;
; ***************************************************************************
; ***  Bubble Software Parallax to PIC Source Converter. Copyright 1999.  ***
; ***  http://www.picnpoke.com                 email: sales@picnpoke.com  ***
; ***************************************************************************
;
; ADD/SUBTRACT n1, n2
; Add two 16-bit numbers to produce a 16-bit result, permitting overflow. 
; Add the two's complement of one 16-bit number to another 16-bit number
;  to perform subtraction, permitting underflow. 

; Addition: n1 = n1 + n2
; Subtraction: n1 = n1 - n2

; Device data and reset vector
	P = pic16c55
	#include <16c55.inc>   ; processor assembler definitions
	_CONFIG _xt_osc & _wdt_off & _protect_off
        reset   start

        org     8
n1H     Res      d'1'       ; MSB of n1. 
n1L     Res      d'1'       ; LSB of n1. 
n2H     Res      d'1'       ; MSB of n2.
n2L     Res      d'1'       ; LSB of n2.

        org     0

start        MOVLW d'0'                 ; Outputs for LEDs to 
             TRIS 6h
             MOVLW d'0'                 ; display 16-bit result.
             TRIS 7h
             MOVLW 0x00C8               ; Problem: C836h-2575h
             MOVWF n1H
             MOVLW 0x36                 
             MOVWF n1L
             MOVLW 0x25                 
             MOVWF n2H
             MOVLW 0x75                 
             MOVWF n2L
             CALL Sub16                 ; n1 = n1 - n2
             MOVF n1L,w                 ; Show result in binary on 
             MOVWF 6h
             MOVF n1H,w                 ; LEDs connected to ports.
             MOVWF 7h
             GOTO $                     ; Endless loop

Sub16        COMF n2H                   ; Take two's complement
             COMF n2L                   ; of n2. If zero, n2L
             BTFSC status,z             ; overflowed, so carry 
             INCF n2H                   ; into n2H.
Add16        MOVF n2L,w                 ; Add the LSBs. If carry,
             ADDWF n1L
             BTFSC status,c             ; increment MSB of sum. 
             INCF n1H                   
             MOVF n2H,w                 ; Add the MSBs  
             ADDWF n1H
             RETLW 0h                   
             
             
             end