When the first Centrino notebooks were introduced in 2003, they were an immediate hit. The name Centrino is really just a marketing term for Intel Pentium M mobile CPUs sold in conjunction with 802.11 wireless modules, and the Pentium M CPUs were what really made Centrino notebooks shine. Code-named Banias, the first Pentium M processors were something of a hybrid between the Pentium III and Pentium 4. Designed to maximize per-clock efficiency and aggressively manage power consumption, they delivered quite good performance.
Then came Dothan, a major update to the Pentium M architecture that really brought the chips into their own. Using a smaller 90nm manufacturing process, Intel managed to double the L2 cache and turn up the clock rate without eating up more power. For more on the change from Banias to Dothan, read our previous coverage:
With its extremely reduced power draw and impressive performance-per-clock, some users have been clamoring for Pentium M-based desktop computers. AOpen now offers a motherboard that makes just that possible. The i855GMEm-LFS is a microATX motherboard that lets you build a regular desktop PC with a Pentium M CPU, complete with standard AGP and PCI cards. We built and tested one to show you how this setup stacks up against standard desktops.
Before we get into AOpen’s motherboard, a quick recap of the Pentium M “Dothan” architecture is in order. The Dothan version of Pentium M is an update to the previous “Banias” core. The primary differences are a shrink from a 130nm manufacturing process to 90nm, and a doubling of the L2 cache from 1MB to 2MB.
The Pentium M architecture in general is a hybrid of Pentium III and Pentium 4. The standard computational logic is similar to that of a Pentium III, with superior branch prediction and more aggressive power management features, such as gating off areas of the chip to dynamically power them down when not in use. The Pentium M has more pipeline stages than a Pentium III CPU (which has 10), helping to scale the clock speed up, but it has far fewer stages than current Pentium 4 CPUs—31 in Prescott. Intel has not yet disclosed the exact number of pipeline stages in Pentium M chips.
The front-side bus is based on that of the Pentium 4. However, Pentium 4 CPUs now run at a FSB clock speed of 800MHz, with new
Nick Stam’s excellent recap of an Intel presentation about Centrino in 2002 is a great place to read up on the basic architectural details of the CPU. Check out
Why would you even want to run a Pentium M on the desktop? The obvious reasons are noise and size. The low power-draw of Pentium M chips makes them easy to cool without a bunch of noisy fans. The CPU cooler on our test system is about half the height of those for average desktop CPUs, and its fan spins slowly and quietly. If we hadn’t put in a high-end graphics card, it would have been whisper-quiet. This could make the chip ideal for home-theater PCs.
Visually, AOpen’s motherboard based on the i855 chipset looks a lot like most other microATX boards. The most readily apparent difference between it and most boards made for desktop chips is the 45-degree angle rotation of the CPU socket. The heat sink retention mechanism is still at right angles to the rest of the motherboard, though. So with the CPU cooler applied, it really doesn’t look much different from any other microATX board.
In many ways, the 855 chipset cripples this system relative to other desktop offerings. RAM support only goes up to DDR 333 (we used DDR400 modules in our testing, but they ran at DDR333 speeds). It’s a bit disappointing to see only two RAM slots, but this isn’t unusual with microATX boards. Another inherent limitation of the 855G chipset is that the AGP slot is only 4X AGP, while current desktop motherboards use 8X AGP or better yet, PCI Express.
Three PCI slots are onboard—standard fare for microATX motherboards. The ICH4-M southbridge is quite limited compared with the ICH5 and ICH6 southbridge chips on Intel motherboards for desktop CPUs, but AOpen compensates by adding a Promise SATA controller and dual Marvell Gigabit Ethernet jacks. Onboard 5.1 audio is provided by a Realtek AC’97 codec, but we’ll use a sound card for testing.
The 2.0GHz Pentium M we’re testing with today is nearly the fastest chip of its type. For comparison, we’ll put it up against some of the fastest Pentium 4 and Athlon 64 systems available. This way, we can see what you really have to sacrifice in terms of performance to put made-for-notebook CPUs on the desktop.
| Dothan-based desktop computer | Pentium 4-based computer | Athlon 64-based computer | |
| CPU: | Pentium M 755 (2.0GHz Dothan) | Pentium 4 560 (3.6GHz) | Athlon 64 FX-53 (2.4GHz) |
| Motherboard and chipset: | AOpen i855GMEm-LFS (Intel i855) | Intel D925XCLVK (Intel 925x) | Asus A8V Deluxe (VIA K8T800 Pro) |
| Memory: | 2 x 512MB DDR400 (2-2-2-5 timings, running at 333MHz) | 2x 512MB DDR533 | 2 x 512MB DDR400 (2-2-2-5 timings) |
| Graphics: | Radeon X800XT (AGP) |
Radeon X800XT (PCIe) check prices | Radeon X800XT (AGP) |
| Hard drive: | Seagate Barracuda 160GB SATA |
Seagate Barracuda 160GB SATA | Seagate Barracuda 160GB SATA |
| Optical drive: | ATAPI DVD-ROM Drive | ATAPI DVD-ROM Drive | ATAPI DVD-ROM Drive |
| Audio: | Sound Blaster Audigy 2 |
Sound Blaster Audigy 2 | Sound Blaster Audigy 2 |
| Operating system: | Windows XP Professional with SP2 | Windows XP Professional with SP2 | Windows XP Professional with SP2 |
We kept the systems configured as closely as possible, but it’s easy to see that the Dothan Desktop Machine is going to be at a disadvantage here: It’s 400MHz behind the Athlon 64 and a whopping 1.6GHz behind the Pentium 4, and it has a slower front-side bus, a slower memory clock speed, and only an AGP 4X interface. We’d be surprised to see it keep up with the others, but the question isn’t whether it will be slower, but by how much?
