During vertical adjustment mode, if Vertical Total Adjust (R5) is changed with a value less than the current VTAC, then:
* on CRTCs 0/1/2, VTAC overflows and will continue to count up to its maximum value (31) before looping backand counting up again until it reaches the new value of R5
* on CRTCs 3/4, the current line is considered the last one of the current frame and vertical adjustment ends