I just replaced the 120MHz Cyrix 6x86 "P150+" on my motherboard with a 100Mhz
AMD K5 "P133+".

With the Cyrix, STREAM was showing me memory copy bandwidth of about 70MB/sec;
a bit higher if I leave ECC off, but after all I shelled out for the 36bit
memory because I wanted ECC.

Note that the Cyrix processor is running at 60MHz x 2.

With the same memory, and same memory timings in the BIOS, and the AMD
processor, I get about 50MB/sec copy bandwidth.  Not Cool.

Note that the AMD processor is running at 66MHz x 1.5.

Since copy bandwidth ought to be limited by the speed of the memory bus, I
would expect the AMD to do about 10% better than the Cyrix.  What's going on
here?  I'm assuming that there's likely some processor-specific feature of
the K5 that my BIOS isn't setting correctly (IIRC I've seen mention of this
here before) but I can't seem to find one that's a likely candidate.

Anyone seen behaviour like this?

Oh -- by the way -- the current locore *still* doesn't get the Cyrix test
right.  It's not comparing the flags correctly after the divide, so it misses
"modern" Cyrix processors and misidentifies the 5x86 and 6x86 as 486DX chips
instead of (at least as far as we care) Pentium-equivalents.  In practice,
AFAIK this loses only because those chips have a few features, like a PDE
cache, which aren't usually turned on by the BIOS and which might improve
performance.

Thor