Apple II
Technical Notes
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                                                  Developer Technical Support


Apple IIGS
#40:    VBL Signal

Revised by:    Dave Lyons                                           July 1989
Written by:    Rob Moore & Rilla Reynolds                            May 1988

This Technical Note discusses reading the VBL signal to accomplish smooth 
animation.
Changes since November 1988:  Noted that vertical blanking does not begin 
when you might expect on the Apple IIGS and removed references to the Apple 
IIc.
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Applications can accomplish smooth animation on the Apple IIGS and Apple IIe 
by changing the data on the screen during the time the system is tracing the 
unusable area of the display.  This time is called "vertical blanking" or 
"VBL" in this Note.  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.  On 
the IIGS, the screen is blanked when the most significant bit of $C019 is 
high (greater than 127 or $7F), while on the IIe, the screen is blanked when 
the bit is low (less than 128 or $80).

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

On the Apple IIGS, vertical blanking begins at scan line 192 regardless of the 
display mode.  When the Super Hi-Res display is visible, vertical blanking 
begins eight scan lines before the bottom of the display area.  If the VBL 
interrupt is enabled, it triggers at scan line 192.


Further Reference
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    o    Apple IIGS Technical Note #39, Mega II Video Counters