* on CRTCs 0/1/2, VTAC overflows and will count up to its maximum value (31) before looping back and 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 will end
As an exception, on CRTCs 0/2, if VTAC is modified during the last frame line, then the updated value will not be considered for the current frame.
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