Seeing Double: A Primer On Dual-Core CPUs

Dual-core processors hold out the hope of improved performance without a huge heat penalty, but software and technological limitations set practical limits on the theoretical gains.
Both the major manufacturers of processors for the Wintel platform, Intel and AMD, have released lines of dual-core processors (Intel offers two, named Pentium D and Pentium Extreme Edition; AMD’s dual-core line is named Athlon 64 X2.) Both company's products are something like a Russian Nesting Doll. On the outside, it looks like a traditional CPU chip, but below the surface two completely independent CPU cores exist. Theoretically, this allows desktop PC users to perform two CPU-intensive operations simultaneously with almost no adverse impact upon performance.

But the benefits of dual-core chips are still only theoretical for a couple of reasons. Software is one. Very few applications exist to take advantage of multiple processors or multiple processing cores. CPU-intensive applications such as audio and video encoding, 3D rendering, and video editing will benefit most from the first wave of dual-core processors.

The die for AMD's Athlon 64 X2 dual-core processor clearly shows the individual processor cores.
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(Windows XP is a bright spot in this picture: it is already capable of utilizing multiple CPUs, so hyper-taskers who often use numerous applications at the same time will experience increased across-the-board performance in the form of faster switching between apps, and faster performance within those apps. Multi-tasking at the OS level—say encoding a CD while batch-processing a series of screen shots—is a noticeably faster experience.)

Second, technological restraints that range from poor inter-processor communication to lack of memory bandwidth will limit dual-core processor performance for a few years to come.

Up Against the Wall, Single-Cores

Both manufacturers have shifted their product emphasis to multi-core architectures because traditional single-core chips have hit the wall: the only way to make a single core chip perform better is to make it run faster, and running faster consumes more power and produces more heat.

Both AMD and Intel have acknowledged that internal temperatures of desktop PCs are getting too hot to handle. Hotter CPU temperatures mean hotter desktop PCs, which results in decreased power efficiency and stability. Dual-core processors will allow makers to replace a single high-powered CPU with two slower ones. Because these processing cores can operate in tandem, the desktop can provide the user with just as much, if not more, processing power at cooler operating temperatures.

Both Intel and AMD have said they have been able to further reduce temperatures and power consumption of their dual core processors by utilizing technology each company has developed for their mobile CPU lines.

Intel and AMD: Twins, But Not Identical

Intel’s Pentium D and AMD’s Athlon 64 X2 are similar in concept but show distinct differences that will likely affect purchasing decisions:

Functional diagram of the Intel Pentium D Dual-Core Processor

Intel Pentium D, Extreme Edition: Intel has released two lines of dual core processors: the Pentium D and the Pentium Exteme Edition. Both are based on the chipmaker’s 18-month-old Prescott design. This means each has 1MB of L2 cache and runs on an 800MHz front-side bus. The Extreme Edition version, however, has one powerful functionality not found in the Pentium D line. The presence of Hyperthreading allows Extreme Edition dual cores to be recognized by Windows XP as four processors (two are virtual, thanks to Hyper Threading). This allows for extra multi-tasking power.

Industry analysts have joked that Intel’s dual core is basically two CPUs glued together. The architecture bears this out. In both lines, the individual cores cannot directly communicate with each other. Instead, each core transmits data to the other via the front-side bus, which is like sending snail-mail to your next-door neighbor. Further, Intel’s design means that the CPUs share the same voltage, which means that both cores are forced to run at the same power state at all times. This will affect power consumption because neither Windows nor the BIOS will be able to put one core to sleep if it’s not needed.

Functional diagram of the AMD Athlon X2 Dual-Core Processor

AMD Athlon 64 X2 AMD’s first dual-core processor line—the X2—is based on the company’s Athlon 64 line. It features 1MB of L2 cache in each core. However, unlike the Pentium dual cores, AMD’s design utilizes the Athlon 64 (and Opteron) line’s on-die North Bridge and a proprietary technology AMD calls a “system request interface” to transmit data between the two cores expeditiously and efficiently.

On the downside, the X2 series does suffer in comparison to the PD and the PEE in terms of memory bandwidth. While limited by the frontside bus, Intel chipsets provide more memory bandwidth for the two cores than AMD’s. That said, neither is in an ideal position here. In both manufacturers' architectures each of the processor cores is forced to share RAM that was previously designated for only one processor. These circumstances should improve as AMD and Intel research and develop more powerful chipset designs.

Caution: Upgrade Ahead

There are two ways to upgrade to a dual-core processor -- the easy way and the hard way.

If you’re already running an Athlon 64-based system on a Socket 939 motherboard, an upgrade to dual-core is almost completely painless. AMD has released the Athlon 64 X2 series for Socket 939, so you won’t need anything but your new dual core CPU and a BIOS update.

Intel dual-core upgrades aren’t so pretty. You’ll have to upgrade to Intel’s new 955X mobo platform, which will definitely add dollar signs to your upgrade path.

The price may be almost as steep as the upgrade path: Pentium D processors can be purchased from online outlets for as little as $260 for the Pentium D 820 (which runs at 2.8GHz) to 570 for the D 840 to $1020 for the new Pentium D 840 Pentium Extreme Edition. Athlon 64 X2 CPUs range from $585 for the X2 4200+ to $1,065 for the X2 4800+.

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