Mandel-6502

Work-in-progress Mandelbrot fractal viewer for Atari 8-bit home computers. Mostly an excute to write an integer multiplication routine for the 6502 for practice.

Goals:

• have fun learning 6502 assembly
• make an old machine do something inefficient as efficiently as possible.
• post cool screenshots of low-res fractals

Non-goals:

• maintain anything long-term (but feel free to copy/fork if you want to make major changes!)

Enjoy! I'll probably work on this off and on for the next few weeks until I've got it producing fractals.

-- brion, january 2023

Current state

The 16-bit signed integer multiplication seems to be working, though I need to double-check it some more. It takes two 16-bit inputs and emits one 32-bit output in the zero page, using the Atari OS ROM's floating point registers as workspaces. Inputs are clobbered.

The main loop is a basic add-and-shift, using 16-bit adds which requires flipping the sign of negative inputs (otherwise you'd have to add all those sign-extension bits). Runs in circa 450-750 cycles depending on input (I'll run the exact numbers again later).

The mandelbrot loop is partly sketched out but I have future updates to make on that.

I've also sketched out a 16-bit rounding macro, which is not yet committed.

Deps and build instructions

I'm using ca65 as a macro assembler, and have a Unix-style Makefile for building. Should work fairly easily on Linux and Mac. Might work on "raw' Windows but I use WSL for that.

Currently produces a .xex executable, which can be booted up in common Atari emulators and some i/o devices.