=== "Rupture For Dummies" bug ===
CRTC 1 has a bug that occurs when R5 is updated with a non-zero value when HCC=R0 and VLC≠R9, but only when R5 was previously 0. This changes the MA update source to R12/R13 instead of MA'. The CRTC then load loads R12/R13 into MA at the beginning of every scanline, regardless of the VCC value.
The vertical adjustment operates normally after the RFD bug is triggered. If vertical adjustment is not needed, R5 can be reset at any time after the RFD is activated.