The SPUs seem to become VERY irrealistic

Matthias Benkmann matthias at winterdrache.de
Sat Oct 19 09:34:34 PDT 2002


On Sat, 19 Oct 2002 02:38:54 +0200 "Florian Hess (FLoH)" <hipabos at web.de>
wrote:

> All modern intel&Co. processors are based on the i386 architecture.

I'm not talking about the ISA (instruction set architecture). I'm talking
about the transistor level architecture. This varies a big deal among
different processors. For instance Pentiums can execute several
instructions in parallel but an 80386 can't. A 486 takes 1 clock cycle for
an addition whereas a 386 takes 2. So for a program that consists just of
additions a 486 will be twice as fast as a 386 at the same clock speed
(not taking memory access times into account). However input from a port
takes 12 clock cycles on a 386 but 14 on a 486 so for a program consisting
only of port inputs, a 386 of the same clock frequency is faster than a
486. Then there are different branch prediction strategies. A 486 does not
have branch prediction but modern processors do and they suffer greatly
from mispredictions. It would be possible to construct a program with many
conditional branches that will all be mispredicted by a Pentium2,
constantly causing it to run into pipeline stalls. Such a program could
probably run several times as fast on a 486 than on a Pentium 2 of the
same clock frequency.
Each processor has specific strengths and weaknesses.

MSB

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