Brooke Vibber
05133aabdd
previously we were flipping the inputs if negative, and then the output if both inputs were negative turns out you can just treat the whole thing as an unsigned mul and then subtract each term from the high word if the other term is negative. https://stackoverflow.com/a/28827013 this saves a handful of cycles, reducing our runtime to a merge 14.211 ms/px \o/ |
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| .mailmap | ||
| fixed.js | ||
| Makefile | ||
| mandel.s | ||
| readme.md | ||
| sim.html | ||
| sim.js | ||
| tables.js | ||
| testme.js | ||
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 - february 2024
Current state
Basic rendering is functional, but no interactive behavior (zoom/pan) or benchmarking is done yet.
The 16-bit signed integer multiplication works; 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 470-780 cycles depending on input.
The mandelbrot calculations are done using 4.12-precision fixed point numbers. It may be possible to squish this down to 3.13.
Iterations are capped at 255.
The pixels are run in a progressive layout to get the basic shape on screen faster.
Next steps
Add a running counter of ms/px using the vertical blank interrupts as a timer. This'll show how further work improves it!
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.
I may be able to do a faster multiply using tables of squares for 8-bit component multiplication. (done)
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.