68k to Z80
Writes to the Z80 are byte-wide and made through $320000.
Any byte can be sent, their meaning are only determined by the way the Z80 code handles them except for 3 special cases, as seen below.
Z80 to 68k
Port $0C is used to reply to the 68K. The value is also buffered in the same chips, but there's no interrupts generated. The value can be read by using the same register, $320000.
Many drivers acknowledge sound commands by echoing them back with bit 7 set to 1 when they are processed.
Commands $01 and $03 are always expected to be implemented as they are used by the BIOSes for initialization purposes. Typically, if the Z80 doesn't reply to command $01 in time, the "Z80 ERROR" message is displayed and the system locks up.
Command $03 is then sent to ask for a soft reset of the Z80, but no replies are expected.
Command $02 is used by cartridge systems to play the boot music. The pattern (melody) was certainly imposed by SNK, but developers often chose their own instruments parameters. Boot music.
These are sufficient handlers for both init commands:
Command01_Handler: ; with the command in A di ; disable interrupts ld sp, $FFFF ; clear call stack out ($0C),a ; echo the command jr NMI_End ; usually POPs, then RETN
Command03_Handler: ; with the command in A di ; disable interrupts ld sp, $FFFF ; clear call stack ld hl,0 push hl retn ; RETN to 0