On all CRTCs, if Vertical Total (R4) is changed with a value less than VCC, then:
* 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 and counting up again until it reaches the new value of R4
* if this update was done when VCC = R4, the current character line was already decided to be the last one of the current frame. No update to R4 will make the CRTC change its mind for the current frame