61 lines
1.1 KiB
ArmAsm
61 lines
1.1 KiB
ArmAsm
// #LAYOUT# STD * #TAKE
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// #LAYOUT# * KERNAL_0 #TAKE
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// #LAYOUT# * * #IGNORE
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//
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// Routine to lock NMI interrupts, using a trick described here:
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// - https://codebase64.org/doku.php?id=base:nmi_lock
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//
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#if CONFIG_TAPE_HEAD_ALIGN
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// XXX consider broader usage of this method to make IO safer
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nmi_lock:
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// We will have to change NMI vector to point empty routine,
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// which will not acknowledge the interrupt - thus preventing
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// the NMI to be triggered again
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// Our code considers zeropage address invalid, so put
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// it there as a preparation
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ldx #$00
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stx NMINV+1
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// We can now safelly replace the vector
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lda #<return_from_interrupt_rti
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sta NMINV+0
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lda #>return_from_interrupt_rti
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sta NMINV+1
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// Stop timer A
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stx CIA2_CRA // $DD0E
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// Set timer A to 0, after starting NMI will occur immediately
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stx CIA2_TIMALO // $DD04
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stx CIA2_TIMAHI // $DD05
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// Set timer A as source for NMI
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lda #$81
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sta CIA2_ICR // $DD0D
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// Start timer A to trigger the NMI
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lda #$01
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sta CIA2_CRA // $DD0E
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// All done. Unmaskable interrupts are now masked.
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rts
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#endif
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