Today I’ve gave a look at the Sony SMC-777 HW again. The system uses a “timed” bankswitch, presumably involving instruction pre-fetching.

For example (this is BIOS ROM code, located at 0x0000-0x3fff):

3BF3: 3E 10         ld   a,$10
3BF5: D3 1C         out  ($1C),a ;enables RAM bankswitch
3BF7: C7            rst  $00 ;executes this, then switches to RAM mode

Enabling this and adding the unimplemented features makes all of the available programs / games to boot properly with sketchy keyboard and some games with imperfect colors (presumably missing clut):

Fixed address readback in rf5c68 and added a preliminary audio redbook support in Mega CD / Sega CD driver … results are shown in this YT video: