=== Horizontal and Vertical Sync (R3) ===
UM6845Type 0:
*Bits 7..4 define Vertical Sync Width. If 0 is programmed this gives 16 lines of VSYNC.
*Bits 3..0 define Horizontal Sync Width. If 0 is programmed no HSYNC is generated.
UM6845RType 1:
*Bits 7..4 are ignored. Vertical Sync is fixed at 16 lines.
*Bits 3..0 define Horizontal Sync Width. If 0 is programmed no HSYNC is generated.
MC6845Type 2:
*Bits 7..4 are ignored. Vertical Sync is fixed at 16 lines.
*Bits 3..0 define Horizontal Sync Width. If 0 is programmed this gives a HSYNC width of 16.
Pre-ASICTypes 3/ASIC4:
*Bits 7..4 define Vertical Sync Width. If 0 is programmed this gives 16 lines of VSYNC.
*Bits 3..0 define Horizontal Sync Width. If 0 is programmed this gives a HSYNC width of 16.