Gimmick VRC4 Hack: Difference between revisions
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==Trivia== | ==Trivia== | ||
For unknown reason, grounds of 74*157 are not connected to the ground of rest cartridge. | For unknown reason, grounds of 74*157 are not connected to the ground of rest cartridge. | ||
Oddly enough, this is ''not'' [[iNES Mapper 183]]. | |||
Revision as of 17:31, 12 September 2017
It is regular VRC4 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
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++++- 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.
Oddly enough, this is not iNES Mapper 183.
