Here's something different, an in progress attempt to reverse engineer the sound board for the arcade game "Defender" (and others). The sounds are very recognizable, and unique to Williams arcade/pinball machines. They always interested me, so I'm making an attempt to understand them better.
The board is based around a 6808 CPU (relative of the 68000), and a DAC attached to an IO controller. The binary is floating around the web, as is this great disassembler: DASMx. I used it to disassemble the ROM into a code listing, and started commenting it.
Here are the schematics from one of the compatible service manuals:
I was able to use the schematics to figure out the memory map. This helps understand the significance of certain read/writes in the code.
RAM: $0000 - $007F (128 Bytes)
PIA: $0400 - $07FF
ROM: $F800 - $FFFF (2KB)
By loading the ROM into audacity I was able to see some recognizable shapes. These are the waveforms/look up tables stored alongside the code. Some are played directly, while others are used to modulate things like pitch, or volume.
There are some interesting tricks done in the code, and I hope to explain them here one day. Things like dynamically generated delay loops, and something akin to granular synthesis...
Until then, the current version of the commented disassembly lives here.