Z80 port map: Difference between revisions
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Note that the [[M1 ROM|Z80 ROM]] [[Z80 bankswitching|bank selection]] is done by '''reading''' ports. | Note that the [[M1 ROM|Z80 ROM]] [[Z80 bankswitching|bank selection]] is done by '''reading''' ports. | ||
SNK used an obscure feature of the Z80: when accessing ports, the | SNK used an obscure feature of the Z80: when accessing ports, the top address bus byte is set to register C. | ||
The banks can then be chosen by putting the bank number in | The banks can then be chosen by putting the bank number in C, and doing IN A,(port). | ||
This is handled by {{Chipname|NEO-ZMC}} in cartridges. | This is handled by {{Chipname|NEO-ZMC}} in cartridges. | ||
Revision as of 05:01, 24 January 2024
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 top address bus byte is set to register C.
The banks can then 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.