During an HSYNC, if HSYNC Width (R3l) is changed with a value less than the current HSC, then HSC is overflowing. The only exception is for CRTC 1 with a value of 0, which cancels the current HSYNC.
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=== VLC (C9) overflow ===
If Number of Scan Lines (R9) is updated with a value less than the current VLC, then:
* on CRTCs 0/1/2, VLC will count to its maximum value (31) before looping
* on CRTCs 3/4, VLC changes to 0 on the next line
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