Gimmick VRC4 Hack

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Gimmick vrc4 components.jpg
Gimmick vrc4 top.png
Gimmick vrc4 bottom.png
Gimmick vrc4 sch.png

It is regular VRC4e with PRG-ROM mapped also at $6000-$7fff and one additional register at $6000-$7fff (bits of address are latched) 

A~[011. .... .... PPPP] $6000-$7fff
                  ||||
                  ++++- 8kB PRG-ROM bank at $6000-$7fff (only banks 0-15 can be swapped)

This modification was needed because original Gimmick, which is based on mapper FME-7 also uses capability of mapping PRG at $6000-$7fff.

Hardware implementation

It consists of AX5208C (pirate VRC4), 4 bit latch (74*161) and two multiplexers (74*157). VRC's lines controlling WRAM at $6000-$6fff are tied to latch. Because VRC4 decodes RAM only at $6000-$6fff, when CPU accesses $0000-$7fff, one of muxes fools VRC by feeding GND at his A12, so that when CPU access $6000-$6fff or $7000-$7fff, VRC thinks for both it is $6000-$6fff and enables WRAM control line.

Trivia

For unknown reason, grounds of 74*157 are not connected to the ground of rest cartridge.

Contrary to previous opinion, this is the true description of iNES Mapper 183.

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