Intel Core i5 661 £150
19th Jan 2010 | 11:00
Intel fuses CPU and GPU in its latest low-end dual-core chip
We know what you're thinking about the Core i5 661. A new commodity-class CPU from Intel is hardly the stuff of performance PC legends. Especially one with just two execution cores.
Here in the cold environs of early 2010, surely quad-core or more is where it's at?
Perhaps. But you're still going to have to lump it. By 'it' we mean Clarkdale, the first dual-core variant of Intel's new 32nm Westmere processor family. Clarkdale debuts several important new technologies. Most significantly, it's the first PC processor with graphics features integrated into the CPU package itself.
Undeniably, it's a harbinger of things to come, both architecturally and in terms of Intel's manufacturing tech.
Clarkdale delights and disappoints in equal measure. At the least, it challenges conventional wisdom regarding core counts and, in turn, prevailing notions of adequate performance. In fact, even when Clarkdale flops, it does so for intriguing reasons. This is more than just another budget chip.
Let's start with the most obvious novelty: its dual-chip modular architecture. Despite being a lowly dual-core model, each Clarkdale CPU contains two separate chips. Intel has done so-called multi-chip packages before, but previous efforts have always been a quick and dirty ruse, a way of stuffing in more cores and sidestepping the engineering challenges of designing a whole new chip.
Thus, Intel knocked up early dual-core Pentium D processors courtesy of what was literally two separate processor dies concealed in a single CPU package. Likewise, all quad-core processors in the Core 2 family were in fact composed of a pair of dual-core dies.
It wasn't until late 2008 and the arrival of the first Core i7 processor that Intel could lay claim to a 'monolithic' or 'native' quad-core processor.
With Clarkdale, however, multi-chip is all about the drive towards ever greater integration of features into the CPU. For Intel, this process kicked off with the Bloomfield Core i7, its first desktop CPU to pack an on-die memory controller.
Next up was Lynnfield, variously branded both Core i5 and Core i7. To the integrated memory controller, Lynnfield added full system I/O, including the PCI Express bus. In effect, Intel had moved the northbridge chip off the motherboard and into the CPU.
In that context, Clarkdale is both the next logical step and something of a regression. On the one hand, it offers a greater level of integration by virtue of bringing a graphics core into the CPU. On the other, its multi-chip approach is a break from Intel's recent past.
Moreover, given that we're talking about a relatively simple dual-core processor, you might wonder why Intel didn't put the lot into a single chip. Take a closer look at the specifics of the two chips that make up a Clarkdale CPU and the reason hits you smartly between the peepers.
The chip with the two processor cores and cache memory is fabbed courtesy of Intel's new 32nm silicon production technology. But the second chip, which contains the memory controller, system I/O and the integrated graphics core, defaults to the relatively old-school 45nm node. If that strikes you as odd, then you're probably on to something.
Intel's original plan for Clarkdale was almost definitely to produce the whole shebang in a single 32nm die. But even Intel sometimes has to make compromises. In the end, it was probably decided that attempting its first CPU-GPU fusion chip on a new production process was asking too much.
Instead, Intel played safe and stuck the graphics part of the equation on the tried-and-tested 45nm node. A 'true' fusion chip from Intel won't arrive until the end of 2010 and the debut of the new Sandy Bridge architecture. Core details What else can we tell you about Clarkdale?
The cores themselves benefit from the new 32nm Westmere architecture. Well, we say new. Westmere is a mild rehash of the existing Nehalem design. Think 65nm Penryn Core 2 compared to 65nm Conroe Core 2 and you'll get the idea.
Not that there's anything wrong with Nehalem; quite the contrary. Highlights include HyperThreading support, which in the context of Clarkdale means support for a total of four simultaneous software threads. Then there's a high bandwidth DDR3 memory controller, 4MB of on-die cache and Intel's autooverclocking Turbo feature.
Several models will become available over the coming months, ranging from 2.93GHz up to 3.46GHz. Our test chip is a 3.33GHz example that goes under the Core i5 661 moniker and is claimed to be good for up to 3.6GHz in Turbo mode.
The arrival of Clarkdale also brings the first use of the Core i3 brand. The key distinguishing feature is the absence of Turbo Mode – only Core i5 Clarkdales qualify for that particular goodness. Given that Intel has already applied the Core i5 brand to a true quad-core model, it would make a lot more sense for all Clarkdales to be branded Core i3. But, frankly, we've given up hoping for sanity to prevail inside Intel's marketing machine.
As for the graphics core, don't get too excited. It's closely related to Intel's existing G45-era integrated GPU, the main difference being an additional pair of shader cores, for a total of 12. Intel has also upped the graphics clock from 800MHz to 900MHz for the fastest Clarkdales and added some HD video features, such as HDMI 1.3.
All of which leaves the matter of how Clarkdale performs. On the CPU side, it's a tale of two halves. At stock clocks it pretty much hoses dual-core chips from the previous generation, such as the Core 2 Duo E7500.
Granted, that chip runs slightly slower at 2.93GHz, but the gap is bigger than the clocks alone can account for.
It compares pretty well to low-end quads, too. The bad news is that our initial Clarkdale sample is an extremely poor overclocker. 3.45GHz is the best we could manage. Certainly, we look forward to having another crack at it with a non-Intel motherboard.
For now, however, we wonder if combining processor and graphics in a single package is putting a downer on overclocking headroom.
On the subject of graphics, it stacked up surprisingly well compared to our AMD-integrated and low-end discrete comparison platforms. But Intel has a long way to go regarding stability. Constant crashes and blue screens accompanied our 3D testing.
It's early days for this new core, though, and Intel still has time to tweak drivers before it hits the shelves.
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