;
; ***************************************************************************
; ***  Bubble Software Parallax to PIC Source Converter. Copyright 1999.  ***
; ***  http://www.picnpoke.com                 email: sales@picnpoke.com  ***
; ***************************************************************************
;
; COMPARE n1, n2
; Compares two 16-bit numbers by subtracting one from the other and reports the 
; outcome by a code in the w register. Does not alter n1 or n2. Uses the status
; register bit PA2 to preserve the z flag from subtraction of the LSBs. If you 
; adapt this code for use in PICs other than 16C5x series, make sure that this
; bit is available. If it isn't, use a bit in one of the file registers instead. 

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

Z_      equ        PA2    ; Unused status bit as extra Z flag. 
        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. 
             TRIS 6h
             MOVLW 0x00C8               ; Compare 0C836h to
             MOVWF n1H
             MOVLW 0x36                 ; 2575h. 
             MOVWF n1L
             MOVLW 0x25                 
             MOVWF n2H
             MOVLW 0x75                 
             MOVWF n2L
             CALL Comp16                ; Call the subroutine. 
             MOVWF 6h                   ; Show result on LEDs
             GOTO $                     ; Endless loop

; Upon return, a code in w will indicate the outcome of the comparison. 
; If n1 > n2 then w = 1. If n1 < n2 then w =2. If n1 = n2 then w = 0. 
Comp16       BCF status,pa2             ; Clear aux Z bit.
             MOVF n2L,w                 ; w = n2L
             SUBWF n1L,0		; w = n1L-n2L <Microchip instruction>
             BTFSC status,z             ; Copy z bit to Z_. 
             BSF status,pa2             
             MOVF n2H,w                 ; If n1L-n2L underflowed, 
             BTFSC status,c             ; then increment n2H (same
             GOTO Comp16_cont
             INCF n2H,w                 ; as decrementing n1H).
             BTFSC status,z             ; If n2H overflows, n1 must be < n2
             RETLW d'2'                 ; so return with 2 in w.
Comp16_cont  SUBWF n1H,0            	; w = n1H-n2H  <Microchip instruction>
             BTFSS status,c             ; If n1H underflows, n1<n2 retw 2 ; so return with 2 in w. jnb Z_,:cont2 ; By now we're sure n1'>=n2. 
             BTFSC status,z             ; If both z and Z_ are set, 
             RETLW d'0'                 ; both subtractions resulted in 0, so
Comp16_cont2 RETLW d'1'                 ; n1=n2. Otherwise n1>n2.
             
             
             end