VT02+ MMC3 Compatibility Registers

In OneBus mode, VT02+ consoles combine the Famicom cartridge connector's CPU and PPU address lines into one 32 MiB address space, with separate CPU and PPU bankswitch registers pointing to appropriately-placed code and picture data. The bankswitching scheme is based on, and indeed backwards-compatible to, the Nintendo MMC3's. Backwards compatibility is realized by way of forwarding CPU writes to $8000-$FFFF to the appropriate VT02+ register. V.R. Technology's data sheets call this the "old compatible mode". Forwarding can be disabled by setting register $401B bit 3 (FWEN).

When $410B bit 3 (FWEN) is cleared, the MMC3's registers are forwarded to VT02+ registers as follows:

MMC3 register   VT02+ register          Meaning
-------------   --------------          -------
$8000           $4105, except bit 5     MMC3 Index Register ("Pointer"), PRG A14 inversion, CHR A12 inversion
$8001  reg
        0       $2016                   1 KiB CHR Bank Number at PPU $0000-$03FF (AND $FE) and PPU $0400-$07FF (OR $01)
        1       $2017                   1 KiB CHR Bank Number at PPU $0400-$07FF (AND $FE) and PPU $0800-$0FFF (OR $01)
        2       $2012                   1 KiB CHR Bank Number at PPU $1000-$13FF
        3       $2013                   1 KiB CHR Bank Number at PPU $1400-$17FF
        4       $2014                   1 KiB CHR Bank Number at PPU $1800-$1BFF
        5       $2015                   1 KiB CHR Bank Number at PPU $1C00-$1FFF
        6       $4107                   8 KiB PRG Bank Number at CPU $8000-$9FFF
        7       $4108                   8 KiB PRG Bank Number at CPU $A000-$BFFF
$A000           $4106, only bit 0       Mirroring select
$A001           not forwarded           WRAM Enable/Write Protect
$C000           $4101                   IRQ Latch
$C001           $4102                   Reload IRQ Counter
$E000           $4103                   Disable/Acknowledge IRQ
$E001           $4104                   Enable IRQ

Notes:

• The table disregards mirrored MMC3 addresses.
• Writing a register number to the lower three bits of $4105 also decides which register is affected when writing data to $8001, as $4105 is indeed a mirror of $8000 (excluding bit 5) and vice-versa. EmuVT does not take this into account, resulting in glitches when running games such as Mighty Bomb Jack on the 100-in-1 (D-CAT8) multicart.
• The JungleTac game Bolt Fighter on the Classic Max Lite 120-in-1 multicart writes garbage data to the $8000-$FFFF range during gameplay. This would lead to inadvertent CHR bank changes, were it not for the fact that the multicart menu sets FWEN to 1 to disable forwarding.
• As the VT02+ consoles themselves have no WRAM at $6000-$7FFF, there is no register to which a $A001 write could be forwarded.