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Brooke Vibber 2024-12-30 16:50:25 -08:00
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@ -14,15 +14,18 @@ Non-goals:
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
-- brooke, january 2023 - december 2024
## Current state
Basic rendering is functional, but no interactive behavior (zoom/pan) or benchmarking is done yet.
Basic rendering is functional, with interactive zoom/pan (+/-/arrows) and 4 preset viewports via the number keys.
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 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.
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.
* 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. It may be possible to squish this down to 3.13.
@ -30,17 +33,18 @@ Iterations are capped at 255.
The pixels are run in a progressive layout to get the basic shape on screen faster.
## Next steps
There is a running counter of ms/px using the vertical blank interrupts as a timer, used to track our progress. :D
Add a running counter of ms/px using the vertical blank interrupts as a timer. This'll show how further work improves it!
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.
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)
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`.