On Thu, Apr 15, 1999 at 02:04:11PM -0700, bwoodard@cisco.com wrote:
> > The ix86 insn set is hard to support with a simple design. Actually, wh=
at
> > K6, PII and PIII do is to translate those insns in internal RISC like i=
nsns
> > which can be nicely handled by the execution units.
> >=20
> > If you want to get rid of that much silicone, you have to dwitch
> > architecture and use PPC, ARM, Alpha or whatever ...
> >=20
> > On the other hand, the x86 code is very dense, resulting in relatively =
short
> > executables.
> >=20
>=20
> One more question then. Since ix86 instruction set is so easy to
> support what takes up all the die space?
Sorry? Did you read what I wrote?
I said: The ix86 insns set is hard to support.
You got a lot of instructions with a lot of modes and different lengths with
few registers. That's an old CISC design.
> Also I still don't understand why chip designers don't design simpler
> chips that behave like multiple processors rather than going to all
> the work of having deep pipelines and all the complexity associated
> with that e.g. speculative execution, out of order execution...=20
> i.e. Is there some reason they don't make a chip which is essentially
> four 486's, cache and all the glue circuitry needed to make them
> capable of running SMP.
Well. Most OSes out there don't support SMP. Also, there are apps, which
don't profit much from multiple processors. Also note, that if 10% of your
app is not parallelizable, you will at most get 10 times the performance of
a single CPU by using multiple. Not counted communication overhead.
Maybe processor designers find it
more exciting to have those nighty features like specul. execution etc.=20
One goal is to build a single very performing CPU. Consider the AXP-21264,
which I consider the most performant affordable chip out there. The AXp is a
RISC design, no question. But with the 21264, a lot of extra logic, such as
spec. execution, more than one insns per cycle, etc. was added. So, the
21264-500 outperforms the 21164-633 by a factor of 1.5 to 2.0 (!)
If you need plenty of performance, you will get several of those.
Regards,
--=20
Dipl.Phys. Kurt Garloff <kurt@garloff.de> [Wuppertal, FRG]
Plasma physics, high perf. computing [Linux-ix86,-axp, DUX]
PGP key: see mailheader / key servers [Linux SCSI driver: DC390]
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