mirror of https://github.com/A2osX/A2osX.git
30 lines
1.4 KiB
Plaintext
30 lines
1.4 KiB
Plaintext
NEW
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AUTO 3,1
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.LIST OFF
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*--------------------------------------
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* The action taken for the soft switches
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* from $C058 ... $C05F is only taken if access
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* has first been enabled by writing to IOUDISOFF ($C07F)
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*--------------------------------------
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* Apple IIc Tech Note #9 : Detecting VBL
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* Written by Dan Strnad (November 1989)
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* This Technical Note describes how the VBLInt flag at $C019 behaves differently
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* 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
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* $C019 reads and resets the VBLInt flag. This is not correct.
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*
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* After reading $C019 once the high bit has been set to flag VBL,
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* the high bit remains set. A program polling VBL at $C019 would have to access
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* either PTrig at $C070 or RdIOUDis at $C07E to reset the high-bit for $C019.
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* Note that IOUDis must have been turned off by writing to $C07F
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* then ENVBL accessed at $C05B in order to poll for $C019 on the IIc.
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*--------------------------------------
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IO.IIC.RDVBLMSK .EQ $C041 R, B7=1: VBL interrupts enabled
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*--------------------------------------
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IO.IIC.DISVBL .EQ $C05A W, Disable VBL interrupts
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IO.IIC.ENVBL .EQ $C05B W, Enable VBL interrupts
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IO.IIC.CLRVBLIRQ .EQ $C070 R, Clear the VBL interrupt condition
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*--------------------------------------
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MAN
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SAVE INC/IO.IIC.I
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