In our review of AMD's "Bulldozer" desktop processors, we relayed a bit of information from AMD that wasn't quite right. The company initially told us, along with virtually everyone else in the press, that Bulldozer/Zambezi chips were made up of two billion transistors. That is a very big number, and honestly, it took us some time to process. I'll even admit to spending the better part of a day talking to people about how extensively AMD used logic synthesis in Bulldozer, trying to figure out where all of those transistors came from.
Turns out the answer to our question was relatively simple: the transistor count number AMD supplied to the press was incorrect, as an article at AnandTech pointed out today. After reading that, we spoke with AMD PR rep Phil Hughes, who confirmed that the correct number for Bulldozer's transistor count is 1.2 billion. Hughes told us the initial figure was a very early and rough estimate that was simply not accurate, and he said he'd triple-checked the new number with engineering.
|Gulftown||Core i7-970, 990X||6||12||12 MB||32||1168||248|
|Sandy Bridge-E||Core-i7-39xx||8||16||20 MB||32||2270||435|
|Thuban||Phenom II X6||6||6||6 MB||45||904||346|
As you may know, transistor counts for chips are generally estimates, and the methods of making those estimations can vary. Also, the relationship between the physical die area and the number of transistors on a chip is complex, because different types of transistors can occupy more or less space. Still, this new transistor count for Bulldozer makes a lot more sense, especially when one considers that Intel's Sandy Bridge-E has 2.27 billion transistors (many of them in very dense SRAM cells) and is 120 mm² larger than Bulldozer.
Although die area is generally more important than transistor counts in the grand scheme, the more modest number of transistors in Bulldozer does improve our sense of its prospects somewhat. The chip's performance still isn't what it should be by any means, but its architectural efficiency now doesn't seem quite so dire.
Naturally, with any change like this one, some folks on the Internet are bound to speculate about what really happened behind the scenes and whether AMD is, for some reason, not being entirely straightforward. The ball really got rolling this time with Dr. Schuette at LostCircuits suggesting the chip's cache alone would occupy nearly 1.2 billion transistors. However, his first crack at the math assumed an eight-transistor SRAM cell, and those cells are almost certainly 6T, not 8T. We've pinged AMD to confirm. For further background, I also called up David Kanter of RealWorldTech, who wound up posting his own quick analysis of Bulldozer's transistor count. He thinks Bulldozer may be closer to 1.38 billion transistors, based on various sources and informed speculation, but he acknowledges that he may be wrong or that variance in counting methods may account for some of the difference from AMD's result.
Bottom line, in my view: the new number makes a lot more sense, and I think it's probably about as correct as most of the others we've put into these tables over the years. We'll be updating our Bulldozer and Sandy Bridge-E reviews with the corrected figure.