Re: IP Checksumming

Richard Henderson (
Fri, 22 Nov 1996 01:26:31 -0600 (CST)

> csum += data[i];
> carry = (csum < data[i]);

This is the only way to do it on the Alpha, as there is no
processor status word in which to store a carry flag.

> does not need branches or other pipeline killers at all on most CPUs.
> In fact on MIPS this even makes really nice code which takes four
> cycles per 4 bytes on 32 bit CPUs or four cycles per 8 bytes on R4000
> (64 bit) CPUs, R5000/R10000 should be able to do this in 2 cycles.

Or look at the csum_ipv6_magic routine I wrote for 2.1.11 wherein
all the carry bits for the 40 byte headers are merged in 3 (ev5)