mandel-6502/readme.md

# 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 6 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 3.13-precision fixed point numbers with 6.26-precision intermediates.

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`.