Isn't RDAT documented on page 216 of the Programmers Reference from June 1984
On another note, vector and hires modes show something - although parameters like direction (DIR) and X/Y (of lines and arcs) seem to be oddly 'reversed'.
I have to override 'm_bitmap_mod' in UPD7220.CPP, otherwise figures never appear. WDAT 0x22 (see above) frequently sets RMW mode to 2, which zaps all bits. Sure, i haven't fully grasped how the extra hardware and the GDC interact.
Mastermind intro. Circles are OK. Arc over text should appear rotated 180 degrees to right (adjacent to character w)
Graphics Demo. Text barely readable (X axis...), lines have wrong direction
(Solitaire, hires, with vector mode)