68k to Z80 (request)
Writes to the Z80 are byte-wide and made through $320000 (REG_SOUND).
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).
(What chip is used on first gen chipset?)
Z80 to 68k (answer)
The Z80 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. During the MVS power up self-tests, if the Z80 doesn't reply to command $01 in time, the "Z80 ERROR" message is displayed and the system locks up.
It is sent by the BIOS when the slot is switched. As the Z80 rom will be swapped, all sounds need to be stopped, NMI needs to be activated, $01 needs to be sent back to the 68k and the Z80 code has to sit in a loop in RAM. After receiving that $01 reply, the BIOS can then switch slot without crashing the Z80.
It is used by cartridge systems to play the boot logo music. The pattern (melody) was certainly imposed by SNK, but developers often chose their own instruments parameters. Boot music. No reply is expected.
It is used to ask for a soft reset of the Z80, which needs to be done under 100ms. No reply is expected.
These are sufficient handlers for both init commands:
Command01_Handler: ; with the command in A di ; disable interrupts ld sp,$FFFF ; clear call stack ld a,1 out ($0C),a ; echo the command back jmp $F800 .org $F800 lp: jr lp
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