swap_word_bytes.s

<B>Digital的Unix操作系统VAX 4.2源码</B>

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/* ------------------------------------------------------------------ */
/* | Copyright Unpublished, MIPS Computer Systems, Inc.  All Rights | */
/* | Reserved.  This software contains proprietary and confidential | */
/* | information of MIPS and its suppliers.  Use, disclosure or     | */
/* | reproduction is prohibited without the prior express written   | */
/* | consent of MIPS.                                               | */
/* ------------------------------------------------------------------ */

 /*
  * Modification History:
  *
  * 	12-May-1990	Paul Grist
  *		Created this file, and the sw byte swap routine for
  *          	user level VME support.  
  */

#include <asm.h>
#include <regdef.h>


/***************************************************************************
 *
 *   swap__word_bytes(buffer);
 *   unsigned long buffer;
 *
 *   RETURNS: resulting byte swapped value
 *
 ***************************************************************************
 *
 *   swap word bytes: will operate on one unsigned 32 bit quantity
 *
 *
 *  			31       |       0
 *			 +---+---+---+---+
 *	start with:	 | a | b | c | d |
 *			 +---+---+---+---+
 *                               |
 *
 *
 *			31       |       0
 *	end with:        +---+---+---+---+
 *			 | b | a | d | c |
 *			 +---+---+---+---+
 *                               |
 *
 ***************************************************************************/


LEAF(swap_word_bytes)
#ifdef MIPSEL
	# a0 has the input, unsigned 32 bit quanity - abcd
	sll	v0,a0,8		# shift left 8      - bcd0  in v0
	and	v0,0xff00ff00   # mask out b,d      - b0d0  in v0
	srl	v1,a0,8		# shift right 8     - 0abc  in v1
	and	v1,0x00ff00ff   # mask out a,c	    - 0a0c  in v1
	or	v0,v1		# v0 <- v0 | v1	    - badc  in v0
#else
	move	v0,a0		# move v0 into ret value
#endif
	j	ra		# return
.end swap_word_bytes