ffsalib.s

vxworks源码源码解读是学习vxworks的最佳途径

/* ffsALib.s - i80x86 find first set function */

/* Copyright 1984-1993 Wind River Systems, Inc. */
	.data
	.globl  _copyright_wind_river
	.long   _copyright_wind_river


/*
modification history
--------------------
01c,01jun93,hdn  updated to 5.1.
		  - fixed #else and #endif
		  - changed VOID to void
		  - changed ASMLANGUAGE to _ASMLANGUAGE
		  - changed copyright notice
01b,13oct92,hdn  debugged.
01a,07apr92,hdn  written based on TRON version.
*/

/*
DESCRIPTION
This module defines an optimized version of the C routine in ffsLib.c.
By taking advantage of the BSR instruction of 80X86 processors, the
implementation determines the first bit set in constant time.
*/

#define _ASMLANGUAGE
#include "vxWorks.h"
#include "asm.h"

#if defined(PORTABLE)
#define ffsALib_PORTABLE
#endif

#ifndef ffsALib_PORTABLE

	/* exports */

	.globl	_ffsMsb

	.text
	.align 4


/*******************************************************************************
*
* ffsMsb - find first set bit (searching from the most significant bit)
*
* This routine finds the first bit set in the argument passed it and
* returns the index of that bit.  Bits are numbered starting
* at 1 from the least signifficant bit.  A return value of zero indicates that
* the value passed is zero.
*
* RETURNS: bit position ...

* int ffsMsb (i)
*     int i;       /* argument to find first set bit in *

*/

_ffsMsb:
	movl	SP_ARG1(%esp),%edx		/* %edx = i */
	bsrl	%edx,%eax
	je	ffsNoBitSet 			/* zeros means no bit is set */
	incl	%eax
	ret

	.align 4,0x90
ffsNoBitSet:
	xorl	%eax,%eax
	ret

#endif /* ! ffsALib_PORTABLE */