Capcom Cx4 Coprocessor
Among the least-understood of the Super Nintendo coprocessors is the Capcom Cx4. Used in Mega Man X2 and Mega Man X3 (Rockman X2/X3 in Japan), it was used to to perform general trigonometric calculations for wireframe effects, sprite positioning and rotation. Back in 2008, the Cx4 was successfully decapped, and as a result, the instruction set was reverse-engineered and the data ROM was dumped. As a result, we now have low-level emulation support for it in higan, but there's still not a whole lot of information out there, so I'm trying to collect as much as I can here. To start out, byuu asked me to build him a re-programmable cart so he could run his own code and test it out, so the first step was to trace out the chip's pinout. I wasn't able to find an existing one, so I started from scratch. I picked up both existing official Cx4 PCB's, the SHVC-1DC0N-01 (Rockman X3), and SHVC-2DC0N-01 (Rockman X2), and my tracing results are recorded here.
Pins 74 and 75 are connected to GND on both official PCB's but are not connected internally to GND. They're most likely inputs of some kind. Maybe a Hi/LoROM switch like pin #10 on the MAD-1, or something like that, but right now, that's pure speculation.
The Cx4 supports either 1x16Mbit ROM or 2x8Mbit ROMs, and 1x256Kbit SRAM (though neither official cart included SRAM).
ROM is mapped to $00-$7F,$80-$FF:8000-FFFF
SRAM is mapped to $70-$77:0000-7FFF