NEW
  AUTO 3,1
				.LIST OFF
*--------------------------------------
* The action taken for the soft switches 
* from $C058 ... $C05F is only taken if access 
* has first been enabled by writing to IOUDISOFF ($C07F)
*--------------------------------------
* Apple IIc Tech Note #9 : Detecting VBL
* Written by Dan Strnad (November 1989)
* 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.
*--------------------------------------
IO.IIC.RDVBLMSK	.EQ $C041				R, B7=1: VBL interrupts enabled
*--------------------------------------
IO.IIC.DISVBL	.EQ $C05A				W, Disable VBL interrupts
IO.IIC.ENVBL	.EQ $C05B				W, Enable VBL interrupts
IO.IIC.CLRVBLIRQ .EQ $C070				R, Clear the VBL interrupt condition 
*--------------------------------------
MAN
SAVE INC/IO.IIC.I
