50 lines
2.3 KiB
Markdown
50 lines
2.3 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 - december 2024
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## Current state
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Basic rendering is functional, with interactive zoom/pan (+/-/arrows) and 4 preset viewports via the number keys.
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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.
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* 16-bit multiplies are decomposed into 4 8-bit unsigned multiplies and some addition
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* an optimized case for squares uses a table of 8-bit squares to reduce the number of 8-bit multiplication sub-ops
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* when expanded RAM is available as on 130XE, a 64KB 8-bit multiplication table accelerates the remaining multiplications
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* without expanded RAM, a table of half-squares is used to implement the algorithm from https://everything2.com/title/Fast+6502+multiplication
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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.
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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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There is a running counter of ms/px using the vertical blank interrupts as a timer, used to track our progress. :D
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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.
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There's some cute color cycling.
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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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## Todo
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See ideas in `todo.md`.
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