Em Fri, Oct 31, 2003 at 04:38:43PM -0800, Stephen Hemminger escreveu:
>
> Okay, here is the standard: (Inside Appletalk)
>
> > The DDP checksum is provided to detect errors caused by faulty operation
> > (such as memor
> > data bus errors) within routers on the internet. Implementers of DDP should
> > treat generati
> > the checksum as an optional feature. The 16-bit DDP checksum is computed as
> > follows:
> > CkSum := 0 ;
> > FOR each datagram byte starting with the byte immediately following th
> > Checksum field
> > REPEAT the following algorithm:
> > CkSum := CkSum + byte; (unsigned addition)
> > Rotate CkSum left one bit, rotating the most significant bit in
> > least significant bit;
> > IF, at the end, CkSum = 0 THEN
> > CkSum := $FFFF (all ones).
> > Reception of a datagram with CkSum equal to 0 implies that a checksum is
> > not performed.
>
>
> Here is the old loop:
>
> while (len--) {
> sum += *data;
> sum <<=1;
> if (sum & 0x10000) {
> sum++;
> sum &= 0xffff;
> }
> data++;
> }
>
> My buggy loop is:
>
> while (len--) {
> sum += *data++;
> sum <<= 1;
> sum = ((sum >> 16) + sum) & 0xFFFF;
> }
>
> The problem is the carry from the first addition needs to be dropped
> not folded back (like IP).
>
> Corrected fast code is:
>
> while (len--) {
> sum += *data++;
> sum <<= 1;
> sum = (((sum & 0x10000) >> 16) + sum) & 0xffff;
> }
>
> At least it is correct on the standalone random data test, and the
> new code is 30% faster for the cached memory case (13.7 clks/byte vs 18
> clks/byte).
Testing...
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