Apple II Technical Notes _____________________________________________________________________________ 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. _____________________________________________________________________________ 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 _____________________________________________________________________________ o Apple IIc Technical Reference Manual, Second Edition