[RC5] clocks / key differences

EnoJon enojon at delphi.com
Thu Nov 11 22:53:35 EST 1999

Others with K6 noted this and attribute to AMD having poorer integer
arithmetic processing than Intel and Cyrix.  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.   The same applies to Celerons with only 128k cache.

While the AMD runs cache at full processor speed and the Celeron
uses full speed, the smaller cache size means greater probability
of having a memory reference "missing" from cache.  Compare
the G3's and their upgrade cards with 512k to 1mb L2 cache,
usually at 1/2 to 1/3 of processor speed--greater probabilities
of a mem. ref. hitting cache.  The chip designers opted to use
smaller cache [hence lower cost of production] and sped it up
on the AMD and Celeron models.

At least that's what the 'experts' say in DDJ and other journals.

Also, while the MHz on the AMD is at 450, it only runs slightly
higher than my Cyrix 433 [running at approx 300mhz since the
mainboard bus speed is limited to 83mhz] which attains 677K keys/sec.
Of course, the mainboard also has 512k L2 cache.  Running with
an 233mhz MMX Gen. Intel the keyrate was approx. 499K keys/sec.

When I moved the 233mhz Intel to a board with 256K L2, the
rate hovered around 489K keys/sec [bus speed is 66mhz].

I also add that the operating system also influences keyrate; Win9x
slows keyrate on any platform.  OS/2 and Linux -- with much better
task switch and multitasking ability -- and fewer running services
achieve higher keyrates than WinNT [see the Task Mgr window
and you'll begin to understand].  DOS is the undisputed winner.

Dennis Lubert wrote:

> Does anyone know how many clocks per key the AMD-K6-III needs ? My 200MHz
> MMX needs ~ 463 c/k (keyrate 432000 k/s --> 200 000 000 Hz / 432 000 ~=463)
> but my K6-III @ 450 MHz has a keyrate of 764000 k/s that would result in
> (450 000 000 / 763 000 ~=)590 c/k !!!!Shouldn't the (newer) K6-III have a
> better rate than this ? Or is there something wrong with my PC ? DO I
> normally need to have a keyrate of ~900 000 k/s ?
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