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 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.
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