SpritesMind.Net

About DMA flag. I think, 32X logic monitors the address bus (VA) for a DREQ source address register match and then captures data from the data bus (VD). Hmm, so it only works from a fixed source address then? Looks like an address match serves as a trigger to start the DMA. You can find the source ...

Oh that's bad. I was faking it as always being accessible currently. Initial attempts at it were breaking games and tests. I'll pay closer attention to getting it working then. But, DRAM refresh? Really? So every time there's a CPU DRAM refresh it has to go inaccessible for short bursts? That's ......

Some games require proper FEN flag emulation (including DRAM refresh).
After Burner uses UBC registers as variables for sound processing.

About DMA flag. I think, 32X logic monitors the address bus (VA) for a DREQ source address register match and then captures data from the data bus (VD).

No, just area 0x70-0x73. Maybe it's a separate register.
The same register is in Sega СD and some games change it as needed.

From 32X Overview, the same as in Sega CD.

Some tests of Mars Check Program use the DREQ and almost all 32X hardware. And source code of Mars Check Program is in 32X-DDK.