Proposed mapper extension for 4MB/8MB SuperFX ROMs

[Yoshimaster96]

Not sure where else to post this, so I'll post it here.

According to the SNES Development Manual, SuperFX ROMs can have 6MB of ROM mapped exclusively to the CPU, in addition to the 2MB mapped to both the CPU and GSU. However, no games made use of this, and as such, the exact mapping is not well-defined. What is known however is that it 2MB would have been mapped starting at $808000, and 4MB would have been mapped starting at $C00000.

Similar to what has been done for ExLoROM (another completely custom mapping), I propose the following mapping (note that the GSU can only access the first 2MB for all of these on account of the lack of an A23 pin):

For 2MB SuperFX ROMs (same as original):
2MB mapped to $008000 and $808000 in 32KB chunks (mirrored at $400000 and $C00000 in 64KB chunks).

For 4MB SuperFX ROMs:
2MB mapped to $008000 in 32KB chunks (mirrored at $400000 in 64KB chunks).
2MB mapped to $808000 in 32KB chunks (mirrored at $C00000 in 64KB chunks).

For 8MB SuperFX ROMs:
2MB mapped to $008000 in 32KB chunks (mirrored at $400000 in 64KB chunks).
2MB mapped to $808000 in 32KB chunks.
4MB mapped to $C00000 in 64KB chunks.

If I can figure out how to build Snes9x and bsnes, I'll see if I can make a fork supporting these. I'm curious what your thoughts are. I'll probably use the existing mapping value of $24 for this, since $27 is the only value not used at this point and I don't wanna waste it if I don't have to.

[lidnariq]

Higan manifests already support this, no need for recompilation.

[Yoshimaster96]

Higan manifests already support this, no need for recompilation.

Yeah, I was mainly referring to Snes9X in terms of compilation.