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 back
* if this update was done when VCC = R4, the decision was already made to switch to vertical adjust adjustment mode and no update to R4 will make the CRTC change its mind for the current frame
The only exception when VCC = R4 is for CRTC 1 with a value of 0, which will cause the VCC to overflow.