Dual processor laptops?

[mark at buttery.org (Mark J. Dulcey)]

Scott Ehrlich wrote:
> I've been asked to spec out a dual processor laptop.  What is the 
> difference between Intel's Duo and having dual processors?  I've heard 
> that the Duo Core puts two processors back-to-back, affectively making 
> it a dual processor.  Is this correct?   How does this really differ, if 
> at all, from having two separate processors on the motherboard?
> 
> We are largely a Dell shop, but I don't have to go solely with Dell. 
> Ignore budgets for now.

So far as I know, nobody ever built a laptop that had two separate CPUs. 
All dual-processor laptops you will see actually have a dual-core CPU. 
Yes, putting two CPUs in one effectively makes it a dual processor 
system; two completely separate CPUs might have had a slight advantage 
in memory bandwidth, but a disadvantage in maintaining cache coherency 
(that is, making sure that CPU B isn't working with incorrect data 
because CPU A has something in cache), so it's mostly a wash.

There are two ways to make a dual-core CPU. One way is to put two 
complete processors on one IC die, as the Core Duo 2 and the TurionX2 
do. The other way is to put two IC die inside a single package, like the 
Pentium D; this has a slight performance disadvantage, because 
communications between the two CPUs aren't as fast. (I'm not sure which 
method is used by the original Core Duo.) Yes, that's die, not dice; in 
the IC business, they have the peculiar usage of using "die" as the 
plural of "die". I guess it makes up for the D&D players who speak of 
rolling one "dice".

The fastest available CPU right now is the Core Duo 2. The earlier Core 
Duo isn't quite as fast on a matched-clock basis (maybe 20% slower), but 
if the deal is right, they might be worth buying. AMD's competition is 
the TurionX2; it's about even with the Core Duo (not the faster Core Duo 
2) at the same clock speed. Considerations like size, weight, display 
quality, battery life, and video interface are probably more important 
than the specific CPU in the box.

As for the video interface, if you've been asked to "spec it out", the 
choice likely isn't important, because they won't care about 3D gaming 
performance anyway. But if that DOES matter, systems with an NVidia Go 
7400 or higher, or ATI Radeon Mobility X1600 or higher, are the ones to 
look for. Avoid anything with any form of integrated graphics, including 
"Intel Extreme Graphics".

My advice; first shop for the size and weight of laptop you're looking 
for, then make sure that you can get it with a suitable CPU.

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