Apple II
Technical Notes
_____________________________________________________________________________
                                                  Developer Technical Support


Apple IIGS
#40:    VBL Signal

Revised by:    Rilla Reynolds                                   November 1988
Written by:    Rob Moore & Rilla Reynolds                            May 1988

This Technical Note discusses the reading the VBL signal to accomplish smooth 
animation.
_____________________________________________________________________________

Applications can accomplish smooth animation on the Apple IIGS, Apple IIe, and 
Apple IIc by changing the data on the screen during the time the system blanks 
the screen for vertical retracing.  You can determine the state of the VBL 
signal by reading location $C019.

On the Apple IIGS, the $C019 sense of the VBL signal differs from the IIe and 
IIc.  On the IIGS, the screen is blanked with the most significant bit of 
$C019 is high (greater than 127 or $7F), while on the IIe and IIc, the screen 
is blanked when the bit is low (less than 128 or $80).

A VBL interrupt also is available on all three Apple II systems via the Apple 
IIGS Miscellaneous Tool Set or mouse firmware, the Apple IIe mouse card, and 
the Apple IIc mouse firmware.

