Most people, when they go to build a computer, pick out a nice Pentium 4, Pentium D, or Athlon 64.  They offer good all-around performance out of the box, and have plentiful motherboard support.  However, some overclockers and silent-PC freaks decide to take another route, and they buy laptop processors and put them into their custom builds.

There are several attractive features of mobile processors.  One is that because laptops are designed to be run on battery, mobile chips use less power, including lower voltages, than their desktop counterparts.  Because of this, when run at normal voltages on desktop motherboards, the processors overclock quite nicely.  Another important property is the lower clock speeds and front-side bus speeds that mobile processors use in comparison to their desktop counterparts.  Because the number one technique that overclockers employ is to up the front-side bus, having a low speed one means there is more headroom to bump the speeds up.  The same goes for clock speeds – lower clock speeds mean a lower multiplier, which in turn means that the front-side bus can be upped to a higher level before the overclocker runs into a clock-speed barrier.

A result of both the lower voltages and lower clock speeds employed by mobile processors is lower thermal dissipation – in other words, mobile processors produce less heat than their desktop counterparts.  This is attractive to both overclockers and silent-PC freaks (and in turn people who are building media PCs), because it means less cooling is required both at stock and overclocked speeds.  At stock speeds, thermal dissipation is usually low enough to use a very low-speed fan, thus cutting down on noise.

There’s actually a fairly long-running tradition of people using laptop processors in desktop PCs.  In the early Socket 478 Pentium 4 days, overclockers loved the P4-Ms that ruled the laptop roost, because they had plenty of headroom and fit into standard desktop motherboard sockets.  Later, when the cheaper Mobile P4 came out, the Celeron versions of those chips were especially good overclockers — the 1.6 GHz version could hit 3.2 GHz using standard cooling and desktop-PC voltages, and 2.66 GHz using the normal laptop voltages.

Overclockers used mobile processors on the AMD side, too – the Athlon XP-M was notoriously good at attaining high speeds, and probably the mobile processor used the most of any other mobile processor on desktop systems.  Nowadays, they use Turions and Mobile Athlon 64s in Socket 754 motherboards.

Meanwhile, Intel discovered that the P4-M was not giving them the battery life they wanted on laptops, and so they introduced the lower-clocked Pentium-M (which you’ve read about before in this column).  The Pentium-M posed a problem for people who wanted to use it on their desktops, because it required a special chipset and socket in order to function.  However, eventually AOpen and DFI came out with desktop boards featuring the Pentium-M’s Socket 479 and 855 chipset.  Despite being MicroATX and limited by single-channel DDR333 RAM, 4x AGP, and other chipset quirks, tweakers quickly embraced these niche-market boards.  They discovered that if one overclocked a 1.6 GHz Dothan-core Pentium-M to 2.4 GHz, it would beat the Athlon 64 FX-53 in many benchmarks.

Things got even more interesting when Asus introduced their CT-479 adaptor.  Unfortunately, it came out after LGA775/PCI-E motherboards hit the market, and was only compatible with Asus’s Socket 478 boards, but it allowed people who wanted to use the Pentium-M in their systems to do so with an 865 or 875 chipset motherboard, thus allowing higher overclocks, dual-channel DDR, native Serial ATA, and AGP 8x.  Best of all, while the 855-chipset Pentium-M motherboards were $250 and up, the CT-479 only cost $50. 

At the same time, AOpen came out with a new 915-chipset based motherboard with PCI-Express and dual-channel DDR2.  This was, once again, expensive, and mainly geared towards media-PC applications, but it allowed Pentium-M fanatics to use PCI-E video cards with their processors.

The popularity of this board and adaptor was boosted by the fact that the Pentium 4 Prescott ran hot and had relatively disappointing performance.  The Pentium-M became the best choice for people who wanted to go Intel, but didn’t want a space-heater in a tower that sounded like a vacuum cleaner because of all its fans.

Now that the Core Duo is out, a new round of boards is appearing to take advantage of its benefits.  AOpen and DFI are in the game, and even Asus is getting into the market.  The most notable board, however, is the AOpen i975Xa-YDG.  This is a full-featured ATX board using the Intel 975X chipset; AOpen says it’s compatible with both Crossfire and SLI, and it offers SATA RAID and four memory slots, a first for a Pentium-M board (although it was possible before if you used the CT-479).  It also uses a standard Socket 478 heatsink-mounting bracket, allowing overclockers to use P4 waterblocks and phase-change systems to achieve even higher speed potential.

The i975Xa-YDG is a good motherboard, because it allows more people than ever to take advantage of the Pentium-M’s (now Core Duo’s) performance.  Hopefully AOpen will price it competitively so that it will be a compelling alternative to the Pentium D and Athlon 64 x2.