On all CRTCs, during the vertical display period (ie. when VCC <= R4), 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 back
* if this update was done when VCC = R4, the decision has was already been made to switch to vertical retrace period (non-display) 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.