Spotted at EETimes.com – Odd move: AMD plans three-core CPU . Interesting that someone in the mainstream finally breaks the 1-2-4-8 progression that seems to be the norm.
The reason for using three cores is an interesting one — AMD claims that that is fairly optimal for users using many desktop applications at once. Four cores apparently bring a diminishing benefit currently, and two cores can be fully utilized. So at the moment, three cores might represent a sweet-spot for many normal users. I am thinking of future write-up on capacity computing vs capability computing, and this design does have something to say about capacity computing.
I don’t know of any technical or engineering reason that you have to have cores in numbers that are power-of-two. For shared-memory machines, this does seem to be the norm, even for optimized embedded designs. The only exception I can think off immediately is the Xenon 3 core design used in the Xbox 360. Most odd-core-count chips that I have seen are assymetric or local-memory designs with several independent ARM cores or combinations of ARM and DSPs.
I would venture a guess that the three-core parts are actually four-core chips where one core has been disabled or went bad in the manufacturing process (this is also the opinion of ArsTechnica). That is common practice for embedded SoCs, where the same die is used for a range of products by simply disabling various functions and devices. In this way, you get a number of different products at different cost points, while still obtaining economies of scale in manufacturing and avoiding the verification cost of a variant die.