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Annotation of 43BSD/sys/vax/ns_cksum.c, revision 1.1

1.1     ! root        1: /*
        !             2:  * Copyright (c) 1985, 1986 Regents of the University of California.
        !             3:  * All rights reserved.  The Berkeley software License Agreement
        !             4:  * specifies the terms and conditions for redistribution.
        !             5:  *
        !             6:  *     @(#)ns_cksum.c  7.1 (Berkeley) 6/5/86
        !             7:  */
        !             8: #include "types.h"
        !             9: #include "mbuf.h"
        !            10: 
        !            11: 
        !            12: /*
        !            13:  * Checksum routine for Network Systems Protocol Packets (VAX Version).
        !            14:  *
        !            15:  * This routine is very heavily used in the network
        !            16:  * code and should be modified for each CPU to be as fast as possible.
        !            17:  */
        !            18: 
        !            19: u_short
        !            20: ns_cksum(m, len)
        !            21:        register struct mbuf *m;
        !            22:        register int len;
        !            23: {
        !            24:        register u_short *w;            /* on vax, known to be r9 */
        !            25:        register int sum = 0;           /* on vax, known to be r8 */
        !            26:        register int low = 0;           /* on vax, known to be r7 */
        !            27:        register int mlen = low;        /* want 0, shuts lint up about low */
        !            28: 
        !            29:        for (;;) {
        !            30:                /*
        !            31:                 * Each trip around loop adds in
        !            32:                 * word from one mbuf segment.
        !            33:                 */
        !            34:                w = mtod(m, u_short *);
        !            35:                if (mlen == -1) {
        !            36:                        /*
        !            37:                         * There is a byte left from the last segment;
        !            38:                         * add it into the checksum.  Don't have to worry
        !            39:                         * about a carry-out here because we make sure
        !            40:                         * that high part of (32 bit) sum is small below.
        !            41:                         */
        !            42:                        sum += *(u_char *)w << 8;
        !            43:                        sum += sum;
        !            44:                        w = (u_short *)((char *)w + 1);
        !            45:                        mlen = m->m_len - 1;
        !            46:                        len--;
        !            47:                } else
        !            48:                        mlen = m->m_len;
        !            49:                m = m->m_next;
        !            50:                if (len < mlen)
        !            51:                        mlen = len;
        !            52:                len -= mlen;
        !            53:                /*
        !            54:                 * Force to long boundary so we do longword aligned
        !            55:                 * memory operations.  It is too hard to do byte
        !            56:                 * adjustment, do only word adjustment.
        !            57:                 */
        !            58:                if (((int)w&0x2) && mlen >= 2) {
        !            59:                        sum += *w++;
        !            60:                        sum += sum;
        !            61:                        mlen -= 2;
        !            62:                }
        !            63:                /*
        !            64:                 *
        !            65:                 * We can do a 16 bit ones complement sum using
        !            66:                 * 32 bit arithmetic registers for adding,
        !            67:                 * with carries from the low added
        !            68:                 * into the high (by normal carry-chaining)
        !            69:                 * so long as we fold back before 16 carries have occured.
        !            70:                 *
        !            71:                 */
        !            72:                while ((mlen -= 32) >= 0) {
        !            73:                        /*asm("bicpsw $1");              clears carry */
        !            74: #undef ADD
        !            75: #define ADD asm("movw (r9)+,r7")asm("addl2 r7,r8")asm("addl2 r8,r8")
        !            76: #define FOLD { asm("ashl $-16,r8,r0")asm(" addw2 r0,r8"); \
        !            77:                  asm("adwc $0,r8")asm(" movzwl r8,r8"); }
        !            78:                        FOLD;
        !            79:                        ADD; ADD; ADD; ADD; ADD; ADD; ADD; ADD;
        !            80:                        FOLD;
        !            81:                        ADD; ADD; ADD; ADD; ADD; ADD; ADD; ADD;
        !            82:                }
        !            83:                mlen += 32;
        !            84:                while ((mlen -= 8) >= 0) {
        !            85:                        /*asm("bicpsw $1");              clears carry */
        !            86:                        FOLD;
        !            87:                        ADD; ADD; ADD; ADD;
        !            88:                }
        !            89:                mlen += 8;
        !            90:                /*
        !            91:                 * Now eliminate the possibility of carry-out's by
        !            92:                 * folding back to a 16 bit number (adding high and
        !            93:                 * low parts together.)  Then mop up trailing words
        !            94:                 * and maybe an odd byte.
        !            95:                 */
        !            96:                FOLD;
        !            97:                while ((mlen -= 2) >= 0) {
        !            98:                        ADD;
        !            99:                }
        !           100:                if (mlen == -1) {
        !           101:                        sum += *(u_char *)w;
        !           102:                }
        !           103:                if (len == 0)
        !           104:                        break;
        !           105:                /*
        !           106:                 * Locate the next block with some data.
        !           107:                 * If there is a word split across a boundary we
        !           108:                 * will wrap to the top with mlen == -1 and
        !           109:                 * then add it in shifted appropriately.
        !           110:                 */
        !           111:                for (;;) {
        !           112:                        if (m == 0) {
        !           113:                                printf("idpcksum: out of data\n");
        !           114:                                goto done;
        !           115:                        }
        !           116:                        if (m->m_len)
        !           117:                                break;
        !           118:                        m = m->m_next;
        !           119:                }
        !           120:        }
        !           121: done:
        !           122:        /*
        !           123:         * Add together high and low parts of sum
        !           124:         * and carry to get cksum.
        !           125:         * Have to be careful to not drop the last
        !           126:         * carry here.
        !           127:         */
        !           128:        FOLD;
        !           129:        
        !           130:        if(sum==0xffff) sum = 0;
        !           131:        return (sum);
        !           132: }

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