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 15:01, local time: 2021/10/18 16:19,

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