The NES had a memory-mapped APU[1], so the game just sets sound registers to play the appropriate notes, and ticks down a timer until it's time to switch to the

The NES had a memory-mapped APU[1], so the game just sets sound registers to play the appropriate notes, and ticks down a timer until it's time to switch to the next note: https://gist.github.com/1wErt3r/4048722#file-smbdis-asm-L15610 https://gist.github.com/1wErt3r/4048722#file-smbdis-asm-L156... [1] https://wiki.nesdev.com/w/index.php/APU https://wiki.nesdev.com/w/index.php/APU