please dont rip this site

PIC Microcontoller Math Method

Divide 16 bits by the constant value 15

From: Dmitry Kiryashov

;high=a.b , low=c.d
	movlw   0xF0
	andwf   low,F   ;c.0
	andwf   high,W  ;a.0

; at this point, the low nibble of `low' is zero
; W contains the high nibble of `high'

	xorwf   high,W  ;a.0^a.b
; now W contains low nibble of high, namely 0x0b
;    W  ^ high
;  0xa0 ^ 0xab => 0x0b

	xorwf   high,F  ;a.0
;Now the lower nibble of a is cleared.
;    W  ^ high
;  0x0b ^ 0xab => 0xa0  and this is stored back in high

	xorwf   high,W  ;a.b
;Now W contains the original value of high
;    W  ^ high
;  0x0b ^ 0xa0 => 0xab

	swapf   low,F   ;0.c
	swapf   high,F  ;0.a

	addwf   low,F   ;0.c + a.b
	incf    high,F  ;0.a + carry

;11 clocks/words

Scott Dattalo says:

BTW, you should be aware that Nik's generator is more accurate than what Dmitry and I generated. The algorithm is based on this formula:
  1/(A+B) ~= B/A - (B/A)^2 + (B/A)^3 + ...

Dmitry and I computed the first two terms, Nik does all three in the generator.

The error is on the order of 1/16/16/16 = 2E-4

For example:

suppose you wanted to divide 65535 by 15. The exact answer is 4369. However, using Dmitry's code you'd get: 4350. Nik's produces: 4365 (I think).

But a slight mod will improve Dmitry's

> > ;high=a.b , low=c.d
> >
> >         movlw   0xF0
> >         andwf   low,F   ;c.0
> >         andwf   high,W  ;a.0
> >
> >         xorwf   high,W  ;a.0^a.b
> >         xorwf   high,F  ;a.0
> >         xorwf   high,W  ;a.b
> >
> >         swapf   low,F   ;0.c
> >         swapf   high,F  ;0.a
> >
> >         addwf   low,F   ;0.c + a.b
> >         skpnc
> >         incf    high,F  ;0.a + carry

        movf  high,w
        addwf low,f
         incf high,f

This modification will yield the result: 4366


file: /Techref/microchip/math/div/16byconst15-dk.htm, 3KB, , updated: 2007/9/11 15:59, local time: 2024/3/4 03:36,

 ©2024 These pages are served without commercial sponsorship. (No popup ads, etc...).Bandwidth abuse increases hosting cost forcing sponsorship or shutdown. This server aggressively defends against automated copying for any reason including offline viewing, duplication, etc... Please respect this requirement and DO NOT RIP THIS SITE. Questions?
Please DO link to this page! Digg it! / MAKE!

<A HREF=""> Divide 16 bits by the constant value 15</A>

After you find an appropriate page, you are invited to your to this massmind site! (posts will be visible only to you before review) Just type a nice message (short messages are blocked as spam) in the box and press the Post button. (HTML welcomed, but not the <A tag: Instead, use the link box to link to another page. A tutorial is available Members can login to post directly, become page editors, and be credited for their posts.

Link? Put it here: 
if you want a response, please enter your email address: 
Attn spammers: All posts are reviewed before being made visible to anyone other than the poster.
Did you find what you needed?


Welcome to!


Welcome to!