Gather 'round, gerbils, I've got a tale to tell. See, before AMD's Ryzen Threadripper released, notorious hardware destroyer der8auer ripped apart a chip and found what appeared to be four silicon dice under the lid. This interesting bit of info arrived shortly before AMD said that Threadripper made use of only two eight-core "Zeppelin" dice, and that the two extra bits of silicon were non-functional "dummies."
Despite AMD's statement regarding its "dummy dice," some folks took der8auer's early delidding efforts as evidence that Threadrippers were merely AMD Epyc CPUs by another name. That may have seemed like a reasonable enough conclusion to make. After all, Intel's Core X processors are essentially market-segmented Xeons. der8auer himself could have been among those folks, because he went the extra mile and stripped the substrate from all four dice on another Threadripper CPU that he bought at retail to find that they were in fact fully-etched Zeppelins.
Despite the appearance of functional silicon on all four of Threadripper's dice and their superficial similarity to Epyc packages, AMD maintains that Threadripper CPUs are distinct products from Epyc chips. On Sunday, AMD's James Prior tweeted the following info to that effect:
Threadripper is not a Epyc processor. Different substrate, different dies. 2 dies work, other 2 have no path to operation. Basically rocks.— James Prior (@cavemanjim) September 17, 2017
As Prior says, the two non-functional dice in a Threadripper processor are "basically rocks" with "no path to operation." That's to say that even if they were working processors, there are no connections to them in the Threadripper package. That also means the extra dice in the Threadrippers shipping today can never be enabled through some kind of BIOS hack, dashing the hopes of some optimists who thought they could turn them on somehow.
While der8auer's findings might seem to contradict AMD's earlier statements regarding the nature of these dice, the revelation ultimately makes good sense. AMD's yields on the Zeppelin die are purportedly better than 80%, but that still leaves a whole pile of nonfunctional dies to use as spacers in Threadripper CPUs. It's nice to have confirmation on the exact nature of the extra dice, so hats off to der8auer for his hard work.