Specifications

Developer Technical Support September 1989
Apple IIGS
#70: Fast Graphics Hints 5 of 4
By using only the VertCnt register and ignoring the low bit of the 9-bit vertical counter stored
in HorizCnt, you can determine within 2 scan lines which scan line is currently being drawn.
The VertCnt video counter contains the number of the current scan line divided by two, offset
by $80. For example, if the scan-line beam was currently refreshing either scan line four or five,
VertCnt would contain $82 (4/2 + $80 or 5/2 + $80). Vertical blanking happens during
VertCnt values $7D through $7F and $E4 through $FF.