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