2.3 KiB
Mandel-6502
Work-in-progress Mandelbrot fractal viewer for Atari 8-bit home computers. Mostly an excuse 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.
-- brooke, january 2023 - december 2024
Current state
Basic rendering is functional, with interactive zoom/pan (+/-/arrows) and 4 preset viewports via the number keys.
The 16-bit signed integer multiplication 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.
- 16-bit multiplies are decomposed into 4 8-bit unsigned multiplies and some addition
- an optimized case for squares uses a table of 8-bit squares to reduce the number of 8-bit multiplication sub-ops
- when expanded RAM is available as on 130XE, a 64KB 8-bit multiplication table accelerates the remaining multiplications
- without expanded RAM, a table of half-squares is used to implement the algorithm from https://everything2.com/title/Fast+6502+multiplication
The mandelbrot calculations are done using 4.12-precision fixed point numbers with 8.24-precision intermediates. It may be possible to squish this down to 3.13/6.26.
Iterations are capped at 255.
The pixels are run in a progressive layout to get the basic shape on screen faster.
There is a running counter of ms/px using the vertical blank interrupts as a timer, used to track our progress. :D
There's a check for cycles in (zx,zy) output when in the 'lake'; if values repeat, they cannot escape. This is a big time saver in fractint.
There's some cute color cycling.
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.
Todo
See ideas in todo.md.