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 the first any raster line start of the frameincluding the first one, 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 all CRTCs, program R12/R13 when VCC!=0. This will then take effect at the next frame start.