Z80 port map: Difference between revisions

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|rowspan="2"|$0C
|rowspan="2"|$0C
|-
|-
|$04
|$04-$07
|colspan="2"|[[YM2610]] I/O
|colspan="2"|[[YM2610]] I/O
|-
|-
|$08
|$08
|Set [[Z80 memory map|$F000~$F7FF]] bank
|Set [[Z80|window 0]] bank
|rowspan="4"|Enable NMIs
|rowspan="4"|Enable NMIs
|rowspan="4"|$1C
|rowspan="4"|$1C
|-
|-
|$09
|$09
|Set $E000~$EFFF bank
|Set window 1 bank
|-
|-
|$0A
|$0A
|Set $C000~$DFFF bank
|Set window 2 bank
|-
|-
|$0B
|$0B
|Set $8000~$BFFF bank
|Set window 3 bank
|-
|-
|$0C
|$0C
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|}
|}


Note that the [[M1 ROM|Z80 ROM]] bank selection is done by '''reading''' ports.
Note that the [[M1 ROM]] [[Z80|bankswitching]] 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.
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 B, the port number in C and doing IN A,(C).
The banks can 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.
Some [[sound driver]]s 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.


[[Category:Base system]]
[[Category:Base system]]
[[Category:Audio system]]

Latest revision as of 09:19, 28 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
  • 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.