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PIC Microcontroller Basic Math Division Methods

Divide unsigned 8 bit integer in W by the constant value 5

;-------------------------------------------------------------------------------
; U08DIV5	v.20110117 	By Ihsan V. TORE  
;
; 8 bit unsigned integer divide by 5
; Calculated by subtracting 160:80:40:20:10: 5 from the byte successively, and 
; adding result 32:16: 8: 4: 2: 1 respectively if subtractions are positive.
;
; Code : 26 instructions
; Time : 26 cycles
; Ram  :  1 bytes
;
; Args : W        : u08 : dividend
; Retv : result   : u08 : x DIV 5
;        W        : u08 : x MOD 5 
;
; Note : Heavily optimized for speed.
; Lic. : MIT.		  
;-------------------------------------------------------------------------------
U08DIV5:
    clrf    result          ; result = 0
  
    addlw   .256-.160       ; W -= 160
    rlf     result,    f    ; load cf to result bit
    btfss   result,    0    ; skip if positive (CF == 1)    
    addlw   .160            ; else reload W    (CF == 0)
  
    addlw   .256-.80        ; W -= 80
    rlf     result,    f    ; load cf to result bit
    btfss   result,    0    ; skip if positive (CF == 1)    
    addlw   .80             ; else reload W    (CF == 0)

    addlw   .256-.40        ; W -= 40
    rlf     result,    f    ; load cf to result bit
    btfss   result,    0    ; skip if positive (CF == 1)    
    addlw   .40             ; else reload W    (CF == 0)

    addlw   .256-.20        ; W -= 20
    rlf     result,    f    ; load cf to result bit
    btfss   result,    0    ; skip if positive (CF == 1)    
    addlw   .20             ; else reload W    (CF == 0)

    addlw   .256-.10        ; W -= 10
    rlf     result,    f    ; load cf to result bit
    btfss   result,    0    ; skip if positive (CF == 1)    
    addlw   .10             ; else reload W    (CF == 0)

    addlw   .256-.5         ; W -= 5
    rlf     result,    f    ; load cf to result bit
    btfss   result,    0    ; skip if positive (CF == 1)    
    addlw   .5              ; else reload W    (CF == 0)

    return                  ; return the result

;-------------------------------------------------------------------------------
; Here is the version optimized for size if code space is more important
;-------------------------------------------------------------------------------
; U08DIV5	v.20110116 	By Ihsan V. TORE  
;
; 8 bit unsigned integer divide by 5
; Calculated by subtracting 160:80:40:20:10: 5 from the byte successively, and 
; adding result 32:16: 8: 4: 2: 1 respectively if subtractions are positive.
;
; Code : 12 instructions
; Time : 61 cycles
; Ram  :  2 bytes
;
; Args : dividend : u08 : dividend
; Retv : result   : u08 : x DIV 5
;	 dividend : u08 : x MOD 5 
;
; Note : Heavily optimized for size.
; Lic. : MIT.		  
;-------------------------------------------------------------------------------
U08DIV5:
	clrf	result			; prelude :)
	movlw	.160			; W = subtract = 160
	movwf	subtract
U08DIV5_LOOP:
	subwf	dividend,	W	; W = dividend - subtract
	skpnc				; skip if negative  (CF == 0)
	movwf	dividend		; else dividend = W (CF == 1)
        rlf	result,		F	; load CF to corresponding result bit	
U08DIV5_NEXT:				; CF == 0 here always
	rrf	subtract,	F	; subtract /= 2
	skpnc				; if carry return
	return
	movfw	subtract		; W = subtract
	goto	U08DIV5_LOOP		; loop

+

Ihsan Volkan Tore Says:

This might easily be "reinventing the wheel". I have not seen any routine like this before (probably because I did not sought enough) :). But I think this will be wellcome by all lazy coders like me when needed. Please note that, the first version without the loop can be useful for divisions by 10,20,40 and 80. You can accomplish this by returning after the reloader addlw instructions of respective subtractions. You can omit the last btfss and addlw instructions if you do not need the modulo. Cut paste run and have fun...
+

Ihsan Volkan Tore Says: " Yes, digging into piclist, I've found that this algorithm is called kenyan method. I reinvented the wheel...again... :) " +

See also:


file: /Techref/microchip/math/div/8_bits_by_the_constant_5.htm, 4KB, , updated: 2011/1/17 23:42, local time: 2020/2/20 21:53, owner: IVT-TORETEK-TAA,
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