On CRTC 2, at the beginning of every raster line of the frame (including the first one), MA is loaded with MA'.
On CRTC 1, at the beginning of every raster line of the first character line of the frame (ie. when VCC=0), MA is loaded with R12/R13 instead of MA'. '''This is a major source of incompatibility if the programmer does not take care.''' In demos and games, to be compatible with all CRTCs, program R12/R13 only when VCC != 0. This will then take effect at the next CRTC frame start.
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