On CRTCs 0/3/4, even if the number of lines of VSYNC is set to 1 in R3, the Gate Array will perform a full VSYNC of 26 lines (including 4 lines of C-VSYNC to the monitor). This is also true if the duration of the CRTC VSYNC is reduced to 2 µseconds.
Bit0 of port B of the PPI is directly connected to the VSYNC signal that is produced by pin of the CRTC.
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