Changes

Bit0 of port B of the PPI is directly connected to the VSYNC pin of the CRTC.

On CRTCs 0/1/2, the sole condition to trigger a VSYNC is that VCC=R7. While on CRTCs 3/4, it is necessary to have VCC=R7 and HCC=0 and VLC=0 to trigger a VSYNC.