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 | 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.