On all CRTCs, if Vertical Total (R4) is updated with a value less than VCC:
* if this update was done when VCC < R4, then VCC overflows and will continue to count up to its maximum value (127) before looping
* if this update was done when VCC >= R4, the decision has already been made to switch to vertical adjustment or vertical border mode and no update to R4 will make the CRTC change its mind for the current frame.
The only exception is for CRTC 1 with a value of 0 when VCC = R4, which will cause the VCC to overflow.