Microchip Seepicsrc Psbpix Division.asm

; 
; ***************************************************************************
; ***  Bubble Software Parallax to PIC Source Converter. Copyright 1999.  ***
; ***  http://www.picnpoke.com                 email: sales@picnpoke.com  ***
; ***************************************************************************
;
; DIVIDE
; Divide one 16-bit number into another, returning the 16-bit result and 
; the remainder. Upon entry, the top of the division fraction (the dividend)
; must be in topH and topL, and the bottom (divisor) in btmH and btmL. 
; If division by zero is attempted, the routine will return immediately with 
; the error code of 0FFh in w. Otherwise, it will perform the division, leaving 
; the remainder in topH and topL and the result (quotient) in qH and qL. 
; Upon return from a successful division, w contains 0. 

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

        org     8
topH    Res      d'1'       ; MSB of top of fraction. 
topL    Res      d'1'       ; LSB of top of fraction. 
btmH    Res      d'1'       ; MSB of bottom of fraction.
btmL    Res      d'1'       ; LSB of bottom of fraction.
qH      Res      d'1'       ; MSB of quotient. 
qL      Res      d'1'       ; LSB of quotient. 
count   Res      d'1'       ; temporary counter
index   Res      d'1'       ; temporary counter

        org     0
start        MOVLW d'0'                 ; Set to outputs to display
             TRIS 6h
             MOVLW d'0'                 ; results on LEDs. 
             TRIS 7h
             MOVLW 0x33                 ; Problem: divide
             MOVWF topH
             MOVLW 0x0A                 ; 330Ah by 00A3h. 
             MOVWF topL
             MOVLW 0x00                 
             MOVWF btmH
             MOVLW 0x00A3               
             MOVWF btmL
             CALL divide                
             MOVF qL,w                  ; Display quotient in binary
             MOVWF 6h
             MOVF qH,w                  ; on LEDs connected to rb and rc. 
             MOVWF 7h
             GOTO $                     ; Endless loop

Divide       MOVF btmH,w                ; Check for division by 0.
             IORWF btmL,w               
             BTFSC status,z             
             RETLW d'255'               ; Error code= 255: return. 
             MOVLW d'1'                 ; Otherwise, initialize variables
             MOVWF count
             CLRF index                 ; for the division. 
             CLRF qH                    
             CLRF qL                    

Divide_sh_loop  BTFSC btmH,d'7'         ; Shift divisor left
             GOTO Divide_d1
             BCF status,c               ; until msb is in 
             RLF btmL                   ; btmH.7. 
             RLF btmH                   ; count = no. of shifts+1. 
             INCF count                 
             GOTO Divide_sh_loop        
Divide_d1    BCF status,c               
             RLF qL                     ; Shift quotient left.
             RLF qH                     
             MOVF btmL,w                ; top = top - btm. 
             SUBWF topL
             BTFSC status,c             ; If top - btm < 0 then 
             GOTO Divide_d2
             MOVLW d'1'                 ; top = top + btm
             SUBWF topH
             BTFSC status,c             ; The idea is to do the 
             GOTO Divide_d2
             INCF topH                  ; the subtraction and comparison
             MOVF btmL,w                ; (top > btm?) in one step. 
             ADDWF topL
             goto Divide_reentr		; Then, if btm > top, undo <Microchip instruction>
Divide_d2    MOVF btmH,w                ; the subtraction by adding
             SUBWF topH
             BTFSS status,c             ; top and btm back together
             goto Divide_less      	; <Microchip instruction>
             BSF qL,d'0'                
Divide_reentr
             BCF status,c               
             RRF btmH                   
             RRF btmL                   
             DECFSZ count               
             GOTO Divide_d1
             RETLW 0h                   ; Return w/ remainder in top
					; and result in q. 
Divide_less  MOVF btmL,w                ; btm > top, so 
             ADDWF topL
             BTFSC status,c             ; undo the subtraction by
             INCF topH                  ; adding them back together. 
             MOVF btmH,w                
             ADDWF topH
             goto    Divide_reentr    	; <Microchip instruction>

             
             
             end

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