OpenCP/M +3

Clean-room CP/M 3.1 implementation for the ZX Spectrum +3.

All code is original Z80 assembly — the BIOS, BDOS, CCP, drivers, and tools are written from scratch.

Status

Currently experimental, not ready for any kind of real use.

Features

• 51×24 and 32x24 screen modes
• Full BDOS with console I/O and file operations
• uPD765A FDC driver with 512→128 byte sector deblocking
• Banked memory — TPA mode (~62K user space) and system mode with bank switching
• MiniCCP with built-in DIR, TYPE, VER, HELP, DUMP commands and .COM loading
• Z80 linter (z80lint.py) that validates stack balance and register clobbering across all return paths
• ZPA preprocessor for source-level abstractions on top of Z80 assembly
• RTC clock although stalls during disk-IO like Locomotive version

Compatibility

• BBCBASIC.com
• Mallard BASIC
• Ed
• Pip
• B: drive
• Custom formats (CPC + XDPB formats like PCW 720K)
• Various other simple CP/M tools

Tools

• Date.com with Y2K compliance
• Dump.com with screen-width adaption
• Showxdpb.com - Show the eXtended Disk Parameter Blocks

Test suites

• bdostest.com - Testing the CP/M BDOS functions 1-35
• termtest.com - Testing the terminal emulation/ESC codes
• disktest.com - Testing file and disk operations

Wishlist/TODO

• .SUB execution
• Format command
• RAMdisk support
• 64-column mode
• User area/file spec on command lines

Out of scope

• Serial I/O
• Printer/parallel I/O
• Disckit.com

Technical

Memory Layout

Range	TPA Mode	System Mode
0000–00FF	Page zero (vectors)	Bank 4 — BIOS, BDOS, drivers
0100–F9FF	~63K user TPA
FA00–FFFF	Common stub (bank 3)	Common stub (bank 3)

Banking is controlled via port 0x1FFD. See AGENTS.md for full architectural details.

Building

Requires sjasmplus and Python 3.

# Lint the source
python3 tools/z80lint.py

# Build the disk image
build.cmd

# Build with additional files
build.cmd --add FILE ...

Output: build/cpm3.dsk — a standard +3 disk image ready for emulation.

Testing

Use the zx84 MCP emulator:

model +3
load build/cpm3.dsk
disk_boot
run 1000

Project Structure

src/
  boot/     Boot sector and stage-2 loader
  bios/     BIOS, screen/keyboard/FDC drivers, common memory stub
  bdos/     BDOS (console + file I/O)
  ccp/      MiniCCP command processor
  util/     CP/M utilities (.COM programs)
  test/     Test programs
tools/      Build tools (mkdsk, zpa preprocessor, linter, patchsum)
docs/       API reference, filesystem documentation

Documentation

• CP/M 3.1 BDOS API Reference
• Filesystem Layout
• zx84 MCP Smoke Test

License

This project is licensed under the MIT License.