== HSYNC and VSYNC ==
On CPC, HSYNC and VSYNC from the CRTC are passed into to the [[Gate-Array]] for further modification. See its wiki page.
When The HSYNC width value is active Gate-Array outputs the palette colour blackinterpreted differently between CRTCs. If the On CRTCs 0/1, if 0 is programmed no HSYNC is set to 14 characters then black will be output for 14usgenerated. On CRTCs 2/3/4, if 0 is programmed this gives a HSYNC width of 16.
The HSYNC is modified before being sent to the monitor. It happens 2us after the HSYNC from the CRTC and lasts 4us when HSYNC length is greater or equal to 6. If R2=46, and HSYNC width is 14 then monitor hsync starts at 48 and lasts until 51. On a CPC monitor, the HSYNC is rendered in "absolute black". It is darker than the black output by the Gate-Array. 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. CRTCs 1 and /2 have a fixed VSYNC width value of 16. VSYNC width can be configured with Register 3 on CRTCs 0, /3 and /4. If 0 is programmed this gives 16 lines of VSYNC.
The bit 0 of port B of the PPI changes to 1 as soon as the VSYNC signal is produced by the CRTC.
|2||Horizontal Sync Position ||00000000||46||When to start the HSync signal.
|-
|3||Horizontal and Vertical Sync Widths||VVVVHHHH||128+14||HSync pulse width in characters (0 means 16 only on some CRTCs. Needs to be at least 2 for Gate Array to change the video mode); VSync width in scan-lines (0 always means 16. Not present on all CRTCs, fixed to 16 lines on these).
|-
|4||Vertical Total (-1)||x0000000||38||Height of the screen, in characters.
