User:Zzo38/Mapper A

This is the "AY-3-8910 only" mapper. Registers are mapped to the PPU address space $3000-$3FFF, and only the address is used; the data isn't used for writing the mapper registers. This can be accessed by writing $2006.

Submappers

Bit0 specifies the mirroring mode. If clear, uses hardwired H or V mirroring. If set, it specifies software-controlled one-screen mirroring; the mirroring bit in the iNES header is then used to specify which bit of which register is used to control it (H=bit7 of port A, V=bit7 of port B).

The other bits are used to select the output levels and filters of each channel output; currently only zero is defined, meaning that all three channels are mixed with the 2A03 and microphone audio at equal volume.

Registers

Registers are write-only and are not accessible during rendering. To write to a register: Set the PPU address to $300x or $330x, where x is the register number. Set the PPU address to $32xx, where x is the value to write.

These are the registers of the AY-3-8910 when the BDIR pin is fixed high.

The bankswitching is done using the two I/O ports of the AY-3-8910. The mapper will not work properly unless both ports are in output mode, so after reset this should be done at first. All 32K PRG ROM banks should contain the reset code due to this, since otherwise the I/O ports are in input mode and won't assert any logic signals.

• 32K PRG ROM banks are selected by writing the bank number to the low bits of port A.
• 8K PRG RAM banks are mapped to $6000-$7FFF and are selected by writing the bank number to the high bits of port A.
• 8K CHR ROM or CHR RAM banks are selected by writing the bank number to the low bits of port B.
• If both CHR ROM and CHR RAM is present, and four-screen mirroring is specified in the iNES header, then there is 8K RAM split at $0000-$0FFF and $2000-$2FFF, and 8K ROM split at $1000-$1FFF and $3000-$3EFF. The RAM bank is then selected by the high bits of port B.
• If both CHR ROM and CHR RAM is present, and four-screen mirroring isn't specified in the iNES header, then the high bit of port B selects between ROM and RAM in $0000-$1FFF.