A2osX/INC/IO.IIC.I.txt
2020-02-17 17:55:02 +01:00

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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
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