=== "Rupture For Dummies" bug ===
CRTC 1 has a bug that occurs when R5 is updated with a non-zero value on specific HCC and VLC values, but only when R5 was previously 0. This disrupts the normal VMA update MA reload process, causing the CRTC to load R12/R13 into MA at the beginning of every scanline, regardless of the VCC value.
But if R5 is set back to 0 before HCC reaches R0 on the last scanline of the character line (when VLC=R9), the bug won't trigger.