AVR-Chat

Hi,

> The following comninations will also yield a 1:
> 
> 0111
> 1011
> 1101
> 1110
> 
> I think that David made a mistake on his 0111 entry.
> 
> To check that only one bit is set is the same thing as saying 
> that a number is a power of two.
> 
> If the expression ( x & ( x - 1 )) yields zero, then x is a 
> power of two.

To be completely pedantic, that isn't quite true when x==0 as zero is
not a power of two.

What you could also point out is that x & (x-1) removes the rightmost 1
bit of x and can easily be used to accelerate a "bit count" subroutine:

unsigned bitcnt(unsigned x)
{
  unsigned n;
  if (x == 0)
    return 0;
  n = 1;
  while ((x &= x-1) != 0)
    ++n;
  return n;
}

--
Paul Curtis, Rowley Associates Ltd http://www.rowley.co.uk
CrossWorks for MSP430, ARM, and (soon) Atmel AVR processors