80 core is by Nicolas Allemand, MIT, see src/LICENSE.z80.

The boot ROM, BIOS, formatter and transfer utilities are Grant Searle's work and are deliberately not redistributed here — his terms ask that they not be republished elsewhere without permission. fetch-roms.sh downloads them at build time from the MiSTer ports that already carry them. Original design and documentation: http://searle.x10host.com/Multicomp/. No disk images with third-party CP/M software are included either.

RTL used as reference: MiSTer-devel/MultiComp_MiSTer and RW-FPGA-devel-Team/Mister_mc-2g-1024. CP/M 2.2 is Digital Research's.