in_checksum.s

<B>Digital的Unix操作系统VAX 4.2源码</B>

/*	@(#)in_checksum.s	4.1	(ULTRIX)	7/2/90	*/
/************************************************************************
 *									*
 *			Copyright (c) 1988 by				*
 *		Digital Equipment Corporation, Maynard, MA		*
 *			All rights reserved.				*
 *									*
 *   This software is furnished under a license and may be used and	*
 *   copied  only  in accordance with the terms of such license and	*
 *   with the  inclusion  of  the  above  copyright  notice.   This	*
 *   software  or  any  other copies thereof may not be provided or	*
 *   otherwise made available to any other person.  No title to and	*
 *   ownership of the software is hereby transferred.			*
 *									*
 *   This software is  derived  from  software  received  from  the	*
 *   University    of   California,   Berkeley,   and   from   Bell	*
 *   Laboratories.  Use, duplication, or disclosure is  subject  to	*
 *   restrictions  under  license  agreements  with  University  of	*
 *   California and with AT&T.						*
 *									*
 *   The information in this software is subject to change  without	*
 *   notice  and should not be construed as a commitment by Digital	*
 *   Equipment Corporation.						*
 *									*
 *   Digital assumes no responsibility for the use  or  reliability	*
 *   of its software on equipment which is not supplied by Digital.	*
 *									*
 ************************************************************************/
#include <machine/asm.h>
#include <machine/regdef.h>


/*
 * in_checksum(addr, len, prevcksum)
 *
 * Calculates a 16 bit ones-complement checksum.
 * Note that for a big-endian machine, this routine always adds even
 * address bytes to the high order 8 bits of the 16 bit checksum and
 * odd address bytes are added to the low order 8 bits of the 16 bit checksum.
 * For little-endian machines, this routine always adds even address bytes
 * to the low order 8 bits of the 16 bit checksum and the odd address bytes
 * to the high order 8 bits of the 16 bit checksum.
 */
LEAF(in_checksum)
	move	v0,a2		# copy previous checksum
	beq	a1,zero,4f	# count exhausted
	and	v1,a0,1
	beq	v1,zero,2f	# already on a halfword boundry
	lbu	t8,0(a0)
	addu	a0,1
#ifdef MIPSEL
	sll	t8,8
#endif MIPSEL
	addu	v0,t8
	subu	a1,1
	b	2f

1:	lhu	t8,0(a0)
	addu	a0,2
	addu	v0,t8
	subu	a1,2
2:	bge	a1,2,1b
	beq	a1,zero,3f	# no trailing byte
	lbu	t8,0(a0)
#ifdef MIPSEB
	sll	t8,8
#endif MIPSEB
	addu	v0,t8
3:	srl	v1,v0,16	# add in all previous wrap around carries
	and	v0,0xffff
	addu	v0,v1
	srl	v1,v0,16	# wrap-arounds could cause carry, also
	addu	v0,v1
	and	v0,0xffff
4:	j	ra
	END(in_checksum)