Apple IIgs Especificaciones descargar pdf (Pagina 29)

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.

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Apple IIgs Especificaciones Pagina 29