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 skpnc 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 = 2E4
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 skpnc incf high,fThis modification will yield the result: 4366
Code:
Here is a 9 instruction version of Dmitry's code:+
swapf low,w ; W = d.c
andlw 15 ; W = 0.c
addwf high,w ; W = 0.c + a.b
movwf low ; W > low byte
swapf high,w ; W = b.a
andlw 15 ; W = 0.a
movwf high ; W > high byte
btfsc status,c ; check carry from add
incf high,f ; 0.a + carry
file: /Techref/microchip/math/div/16byconst15dk.htm, 3KB, , updated: 2007/9/11 15:59, local time: 2024/11/5 18:23,
©2024 PLEASE DON'T RIP! THIS SITE CLOSES OCT 28, 2024 SO LONG AND THANKS FOR ALL THE FISH!

©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? <A HREF="http://www.piclist.com/techref/microchip/math/div/16byconst15dk.htm"> Divide 16 bits by the constant value 15</A> 
Did you find what you needed? 
PICList 2024 contributors:
o List host: MIT, Site host massmind.org, Top posters @none found  Page Editors: James Newton, David Cary, and YOU! * Roman Black of Black Robotics donates from sales of Linistep stepper controller kits. * Ashley Roll of Digital Nemesis donates from sales of RCL1 RS232 to TTL converters. * Monthly Subscribers: Gregg Rew. ongoing support is MOST appreciated! * Contributors: Richard Seriani, Sr. 
Welcome to www.piclist.com! 
.