45 lines
2 KiB
Markdown
45 lines
2 KiB
Markdown
# Mandel-6502
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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.
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Goals:
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* have fun learning 6502 assembly
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* make an old machine do something inefficient as efficiently as possible.
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* post cool screenshots of low-res fractals
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Non-goals:
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* maintain anything long-term (but feel free to copy/fork if you want to make major changes!)
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Enjoy! I'll probably work on this off and on for the next few weeks until I've got it producing fractals.
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-- brooke, january 2023 - february 2024
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## Current state
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Basic rendering is functional, but no interactive behavior (zoom/pan) or benchmarking is done yet.
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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.
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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.
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The mandelbrot calculations are done using 4.12-precision fixed point numbers. It may be possible to squish this down to 3.13.
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Iterations are capped at 255.
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The pixels are run in a progressive layout to get the basic shape on screen faster.
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## Next steps
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Add a running counter of ms/px using the vertical blank interrupts as a timer. This'll show how further work improves it!
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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.
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I may be able to do a faster multiply using tables of squares for 8-bit component multiplication.
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## Deps and build instructions
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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.
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Currently produces a `.xex` executable, which can be booted up in common Atari emulators and some i/o devices.
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