Am 10.06.2007 um 23:10 schrieb Thomas Gstädtner:
That would explain why Intel sold XScale :)
Exactly :)
IHMO there is a direct relation between the instruction set and the
power consumption:
The more complex a processor is the more power it needs.
Yes.
Let's explain it the easy (and not fully correct) way: When you
want to see the complexity of a microprocessor in numbers you can
take the number of transistors.
ARM9 Chips are under 10 million, VIA C7 about 25 million, Intel
Core2Duo about 300 million, IBM Power6 800 million.
But keep in your calculation that most of these transistors are used
for on-chip caches and internal parallelism (pipelining). You can
have a processor that executes the same code (i.e. same instruction
set!) at a lower speed with much less transistors. Wikipedia tells
that the original 8086 did have just 29.000 transistors and the 8087
FPU did have 45.000. So, I would assume that the instruction set
extensions since the 8086 area (i.e. 32 bit, etc.) does not make up
more that 1 Mio transistors out of the 300 millons. So, by
sacrificing speed, leaving out on-chip-caches etc. and most
importantly using a much lower clock rate can reduce power
consumption drastically.
This all are modern (except the ARM) and powerful processors and
the more transistors they have, the more power they need.
Of course there are chips with less transistors needing much more
power, there are several reasons for this.
But as long x86 is more complex than other architectures it will
need more power.
Well, I'm definately not a expert, if one reads this, maybe he can
explain :)
P.S. Current XScale are still producesd in 180nm, Intel is down to
65/35nm. The leakage current may be higher in ARM/XScale devices
than it could (or should) be.
From chip design courses (it is long time ago) I remember that most
of a CMOS power consumption comes from charging/discharging the gates
of the transistors (i.e. small capacitors). The lower the clock
frequency and the smaller the capcity (which depends on the
dimensions of the transistors, i.e. the 90/60/45nm technology) the
lower the power consumption.
So, I think there are enough tricks to make low power x86 compatible
processors...
Nikolaus Schaller
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