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46 lines
1.7 KiB
ArmAsm
46 lines
1.7 KiB
ArmAsm
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; ---------------------------------------------------------------------------
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; interrupt.s
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; ---------------------------------------------------------------------------
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;
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; Interrupt handler.
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;
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; Checks for a BRK instruction and returns from all valid interrupts.
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.import _stop
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.export _irq_int, _nmi_int
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.segment "CODE"
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.PC02 ; Force 65C02 assembly mode
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; ---------------------------------------------------------------------------
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; Non-maskable interrupt (NMI) service routine
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_nmi_int: RTI ; Return from all NMI interrupts
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; ---------------------------------------------------------------------------
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; Maskable interrupt (IRQ) service routine
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_irq_int: PHX ; Save X register contents to stack
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TSX ; Transfer stack pointer to X
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PHA ; Save accumulator contents to stack
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INX ; Increment X so it points to the status
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INX ; register value saved on the stack
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LDA $100,X ; Load status register contents
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AND #$10 ; Isolate B status bit
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BNE break ; If B = 1, BRK detected
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; ---------------------------------------------------------------------------
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; IRQ detected, return
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irq: PLA ; Restore accumulator contents
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PLX ; Restore X register contents
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RTI ; Return from all IRQ interrupts
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; ---------------------------------------------------------------------------
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; BRK detected, stop
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break: JMP break ; If BRK is detected, something very bad
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; has happened, so stop running
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