source: buchla-68k/lib700/ispow2.c@ 7c5def4

/*
   =============================================================================
        ispow2.c -- fast test to see if X is a power of 2
        Version 2 -- 1989-01-24 -- D.N. Lynx Crowe

        Returns:

                TRUE if X is a power of 2,
                FALSE otherwise.
   =============================================================================
*/

#include "ram.h"

int16_t ispow2(int32_t x)
{
        register int16_t i;
        register int32_t k;

        k = 0x00000001L;                /* setup the bit mask in k */

        for (i = 32; i--; ) {           /* check each bit in x ... */

                if ((x & k) NE 0L) {    /* ... for the 1st 1 and ... */

                        if ((x & ~k) NE 0L)     /* ... if there are others ... */
                                return(FALSE);  /* ... it's not a power of 2 */
                        else                    /* ... otherwise .. */
                                return(TRUE);   /* ... it is a power of 2 */
                }

                k = k << 1;             /* examine the next position */
        }

        return(FALSE);                  /* no bits on isn't a power of 2 */
}