Z80 port map

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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
  • Read sound code from 68k
  • Acknowledge NMI
Clear sound code from 68k to $00 $0C
$04-$07 YM2610 I/O
$08 Set window 0 bank Enable NMIs $1C
$09 Set window 1 bank
$0A Set window 2 bank
$0B Set window 3 bank
$0C See SDRD1 Reply to 68k $0C
$18 See address $08 Disable NMIs $1C

Note that the M1 ROM bankswitching is done by reading ports.

SNK used an obscure feature of the Z80: when accessing ports, the top address bus byte is set to register C.

The banks can be chosen by putting the bank number in C, and doing IN A,(port). 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.