;
; Oliver Schmidt, 2012-01-18
;
; void __fastcall__ set_irq (irq_handler f, void *stack_addr, size_t stack_size);
; void reset_irq (void);
;

	.export	   	_set_irq, _reset_irq
	.interruptor    clevel_irq, 1		; Export as low priority IRQ handler
	.import 	popax

	.include        "zeropage.inc"

	.macpack	generic


.proc	_set_irq

	; Keep clevel_irq from being called right now
	sei

	; Set irq stack pointer to stack_addr + stack_size
	sta	irqsp
	stx	irqsp+1
	jsr	popax
	add	irqsp
	sta	irqsp
	txa
	adc	irqsp+1
	sta	irqsp+1

	; Set irq vector to irq_handler
	jsr	popax
	sta	irqvec+1
	stx	irqvec+2 	; Set the user vector

	; Restore interrupt requests and return
	cli
	rts

.endproc


.proc	_reset_irq

	lda  	#$00
	sta	irqvec+2	; High byte is enough
	rts

.endproc


.proc	clevel_irq

	; Is C level interrupt request vector set?
	lda    	irqvec+2	; High byte is enough
	bne	@L1
	clc			; Interrupt not handled
	rts

	; Save our zero page locations
@L1:	ldx     #zpspace-1
@L2:    lda     sp,x
	sta     zpsave,x
	dex
	bpl     @L2

	; Set C level interrupt stack
	lda	irqsp
	ldx	irqsp+1
	sta	sp
	stx	sp+1

	; Call C level interrupt request handler
	jsr	irqvec

	; Copy back our zero page content
	ldx     #zpspace-1
@L3:    ldy     zpsave,x
	sty     sp,x
	dex
	bpl     @L3

	; Mark interrupt handled / not handled and return
	lsr
	rts

.endproc

; ---------------------------------------------------------------------------

.data

irqvec:	jmp	$00FF		; Patched at runtime

; ---------------------------------------------------------------------------

.bss

irqsp:	.res	2

zpsave: .res    zpspace
