pet-development

Developing for the Commodore PET

This skill drives an emulated Commodore PET through the pet command line (or the equivalent pet-tools MCP tools). Full command reference: docs/cli.md. Every command takes --json for machine-readable output.

Using MCP instead of the CLI? The tools map mechanically — pet screen → pet_screen_text, pet break add → pet_break_add, and so on — with the same sessions, semantics, and stopped-state rule. Two differences: pet wait is split into pet_wait_text / pet_wait_mem / pet_wait_break, and wait timeouts return {"fired": null, ...} as data instead of an error.

The loop

Write → run → observe → fix:

1. Write BASIC (.bas) or 6502 assembly (.s).
2. pet run FILE — tokenizes/assembles as needed, loads, and RUNs.
3. Observe with pet screen (decoded screen text) — this is the primary way to see output. Use pet wait --text "..." to block until expected output appears; loading and running take a few emulated seconds even in warp, so never assume a program has finished — wait for a signal.
4. Fix and repeat.

Start a machine with pet session start before anything else, and pet session stop when done.

Sessions and models

pet session start --model pet4032 boots a PET 4032 (the default). Add --warp to run at full speed for automation and --headless to suppress the window (SDL builds of VICE only — on Linux/GTK3 builds a window may still appear; use xvfb-run -a on a display-less machine). Models: pet2001 (BASIC 1.0), pet3032 (BASIC 2.0), pet4032 (BASIC 4.0), pet8032 and pet8296 (BASIC 4.0, 80-column). The 40- vs 80-column split matters when reading the screen; BASIC version matters for which tokens and ROM routines exist.

Writing BASIC

BASIC sources follow the petcat convention: write keywords AND string text in lowercase. Lowercase ASCII maps to unshifted PETSCII, which the PET displays as uppercase — so 10 print "hello" shows on screen as 10 PRINT "HELLO". Writing uppercase in the source produces shifted PETSCII, which shows as graphics characters instead of letters. This is the single most common mistake.