cellphone-sized X86 PC motherboard potential OpenMoko platform?

Dr. H. Nikolaus Schaller hns at computer.org
Sun Jun 10 23:28:03 CEST 2007

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.


> 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  

Nikolaus Schaller

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