pyvacs

A Python assembler and disassembler for the Signetics/Philips 2650 microprocessor family.

pyvacs is a clean-room Python reimplementation of the classic VACS 1.24 assembler by A.C. Verschueren and W.H. Taphoorn (later ported to Windows by D.D. Spreen). It targets the same 2650 instruction set and opcode encodings, and adds a round-trip-safe disassembler so you can take a ROM image apart and put it back together byte-for-byte.

The 2650 powered consoles such as the Emerson Arcadia 2001 and its international clones — including the Australian-market Fountain Force 2 — as well as the Interton VC 4000 and a number of arcade boards.

Features

• Full 2650 instruction set with every addressing mode (inherent, register, immediate, relative, absolute, zero-page branch, and absolute-indexed with auto-increment/decrement).
• Two-pass assembler with labels, EQU/SET, expressions, and the ORG, DB/BYTE, DW/WORD, DD, DS, DBX, DBFILL directives.
• VACS-style number literals ($1F, %1010, &17, 0FFh, 1010b) and a full expression evaluator (+ - * / MOD, shifts, bitwise, comparisons, HI/LO, $ location counter, character constants).
• Predefined condition symbols (eq, gt, lt, un, plus z/p/n).
• Round-trip-safe disassembler: any byte it cannot decode as a faithful instruction is emitted as a db, guaranteeing reassembly reproduces the original bytes.
• Two command-line tools and a small importable API.

Install

pip install pyvacs

Or from a checkout:

pip install -e ".[dev]"

Command line

Assemble source to a binary:

pyvacs-asm program.asm -o program.bin

Disassemble a binary (with an address/hex listing):

pyvacs-dasm program.bin --listing

Disassemble to reassemblable source loaded at a given address:

pyvacs-dasm game.bin --origin 0x0000 -o game.asm

Library

from pyvacs import assemble, to_source

binary = assemble("""
        org $0000
start:  eorz r0
        bctr,un start
        end start
""").to_bytes()

source = to_source(binary, origin=0)   # round-trips back to `binary`

Example

        org $0000
start:  eorz  r0              ; clear register 0
        bctr,un go
        retc,un
go:     lodi,r0 $00
        loda,r0 msg,r1,+      ; load, auto-increment index r1
        comi,r0 $ff
        bcfr,eq done
        bcta,un go
done:   nop
msg:    db    $21,$1e,$ff
        end   start

Development

pip install -e ".[dev]"
pytest                 # tests + coverage (>85% enforced)
black --check src tests
pylint src/pyvacs

ROM test fixtures

The test-suite round-trips real Fountain Force 2 / Arcadia 2001 cartridge ROMs through the disassembler and assembler. The ROM images are not checked into this repository: on first run the tests download the TOSEC Arcadia 2001 set from the Internet Archive and cache it under tests/.romcache/ (git-ignored). If the download is unavailable the ROM-dependent tests skip automatically, so the core suite still runs offline. Set PYVACS_SKIP_ROM_TESTS=1 to skip them explicitly.

License

Apache License 2.0. See LICENSE.

The 2650 instruction encodings are derived by inspection of the public-domain VACS sources; no VACS code is included here.