Lorenz Attraktor on C64

example.mov

Compiling / Development

This project uses the ACME assembler for no particular reason but that it was the least complicated and most easy to find assembler. I used ACME 0.97.

For testing I found it best to use VICE, I had a few problems launching .PRG files with the default configuration so I created my own launch script which enables auto start PRG mode.

Note: the script assumes that ROM files (KERNAL, BASIC, character set) are present in the ROM/ directory. Make sure to place them there. They can be found in the release download of VICE.

Running

cd src
make run

The screen should turn black and green dots should appear.

Note that normally you would need to start the program yourself using a SYS 2049 statement (or whatever the initial address is) to jump into the code. We add a header value that adds a bit of BASIC code that does this for us.

If you want to run a specific version of VICE, set the X64SC environment variable.

Running on C64

I used a SD2IEC adapter and copied the src/myprg.prg file to the root of the SD card. Then it is just a matter of loading the program:

LOAD "myprg.prg",8
RUN

I had problems where the SD2IEC adapter didn't even load the directory list (LOAD "$",8) and simply flashed the error LED. Do diagnose this I ran this code:

10 OPEN 15,8,15: INPUT #15,A$,B$,C$,D$
20 CLOSE 15
30 PRINT A$,B$,C$,D$
RUN

The message I got was "74,Drive not ready,12,0" which was caused by a intermittent fault of the SD card socket (card detect or write protect was not triggered correctly). Re-inserting the SD card a few times solved the issue.

Usage

There are three keys you can press:

• R for clearing the screen and resetting the X/Y/Z values
• N to play a melodic sound based on the current trajectory
• M to play a bass sound based on the current trajectory

Resources

This project used several resources for which I am really grateful:

• 6502.org Instruction reference
• 6502.org Addressing reference
• 6502.org Opcodes reference
• c64-wiki BASIC floating point ops
• c64-wiki Zeropage layout
• codebase64 floating point ops
• codebase64 screen mode doc
• Christian Bauer's VIC-II screen doc (it seems to be the most correct one I found)
• codebase64 micro tracker example
• cbm64 memory assignment map

There are three python scripts that were helpful in the development of the final assembler code.

1. attraktor.py - a reference implementation that got subsequently optimized to bit oeprations
2. encode_xy.py - a way of testing & reverse engineering the bit addressing in the 320x200 (but actually 40x25 byte) screen of the C64
3. cbm_to_float.py - a script of converting C64 floating point numbers (5 byte) to python floats for easy debugging with the VICE monitor