I've seen a handful of posts about PC-98 hacking on here but unfortunately there is very little documentation on the system compared to more popular systems like the NES. Good luck!Not too much is known except, I think, the one tracing emulator available includes little documentation.
From what I recall it sounded like it was largely similar to DOS PCs (using a clone 386? CPU able to address 1MB, first 640K (0xA0000) is RAM, followed by VRAM. Though I don't know how PC VRAM is formatted. Is it linear? I think I read PC-98 uses planer, where bit 1 of the entire frame buffer is all stored first, then bit 2, up to... is it 3 or 4 bits (8 or 16 color)?)