[RC5] clocks / key differences

Mikus Grinbergs mikus at bga.com
Fri Nov 12 13:35:17 EST 1999

On Thu, 11 Nov 1999 22:53:35 -0400 EnoJon <enojon at delphi.com>
posted two statements regarding K6 performance which I thought
could be misleading :

> Others with K6 noted this and attribute to AMD having poorer integer
> arithmetic processing than Intel and Cyrix.

I haven't seen the SPECint marks for the AMD K6 (nor the others),
but the majority of the people comparing CPU peformance do _not_
claim that the AMD "has poorer integer arithmetic processing" !!!

I completely agree that the AMD K6 has poorer RC5 processing than
Intel, but RC5 makes heavy use of the x86 'rotate' instruction
which IMHO is not 'typical' in "integer arithmetic" calculations.
(The K6 *does* have poor performance executing the x86 'rotate'.)

> The K6-III has onboard
> L1 of 64k, L2 of 128k and depending on your motherboard a bus
> speed of 100mhz.   Of course, most 200MHz MMX's run on slower
> bus speed motherboards, but if you have equivalent 100mhz bus
> and 512k L2 [or more on newer mainboards] the Intel chip would
> show more efficient processing because it would lack the memory
> latencies the AMD chip receives because it has a reduced cache
> size.

That last sentence above seems to imply that the K6-III has LESS
cache than the MMX.  But if an ordinary motherboard supplies 512k
(or more) of cache to the MMX, an ordinary motherboard would ALSO
supply 512k (or more) of cache to the K6-III.

The K6-III has ADDITIONAL cache that the MMX does not have.  Its
L1 cache is bigger than the L1 cache of the MMX.  And the K6-III
has an extra 128k cache layer between the CPU and the motherboard.
That layer does not _replace_ the cache on the motherboard.  It
_adds_ function by taking care of some requests that otherwise
would have had to be handled by the cache on the motherboard.

[Do not be confused if the cache on the motherboard is called an
 L3 cache with the K6-III.  It is still a cache, and still works
 exactly the same way as the cache on the motherboard for the MMX.
 (Maybe the extra 128k cache onboard the K6-III should have been
 called L1+ cache - then what was being called an L2 cache on the
 motherboard would not have had its name changed when used with
 the K6-III). ]


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