Implemented preliminary DMAC support, allows Tokyo Nanpa Street to be working (if you know JP, but that's another story).
As a side effect this also fixes the PC-8001 text compatibility mode (N-BASIC), even though screen settings are pretty much busted by now.
fwiw my current docs are very poor on this field, DMAC has 4 address registers, 4 counter registers and a mode register. Basically I just know that the address register should be used as a text vram base and that bit 2 is needed for enabling the transfer, nothing else.
Coming up: fix up text attributes.