Cometsong
-
3-channel Mockingboard music with Lo-res graphics
256-byte Intro for Apple II, Lovebyte 2022
by Deater / dSr

Fitting Mockingboard music in 256B was a challenge.
Note: only plays music out the left stereo channel.
Thanks to mA2E for the music.
Thanks to qkumba for optimization help.

This code does a lot of tricky things.  It loads into the zero page
(which makes memory use less) but the music data overflows into
the low half of the stack (at $100 on 6502).  The rest of the code
avoids the use of the stack, so we can use the "PLA" (pull stack
to accumulator) instruction to do an auto-increment load through
the music.  This trick saved 15+ bytes.