a function that returns zero if INDF is a power of two, else returns nonzero.
Dmitry A. Kiryashov [nzews at MAIL.RU] says:
decf INDF,F ;0/1/others > Z=0/Z=1/Z=0 incfsz INDF,W ;0? andwf INDF,W return
Was =0 returning Z=0
Was !=0 and power of 2 returning Z=1
Was !=0 and not a power of 2 returning Z=0
Disadvantage: INDF is decremented in this case.
Wouter adds:
the formula is( x  1 ) & ( x  0x80 )explanation:
 (x1) leaves all set bits in place, except the rightmost one, which is 'smeared out' over the lower bits.
 Hence (x1)&(x) is 0 when there was only one bit set, OR x was 0.
 From x==0 follows (x1) == 0xFF, and (x1) & (anything<>0) != 0.
 When only one bit is set the highest bit in (x1) will be clear, so it has no effect (except in the all clear case, where it has the desired effect) to use (x0x80).
verification:
#include <stdio.h> int f( int x ){ return ( 0xFF & ( (x  1) & ( x  0x80 ) ) ); } int ones( int x ){ int n = 0; while( x > 0 ){ if( x & 1 > 0 ){ n++; } x = x >> 1; } return n; } int main( void ){ int i, j, k; for( i = 0; i < 256; i ++ ){ j = f( i ); k = ones( i ); printf( "%2X %2X %2X %d\n", i, j, k, ( k == 1 ) == ( j == 0 ) ); } return 0; }
Scott Dattalo says:
But it'd probably make more sense to just make this a macro:is_power_of_2 MACRO x decf x,w bsf x,7 andwf x,w endBoth W and the Z bit contain the result.
If you don't wish the register under test to be modified then this is probably the best solution:
is_power_of_2 MACRO x decf x,w movf x,f skpz andwf x,w endor this:
is_power_of_2 MACRO x movlw 1 addwf x,w skpnc andwf x,w end
See also:
file: /Techref/microchip/pow2chk.htm, 2KB, , updated: 2017/4/10 14:01, local time: 2021/3/5 06:26,

©2021 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/pow2chk.htm"> PIC Microcontoller Power of 2 Check Math Methods </A> 
Did you find what you needed? 
PICList 2021 contributors:
o List host: MIT, Site host massmind.org, Top posters @20210305 Neil, RussellMc, Bob Blick, Harold Hallikainen, Spehro Pefhany, Clint Jay, Jason White, Sean Breheny, David C Brown, Allen Mulvey, * 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! 
.