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Core Duo on the desktop

Asus delivers the first micro-ATX mobo for Intel's mobile gem

THE CORE DUO MAY WELL BE Intel's best microprocessor. Conceived initially as a mobile processor, Core Duo is the dual-core descendant of the Pentium M and the direct predecessor to Intel's next-generation microarchitecture, versions of which are due to arrive in desktops, servers, workstations, and laptops later this year. You can understand, then, why we have been waiting impatiently, wriggling in our chairs, until somebody—anybody—actually delivered a desktop-style motherboard for this puppy. This high-performance, low-power processor could be an ideal centerpiece for a fast but quiet desktop system or a killer home theater PC. Equipped with desktop-class cooling and overclocked accordingly, the Core Duo might turn into a fire-breathing titan of performance—and it could give us something of a glimpse of Intel's future, as well.

One of the first Core Duo desktop mobos, the N4L-VM DH from Asus, finally arrived in Damage Labs just over a week ago. Since then, we've been busily testing this processor against a broad range of potential competitors, from other mobile-on-desktop options like the Turion 64 and Pentium M to the latest dual-core desktop processors like the Pentium Extreme Edition 965 and the Athlon 64 FX-60. Can the Core Duo really hold its own against today's fastest desktop processors? What we found may surprise you.

Core Duo close up
If you've been hanging around here for a while, you may have heard us referring to Core Duo by its code name, Yonah, long before Intel decided to give it a somewhat confusing official name. We previewed Yonah after last fall's Intel Developer Forum, explaining some of the features that make it unique. Like other dual-core processors such as the Pentium D and the Athlon 64 X2, the Core Duo joins together a pair of CPU cores on a single chip. In the case of the Core Duo, those CPU cores are massaged and tweaked versions of the Pentium M processor, familiar as part of Intel's Centrino mobile platform. (The Pentium M itself traces its heritage back through many earlier Pentiums, including the Pentium III and Pentium Pro; we've covered the Pentium M at some length in the past.)

Unlike the Pentium D, however, the Core Duo benefits from a very intentional dual-core design. In fact, the Core Duo's two cores are arguably more tightly integrated than those in AMD's dual-core Athlon and Opteron processors. Each of the Duo's cores has a 32KB L1 cache for data and another of the same size for instructions, but the cores share a common 2MB L2 cache via an internal, on-chip bus. Space in the L2 cache is allocated dynamically, so either core can allocate up to the full 2MB of cache for itself, should the other one not need it. The two cores can effectively share the data in the L2 cache, as well. This use of a single, unified L2 cache greatly simplifies the management of cache coherency, especially in a single-socket system like a laptop or desktop PC. With the Pentium D, by contrast, cache coherency updates must constantly be passed across the system's front-side bus, even in a single-socket system.

Core Duo is more than just a pair of Pentium Ms made to share a cache, though. Intel's Israel-based CPU design team has modified the Pentium M design in order to address some of its performance shortcomings, especially in terms of multimedia performance. Simply going from a single core to two will inevitably help speed up tasks like video encoding, where the software is typically multithreaded. But Yonah also supports the group of 13 new instructions known as SSE3, handles some SSE2 instructing like Shuffle and Unpack up to 30% faster, and is capable of using its instruction-grouping abilities (known as micro-ops fusion) on some SSE instructions, improving overall throughput. These and other enhancements should help alleviate some of the Pentium M's few performance weaknesses compared to today's desktop processors.

Of course, none of these enhancements would matter much if Intel couldn't fit the Core Duo into laptops with about the same size, weight, battery power, and cooling capabilities as the Pentium M. In order to make that happen, Intel has arrayed a number of technologies in Core Duo's favor, not least of which is its 65-nanometer chip fabrication process. The process shrink means Core Duo's 151 million transistors can reside in an area only 90 mm2—barely any larger than the single-core Pentium M "Dothan" at 84 mm2 when manufactured with Intel's 90-nano process. The Core Duo also benefits from the fortuitous effects of multi-core processor designs on power consumption; by keeping clock frequencies relatively low and doubling up on CPU logic, a dual-core CPU can generally achieve better performance per watt than a single-core CPU, provided that multithreaded software is reasonably abundant.

The Core Duo T2600 processor These moves to 65nm and dual cores are big steps in the fight to keep power consumption in check, but Intel didn't stop there. They've also given the Core Duo a range of power management techniques that can reduce power use when part or all of the processor is idle. The two cores can independently manage some of their own traditional low-power states or C-states, such as Halt, Stop-Clock, and Sleep, so that one core could enter a lower power mode while the other cranks away on a thread. More innovatively, the Core Duo can choose to deactivate portions of its shared L2 cache in stages if the current applications don't require full use of the cache. Unneeded parts of the cache are flushed to memory and temporarily shut down. Should the CPU become so idle that the entire L2 cache can be flushed to RAM, the Core Duo will enter what Intel calls an Enhanced Deeper Sleep mode. In this state, without the need to power the L2 cache, the processor can operate at even lower voltages. Such C-state transitions happen in fractions of a second, so entering or recovering from a low-power state should be largely imperceptible to the end user. Core Duo also carries over Intel's Enhanced SpeedStep dynamic clock speed and voltage scaling feature, of course, and it adds new thermal sensors on each CPU core near likely hotspots on the chip.

Thanks to all of these changes, Intel rates the TDP, or thermal design power, of the first wave of Core Duo chips—ranging from 1.66GHz to 2.166GHz—at only 31W. (TDP is a design target for cooling solutions and doesn't necessarily represent the peak power draw of the part.) That's up only slightly from the 27W TDP of the faster variants of the Dothan, like the Pentium M 770 at 2.13GHz. Intel makes a single-core variant of Yonah called the Core Solo, and its TDP is 27W. There are also low-voltage versions of Core Duo with TDPs as low as 15W. All in all, that's a mighty impressive achievement for a processor with this sort of performance.

The Core Duo is packed to the hilt with the latest features, including SSE3, support for the Execute Disable Bit for better antivirus protection, and Intel's new VT virtualization technology. The one omission from this list of CPU enhancements is a notable one, though: Core Duo lacks support for EM64T, Intel's version of the 64-bit extensions to the x86 instruction set pioneered by AMD. Without 64-bit support, the Core Duo can't easily address more than 4GB of memory, and it loses out on the potential performance gains offered by x86-64's additional registers. Being stuck at under 4GB won't matter much for the Core Duo's success in mobile applications, and it's not likely to harm its prospects inside of Intel's Viiv-branded home theater PCs. For really beefy desktops, workstations, and especially servers, though, 64-bit support is becoming much more important with time. We will have to wait for Core Duo's successor, code-named Merom and based on Intel's next-generation Core microarchitecture, for 64-bit capabilities in an Intel mobile processor.

Core Duo is part of Intel's newfangled Centrino platform, code-named Napa. The Core Duo's main companion in the Napa scheme is the mobile version of Intel's 945G core logic chipset, the 945GM. The 945GM north bridge chip is linked to Core Duo by means of a 667MHz system bus, and the 945GM's memory controller supports dual channels of DDR2 memory at up to 667MHz. That gives the chipset twice the effective bandwidth of the front-side bus, a disparity Intel has accepted as normal. In many Centrino-based laptops, the excess bandwidth will be used to feed the 945G's integrated graphics processor, the GMA 950.

In part because it needs this new chipset with support for higher bus speeds, the Core Duo is not a drop-in replacement for the Pentium M. In fact, although it has 479 pins on its belly, the Core Duo uses a different pin layout than the Pentium M, making it physically incompatible with older Socket 479 motherboards.