We ran the AOpen board and Dothan CPU through our usual battery of motherboard tests, which includes:
- BAPCO SYSmark 2004, which replaces our now-obsolete Winstone suite.
- PCMark 2004, to gauge memory and CPU performance.
- Windows Media Encoder 9 video encode, DivX 5.2 video encode, and WMA audio encode, to check throughput digital media encoding.
- Four games: Unreal Tournament 2004, Painkiller, Doom 3, and Halo.
- SPECviewperf 8.01. This is a mostly graphics-intensive test using recordings from a suite of professional 3D applications. But it hammers on the graphics interface, and actually stresses both AGP and PCI Express a bit more than the game benchmarks.
Continued…
The Pentium M-based desktop system doesn’t perform nearly as well as either the Athlon FX-53 or Pentium 4, but we certainly expected lower numbers than this. We thought a 400MHz front-side bus combined with DDR333 RAM would spell disaster for this intensive benchmark, but the Pentium M hangs in there.
PCMark is a different story. The reduced bus and memory speeds trash the score of the memory benchmark, and the low clock speed is a major factor in the Dothan system’s low CPU score. The overall PCMark suffers from these shortcomings as well, though not as badly as we might have thought. Remember, this is a 2.0GHz CPU, and it still manages to turn in a PCMark score over 4300. PCMark is a synthetic test, though, and really only serves to satisfy academic curiosity. It’s best to concern ourselves with real-world applications.
We don’t have performance numbers for the Pentium 4 system in these media encoding tests, but it should perform very close to the Athlon 64 FX-53. Clock for clock, the Pentium M does well here. It is operating at a clock speed 20% slower than the Athlon 64, and has several other architectural disadvantages. Performance is from 17-25% slower, which isn’t half bad.
When it comes to 3D workstation application performance, our laptop architecture just can’t keep up with the fast bus speeds and memory access of the desktop systems. These tests simply require fast and efficient memory access, and a 400MHz front-side bus plus 333MHz RAM are major obstacles.
This is absolutely shocking. Neither Intel CPU can touch the Athlon 64 with its fantastic x86 floating-point math capabilities, per-clock efficiency, and integrated memory controller. That much is to be expected. But look at how well the Pentium M stacks up against the Pentium 4! Despite running at only 55% of the Pentium 4’s clock speed, and with half the front-side-bus bandwidth, the Pentium M manages to run games at the same speed. That’s absolutely incredible. We can only conclude that the Pentium M’s architecture is a game-playing dynamo like that of the Athlon 64, and given enough memory bandwidth, a faster bus, and higher clocks, it could easily give AMD a run for the money.
To help show the impact of the 4x AGP bus, we ran Doom 3 again using the game’s Ultra Quality setting. Typically, we run it at High Quality, which compresses certain textures, such as normal maps, to make sure the graphics assets will fit onto 256MB graphics cards. Ultra Quality doesn’t compress any textures at all, and can actually cause the game to use up so much RAM for textures that they won’t fit on a 256MB graphics card. With this setting, the frame rate dropped from 90 to 85. This isn’t a big hit, and you would think that it means the 4x AGP bus isn’t a problem, but consider that the standard Doom 3 time-demo is impacted by the RAM requirements of Ultra Quality less than many of the levels in regular play.
AOpen certainly deserves credit for trying something different. Making a desktop motherboard for mobile CPUs is an interesting idea, and probably has its place in certain applications. The modest cooling requirements and near-silent operation are a boon to home theater PC enthusiasts, and it’s a neat experiment to build out a notebook PC with desktop components.
Sadly, there are plenty of limitations inherent in the platform. Compared with standard desktop architectures, a 400MHz bus speed, DDR333 memory, and AGP 4X are two generations behind state of the art. AOpen managed to add on SATA and dual gigabit Ethernet, but nothing can be done about the core i855 chipset’s constraints.
Other concerns enter the equation, like price. We were unable to find this motherboard on sale anywhere for less than $265. That’s extremely expensive for any motherboard—you can get entire barebones or small-form-factor PCs for that price! Higher-end Pentium M CPUs are expensive, too. Our 2.0GHz model costs over $400, and the new 2.1GHz is even more. In other words, you have to really want the lower power demands and quieter operation of a desktop PC with a notebook processor. If it were less expensive, this would be a neat idea for home theater PCs, but at these prices, it’s just not worth the sacrifice in speed.
Testing the Dothan version of the Pentium M with desktop components did yield some interesting information, though. The CPU seems remarkably efficient, on a clock-for-clock and watt-for-watt basis, especially when it comes to game performance. It has been rumored that Intel’s upcoming dual-core CPUs would actually be based on this core, not the NetBurst architecture of the Pentium 4. With a much faster front-side bus (or dare we hope—an integrated memory controller) and clock speeds up around 2.6GHz or better, this could be one heck of a great desktop CPU.
| Product: | AOpen i855GMEm-LFS |
| Company: | www.aopen.com |
| Pros: | Modest power needs; whisper quiet; dual gigabit Ethernet. |
| Cons: | Way more expensive than most desktop motherboards; notebook chipset and CPU has performance limits. |
| Summary: | An interesting curiosity and a very expensive but near-silent solution for home theater PCs, but ultimately too much money for too little performance. |
| Price: | $265 |
| Score: | |
| Product | Intel Pentium M 755 (“Dothan” 2.0GHz) |
| Company: | www.intel.com |
| Pros: | Very low power draw; lots of cache; fantastic game performance. |
| Cons: | Bus speeds and clock frequency limit performance in many applications. |
| Summary: | This is a fantastic mobile CPU with a very efficient architecture, but as a desktop CPU where power draw is less of a concern, it needs to be clocked higher and have a faster FSB. |
| Price: | $439 |
| Score: | |