R31 doesn't exist on types 0,2,3,4.
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=== R12/R13 on Type 1 ===
The UM6845R differs to other CRTC in respect of R12/R13.
When VCC=0, R12/R13 is re-read at the start of each line. R12/R13 can therefore be changed for each scanline when VCC=0.
Just like other CRTCs when RC==(R9-1), the current MA is captured for the next char-line.
In demos to make a display compatible with all CRTCs, program R12/R13 when VCC!=0. This will then take effect at the next frame start.
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