The VSYNC is also modified before being sent to the monitor. It happens two lines* after the VSYNC from the CRTC and stay two lines (same cut rule if VSYNC is lower than 4). PAL (50Hz) does need two lines VSYNC_width, and 4us HSYNC_width.
Using CRTC1, CRTCs 1 and 2 have a fixed VSYNC width value 0 means a value of 16. VSYNC width can be configured with Register 3 on CRTCs 0, 3 and 4.
== The 6845 Registers ==