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still_alive: make interrupts work on older Apple IIs
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@ -1,8 +1,8 @@
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Still Alive for the Apple II
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Still Alive for the Apple II
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in roughly 20k
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in roughly 16k
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By Deater (Vince Weaver)
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By Deater (Vince Weaver)
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vince@deater.net
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BACKGROUND
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BACKGROUND
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~~~~~~~~~~
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~~~~~~~~~~
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@ -11,7 +11,11 @@
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; Then it sets up the stack like an interrupt and calls 0x3fe
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; Then it sets up the stack like an interrupt and calls 0x3fe
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interrupt_handler:
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interrupt_handler:
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pha ; save A ; 3
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; pha ; save A ; 3
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txa
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pha
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tya
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pha
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; Should we save X and Y too?
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; Should we save X and Y too?
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; inc $0404 ; debug (flashes char onscreen)
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; inc $0404 ; debug (flashes char onscreen)
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@ -286,6 +290,12 @@ done_interrupt:
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exit_interrupt:
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exit_interrupt:
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pla ; restore a ; 4
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pla ; restore a ; 4
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tay
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pla
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tax
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lda $45 ; restore A saved by firmware
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; needed on older (pre-enhanced IIe)
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rti ; return from interrupt ; 6
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rti ; return from interrupt ; 6
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