On CRTCs 0/3/4, at the beginning of the first raster line of the frame, MA and MA' are loaded with R12/R13. Otherwise, MA is loaded with MA'.
On CRTC 2, at the beginning of each every raster line (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 discrepancy 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 when VCC!=0. This will then take effect at the next frame start.