apple2a/interrupt.s
2018-07-27 22:30:44 -07:00

46 lines
1.7 KiB
ArmAsm

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