I see that there originally was an overscan mode intended. What happened? Is it functional at all?
Sincerly,
John Remensperger
Oakland, California
An attempt to add overscan happened a day or two before final tapeout, but there wasn't enough time to complete it. The register was not removed just in case something useful could be done with it(glitches or not)..
What is known about the register:
Overscan starts fetching data from cycle 8 -> 57 in linear mode.
Enabling overscan for a minimum of 1 cycle after cycle57 and before cycle13 will force DisplayWindowEnable on. DisplayWindowEnable = on in blanking will cause character, color and graphics addresses instead of sprites. It will also start the bitmap pixel shifter.
Matrix base is cleared, linear count A&B are loaded on line 10 instead of 48.
Will set EnableDisplay anywhere in display causing MatrixCounter to increment between cycle 14 and 53. If in chunky LinearCountA will also increment by step rate.
RowCounter will increment on cycle 58
Sprites can collide in border area.