Apple II
Technical Notes
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                                                  Developer Technical Support


Apple IIc
#9:    Detecting VBL

Written by:    Dan Strnad                                       November 1990

This Technical Note describes how the VBLInt flag at $C019 behaves differently
than documented in the Apple IIc Technical Reference Manual, Second Edition,
when being polled.
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The Apple IIc Technical Reference Manual, Second Edition, claims that reading
$C019 reads and resets the VBLInt flag.  This is not correct.  After reading
$C019 once the high bit has been set to flag VBL, the high bit remains set.  A
program polling VBL at $C019 would have to access either PTrig at $C070 or
RdIOUDis at $C07E to reset the high-bit for $C019.  Note that IOUDis must have
been turned off by writing to $C07F then ENVBL accessed at $C05B in order to
poll for $C019 on the IIc.

Developers are encouraged to detect VBL using an interrupt handler with one of
the VBL-based mouse modes on the IIc, if possible.


Further Reference
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  o  Apple IIc Technical Reference Manual, Second Edition

