Z80 port map

Some infos from User:kyuusaku

The decode mask for reading ports is always $0C. The one for writes is indicated for each case.

Address	Read	Write	Decode mask
$00	Read sound code from 68k Acknowledge NMI	Clear sound code from 68k to $00	$0C
$04-$07	YM2610 I/O
$08	Set $F000~$F7FF bank	Enable NMIs	$1C
$09	Set $E000~$EFFF bank
$0A	Set $C000~$DFFF bank
$0B	Set $8000~$BFFF bank
$0C	See SDRD1	Reply to 68k	$0C
$18	See address $08	Disable NMIs	$1C

Note that the Z80 ROM bank selection is done by reading ports.

SNK used an obscure feature of the Z80: when accessing ports, the entire address bus is set, not only the lower 8 bits.

The banks can then be chosen by putting the bank number in A, the port number in C and doing IN A,(C). This is handled by NEO-ZMC in cartridges.

Some sound drivers use port $C0 (and others ?) in their code. Since only SDA2 and SDA3 are used for port decoding, port $C0 maps to a mirror of port $00. It's believed to be a remnant of some development tool.