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Annotation of xinu/sys/cksum.s, revision 1.1.1.1

1.1       root        1: /* cksum.s - cksum */
                      2: 
                      3: /*
                      4:  *----------------------------------------------------------------------------
                      5:  * cksum -- compute ones complement of 16-bit ones complement checksum
                      6:  *----------------------------------------------------------------------------
                      7:  * This routine computes the one's complement of the 16 bit two's complement
                      8:  * checksum of an array of words, and is thus suitable for computing the
                      9:  * checksum of various levels of the DARPA InterNet protocols.
                     10:  *
                     11:  *  C Calling sequence:
                     12:  *
                     13:  *     check = cksum(&buf, nwords)
                     14:  *
                     15:  *  where arguments are declared as:
                     16:  *
                     17:  *     int     buf[];           array of words to be summed
                     18:  *     int     nwords;          number of 16-bit words in buf
                     19:  *     int     check;           returned value
                     20: */
                     21:  
                     22:        .align  1
                     23:        .text
                     24:        .globl  _cksum
                     25: _cksum:
                     26:        .word   0x4                     /* save r2                      */
                     27:        calls   $0, _ovfloff            /* clear overflow bit           */
                     28:        movl    4(ap),  r1              /* r1 = address of buf          */
                     29:        movl    8(ap),  r2              /* r2 = nwords                  */
                     30:        clrl    r0
                     31: loop:                                  /* no  ==> compute cksum        */
                     32:        addw2   (r1)+,  r0
                     33:        bcc     decrmnt                 /* no carry ==> continue        */
                     34:        incw    r0                      /* add carry to r0              */
                     35: decrmnt:
                     36:        sobgtr  r2,     loop            /* not done? go back to loop    */
                     37:        mcomw   r0,     r0              /* compute one's complement     */
                     38:        ret

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