On CRTCs 1/2/3/4, R0 accepts all values without causing any problems for counters.
On CRTC 0, :* setting R0 to 0 will cause numerous issues that stem from the fact that VLC is frozen. As HCC never reaches 1, the logic that increments VLC is never triggered.* The logic that triggers vertical adjustment happens when C0=2 on the last line of the frame. If R0<2, this test never happens.
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