47 lines
1.1 KiB
ArmAsm
47 lines
1.1 KiB
ArmAsm
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irq_handler:
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// The IRQ is a commonly messed with thing on the C64,
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// so we need to handle entry points that are commonly
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// relied upon, including:
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// $EA31 - Standard IRQ routine
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// $EA61 - Check keyboard, but don't update timer? https://github.com/cc65/cc65/issues/324
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// $EA81 - https://www.lemon64.com/forum/viewtopic.php?t=2112&sid=6ea01982b26da69783120a7923ca46fb
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// Also the $0314 vector is widely used (e.g., https://www.lemon64.com/forum/viewtopic.php?t=2112&sid=6ea01982b26da69783120a7923ca46fb)
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// Save registers, sequence according to Compute's Mapping the Commodore 64, page 73
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pha
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txa
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pha
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tya
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pha
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// Check if caused by BRK
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tsx
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lda $0104, x // get the pre-interrupt processor state
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and #$10
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bne irq_handler_brk
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// Not caused by BRK - call interrupt routine (only if initialised)
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lda CINV
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ora CINV+1
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beq !+
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jmp (CINV)
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!:
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// Vector not initialized - call default interrupt routine
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jmp default_irq_handler
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irq_handler_brk:
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// Store BRK adddress, to be displayed
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sec
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lda $0105, x
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sbc #$02
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sta CMP0+0
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lda $0106, x
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sbc #$00
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sta CMP0+1
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// Interrupt caused by BRK
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jmp (CBINV)
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