On all CRTCs, while an HSYNC is ongoing, the condition HCC=R2 is ignored. So we cannot trigger a new HSYNC during an HSYNC.
The HSYNC width value is interpreted differently between CRTCs. On CRTCs 0/1, if 0 is programmed then no HSYNC is generated (and therefore, no interrupts). On CRTCs 2/3/4, if 0 is programmed then this gives an HSYNC width of 16.
=== Contiguous HSYNCs ===
