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I am working on code for the circuitry DEC apparently added to the Graphics Option.

Six ports (0x50 to 0x55) seem to add functionality not present on the original NEC 7220, for example up to 4 bit planes, a color map, a scroll map (and more).

https://dl.dropboxusercontent.com/u/37819653/BANNISTER/GDC_and_EXTRA_PORTS.jpg

Currently, i dont know how to implement the Readback Latch (see path in red at right top) which provides 16 bits to the FIFO.

I see that Read, modify, write (RMW) cycles are unimplemented in upd7220.cpp... There must be a way to pipe back display data (orange) to the GDC. How is it done correctly?

There is also some logic involving a "pixel mask" - data apparently coming from the GDC (green arrow).

Thanks for insights,
Karl

https://dl.dropboxusercontent.com/u...on_Programmers_Reference_Guide_Jun84.pdf

[Click to enlarge block diagram]