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.

Wouter adds:

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 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

file: /Techref/microchip/pow2chk.htm, 2KB, , updated: 2017/4/10 15:01, local time: 2024/6/21 22:50, |

©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.massmind.org/techref/microchip/pow2chk.htm"> PIC Microcontoller Power of 2 Check Math Methods </A> |

Did you find what you needed? |

## Welcome to massmind.org! |

## Welcome to www.massmind.org! |

.