On CRTCs 0/3/4, at the first raster line start of the frame, MA and MA' are loaded with R12/R13.
On CRTC 2, at any raster line start of the frame , including the first one, MA is loaded with MA'.
On CRTC 1, at 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 all CRTCs, program R12/R13 when VCC!=0. This will then take effect at the next frame start.