# PICMicrocontoller Power of 2 Check MathMethods

## Test if a value is a power of two.

a function that returns zero if INDF is a power of two, else returns non-zero.

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.

the formula is
```( x - 1 ) & ( x | 0x80 )
```

explanation:

• (x-1) leaves all set bits in place, except the rightmost one, which is 'smeared out' over the lower bits.
• Hence (x-1)&(x) is 0 when there was only one bit set, OR x was 0.
• From x==0 follows (x-1) == 0xFF, and (x-1) & (anything<>0) != 0.
• When only one bit is set the highest bit in (x-1) will be clear, so it has no effect (except in the all clear case, where it has the desired effect) to use (x|0x80).

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

end
```

Both 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
end
```

or this:

```is_power_of_2  MACRO  x

movlw  -1
skpnc
andwf x,w

end
```

 file: /Techref/microchip/pow2chk.htm, 2KB, , updated: 2017/4/10 15:01, local time: 2024/7/18 06:12, TOP NEW HELP FIND:  3.236.112.70:LOG IN

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