;********************************************************
;Int_context
;This program demonstrates need for context saving. 
;Intended for simulation.

;TJW 15.4.05				Tested 17.4.09
;********************************************************
;
;Port A not used. Port B bit 0 used for ext.interrupt ip.
	#include p16f84A.inc
;
rhi	equ	10
rlo	equ	11
phi	equ	12
plo	equ	13
qhi	equ	14
qlo	equ	15  

    	org 	00
    	goto 	start
	org 	04		;here if interrupt occurs
	goto 	Int_Routine
;
start   org 0010
	bsf	intcon,inte 	;enable external interrupt
	bsf	intcon,gie  	;enable global int
loop	movlw 	99
	movwf 	phi
	movwf 	plo
	movwf 	qhi
	movwf 	qlo
	call 	Double_add
	movlw 	00
	movwf 	rhi
	movwf 	rlo
	goto 	loop
;This subroutine adds two 16-bit numbers, stored in phi-plo, and qhi-qlo,
;and stores result in rhi-rlo. 16-bit overflow in Carry flag at end.
Double_add
	movf	plo,0		;move plo to the W reg
	addwf 	qlo,0		;add lower bytes
	movwf	rlo
	btfsc 	status,0
	incf	phi,1		;add in Carry
	movf	phi,0
	addwf 	qhi,0		;add upper bytes
	movwf	rhi
	return
Int_Routine
	bcf		status,0	;clear the Carry flag
	movlw	0ff			;change W reg value
	bcf		intcon,intf
	retfie
	end