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Annotation of 42BSD/sys/vax/in_cksum.c, revision 1.1

1.1     ! root        1: /*     in_cksum.c      6.1     83/07/29        */
        !             2: 
        !             3: #include "../h/types.h"
        !             4: #include "../h/mbuf.h"
        !             5: #include "../netinet/in.h"
        !             6: #include "../netinet/in_systm.h"
        !             7: 
        !             8: /*
        !             9:  * Checksum routine for Internet Protocol family headers (VAX Version).
        !            10:  *
        !            11:  * This routine is very heavily used in the network
        !            12:  * code and should be modified for each CPU to be as fast as possible.
        !            13:  */
        !            14: 
        !            15: in_cksum(m, len)
        !            16:        register struct mbuf *m;
        !            17:        register int len;
        !            18: {
        !            19:        register u_short *w;            /* on vax, known to be r9 */
        !            20:        register int sum = 0;           /* on vax, known to be r8 */
        !            21:        register int mlen = 0;
        !            22: 
        !            23:        for (;;) {
        !            24:                /*
        !            25:                 * Each trip around loop adds in
        !            26:                 * word from one mbuf segment.
        !            27:                 */
        !            28:                w = mtod(m, u_short *);
        !            29:                if (mlen == -1) {
        !            30:                        /*
        !            31:                         * There is a byte left from the last segment;
        !            32:                         * add it into the checksum.  Don't have to worry
        !            33:                         * about a carry-out here because we make sure
        !            34:                         * that high part of (32 bit) sum is small below.
        !            35:                         */
        !            36:                        sum += *(u_char *)w << 8;
        !            37:                        w = (u_short *)((char *)w + 1);
        !            38:                        mlen = m->m_len - 1;
        !            39:                        len--;
        !            40:                } else
        !            41:                        mlen = m->m_len;
        !            42:                m = m->m_next;
        !            43:                if (len < mlen)
        !            44:                        mlen = len;
        !            45:                len -= mlen;
        !            46:                /*
        !            47:                 * Force to long boundary so we do longword aligned
        !            48:                 * memory operations.  It is too hard to do byte
        !            49:                 * adjustment, do only word adjustment.
        !            50:                 */
        !            51:                if (((int)w&0x2) && mlen >= 2) {
        !            52:                        sum += *w++;
        !            53:                        mlen -= 2;
        !            54:                }
        !            55:                /*
        !            56:                 * Do as much of the checksum as possible 32 bits at at time.
        !            57:                 * In fact, this loop is unrolled to make overhead from
        !            58:                 * branches &c small.
        !            59:                 *
        !            60:                 * We can do a 16 bit ones complement sum 32 bits at a time
        !            61:                 * because the 32 bit register is acting as two 16 bit
        !            62:                 * registers for adding, with carries from the low added
        !            63:                 * into the high (by normal carry-chaining) and carries
        !            64:                 * from the high carried into the low on the next word
        !            65:                 * by use of the adwc instruction.  This lets us run
        !            66:                 * this loop at almost memory speed.
        !            67:                 *
        !            68:                 * Here there is the danger of high order carry out, and
        !            69:                 * we carefully use adwc.
        !            70:                 */
        !            71:                while ((mlen -= 32) >= 0) {
        !            72: #undef ADD
        !            73:                        asm("clrl r0");         /* clears carry */
        !            74: #define ADD            asm("adwc (r9)+,r8;");
        !            75:                        ADD; ADD; ADD; ADD; ADD; ADD; ADD; ADD;
        !            76:                        asm("adwc $0,r8");
        !            77:                }
        !            78:                mlen += 32;
        !            79:                while ((mlen -= 8) >= 0) {
        !            80:                        asm("clrl r0");
        !            81:                        ADD; ADD;
        !            82:                        asm("adwc $0,r8");
        !            83:                }
        !            84:                mlen += 8;
        !            85:                /*
        !            86:                 * Now eliminate the possibility of carry-out's by
        !            87:                 * folding back to a 16 bit number (adding high and
        !            88:                 * low parts together.)  Then mop up trailing words
        !            89:                 * and maybe an odd byte.
        !            90:                 */
        !            91:                { asm("ashl $-16,r8,r0; addw2 r0,r8");
        !            92:                  asm("adwc $0,r8; movzwl r8,r8"); }
        !            93:                while ((mlen -= 2) >= 0) {
        !            94:                        asm("movzwl (r9)+,r0; addl2 r0,r8");
        !            95:                }
        !            96:                if (mlen == -1) {
        !            97:                        sum += *(u_char *)w;
        !            98:                }
        !            99:                if (len == 0)
        !           100:                        break;
        !           101:                /*
        !           102:                 * Locate the next block with some data.
        !           103:                 * If there is a word split across a boundary we
        !           104:                 * will wrap to the top with mlen == -1 and
        !           105:                 * then add it in shifted appropriately.
        !           106:                 */
        !           107:                for (;;) {
        !           108:                        if (m == 0) {
        !           109:                                printf("cksum: out of data\n");
        !           110:                                goto done;
        !           111:                        }
        !           112:                        if (m->m_len)
        !           113:                                break;
        !           114:                        m = m->m_next;
        !           115:                }
        !           116:        }
        !           117: done:
        !           118:        /*
        !           119:         * Add together high and low parts of sum
        !           120:         * and carry to get cksum.
        !           121:         * Have to be careful to not drop the last
        !           122:         * carry here.
        !           123:         */
        !           124:        { asm("ashl $-16,r8,r0; addw2 r0,r8; adwc $0,r8");
        !           125:          asm("mcoml r8,r8; movzwl r8,r8"); }
        !           126:        return (sum);
        !           127: }

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