entry = $200 irq_vec = 144 org $ffc0 reset cld ldx #$ff txs lda #lo(isr) sta irq_vec lda #hi(isr) sta irq_vec+1 jmp entry irq pha txa pha tya pha jmp (irq_vec) isr pla tay pla tax pla rti nmi rti org $fffa dw nmi dw reset dw irq