ffs.S

/* Copyright (c) 2005, 2007 Dmitry Xmelkov
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   * Neither the name of the copyright holders nor the names of
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     from this software without specific prior written permission.

  THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
  AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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  ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
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/* $Id$ */

/** \file */

/** \ingroup avr_string
    \fn int ffs (int val);

    \brief This function finds the first (least significant) bit set in the input value.

    \returns The ffs() function returns the position of the first
    (least significant) bit set in the word val, or 0 if no bits are set.
    The least significant bit is position 1.

    \note For expressions that are constant at compile time, consider
    using the \ref _FFS macro instead.
*/

#ifndef __DOXYGEN__

#include "macros.inc"

#define val_lo  r24
#define val_hi  r25
#define cntr    r30

    ASSEMBLY_CLIB_SECTION
    .global     _U(ffs)
    .type       _U(ffs), @function
_U(ffs):

        ldi     cntr, 1
        tst     val_lo
        brne    2f
        or      val_lo, val_hi
        breq    9f
        ldi     cntr, 8
1:      inc     cntr
2:      ror     val_lo
        brcc    1b
        mov     r24, cntr
        clr     r25
9:      ret

    .size  _U(ffs), . - _U(ffs)
    .end
#endif  /* not __DOXYGEN__ */