Commodore 64 Extended BASIC

Flashy Bars

This is kind of the assembler equivalent of “Hello World” in my opinion. It’s a small program which flashes colours on the screen quickly enough to display coloured bars. Such tricks were common for custom tape loaders, to give the impression something was loading.

Extended BASIC

Commodore 64 BASIC was never up to much in my opinion. It seemed rushed into production, and lacked features of other implementations such as graphics and sound.

One trick people used to pull off was to add new commands to BASIC by intercepting certain points in the interpreter’s lexical analyser.

I’ve had a go at this myself, using disassembled listing of the BASIC code to guide me, but it’s very much a work in progress. So far I can:

  • Copy the BASIC ROM to RAM (this is where you see the flashy colours)
  • Switch bank (so BASIC is now read from RAM)
  • Edit the welcome message and colour scheme
  • Restart BASIC from RAM

Still to do was figure out an intercept point to implement my own commands.

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