Emulate Capcom CX4 SNES enhancement chip

[cuavas]

Segher, Overload and Ikari have reverse-engineered enough of the CX4’s behaviour to get the games that use it running (Mega Man X2 and Mega Man X3).

@dinkc64 has written an emulator for it in the style of old, pre-C++ MAME CPU cores: https://github.com/dinkc64/LakeSnes/blob/10f4a9dcc36d173057df2b01bf1a2dbb71c08abe/snes/cx4.c

It might be a fun task to make a MAME CPU core for it. @dinkc64’s code could be used as a starting point:

• It obviously needs to be turned into a C++ cpu_device implementation rather than a collection of mostly static things.
• The instruction decoding and execution parts are a pretty good starting point.
• Memory and SFR access stuff needs to make use of address maps, etc.
• Timer stuff should use MAME timers or work off the execute loop, etc.