Ask Dan: Why's it so hard to get a PC with a laptop CPU?

Date: 9 September 2007
Last modified 03-Dec-2011.


I'm wondering what's happened to using Core 2 mobile CPUs in desktop PCs.

For a quiet, efficient desktop and for a nice Home Theatre PC, mobile looked like a great way to go. I was holding my breath imagining a new generation of microATX boards using the Santa Rosa chipset and newer mobile graphics processors.

Instead, most mobile-for-desktop motherboards seem to have have disappeared, and the one remaining for sale at AusPC is a bit thin in the graphics department; Intel GMA isn't a great match for Vista.

Any thoughts about where the mobile/desktop marriage is headed? Or about the best CPU/mobo choices for a small, fast, cool and quiet HTPC?



As I mentioned in my Ask Dan about "low voltage" processors, this is not exactly a vibrant market segment.

This is because of the usual mobile-on-the-desktop situation, which isn't actually much worse at the moment than it's even been.

In brief, the laptop chips aren't available in versions that plug into the same socket the desktop chips use (the vast majority of laptop CPUs aren't socketed at all, but are meant to be soldered onto a board). So motherboard manufacturers have to make special boards for them. And the laptop chips also cost more, so there's no demand for them from the mainstream market.

And then there are Via's very low power options, which don't have the muscle to cut it as a home theatre or gaming PC, but which cut another chunk out of the power-saving market.

Result: Many dealers currently sell dozens of different Intel-chip LGA775 and AMD-chip Socket AM2 motherboards, and a few of the cute little Vias, but they often don't have even one motherboard that accepts Core 2 Duo mobile chips.

Aus PC Market do, at least, have one. The Abit iL-90MV you mention isn't listed in Abit's CPU Support List at all, so I wasn't sure what kind of socket the bleeding thing actually had. But apparently yes, it really is a proper Core 2 Duo laptop-on-desktop board. Probably the only one that's even in production at the moment, as it happens.

Aus PC Market have whipped up a HTPC system package that includes the iL-90MV, a 1.83GHz Core 2 Duo T5600 CPU, a slimline hif-fi-component-style case, 2Gb of RAM, a 500Gb hard drive and a DVD burner for $AU1261.70, which looks like a pretty good deal to me. (Aussies who'd like to order it can click here to do so!)

The reason for this strange only-one-motherboard situation is that Intel haven't released any Merom (Core 2 Mobile) processors that use the LGA775 socket ("Socket T", not to be confused with the abovementioned older Pentium M Socket 479) that the regular Core chips use.

Instead, Core 2 Mobiles come with another three new and wonderful connector types.

Thankfully, I think only one of those is applicable for desktop-computer use. It's Socket M, also known as FCPGA6.

This wouldn't in itself be a huge obstacle if the processors were cheap. But, as usual, they aren't.

A Core 2 Duo T7200, using the 34-watt Thermal Design Power Merom mobile core, is currently selling from Aus PC Market for $AU445.50 delivered.

An LGA 775 Core 2 Duo E4400 runs at the same speed but has the 65-watt-TDP Conroe core. It's currently $AU192.50.

The mobile chip costs almost $AU50 more than the quad-core Q6600, each of whose cores is 2.4GHz!

The T5600 CPU that Aus PC put in their HTPC system package isn't a whole lot cheaper than the T7200 - it's $AU368.50 by itself.

I doubt I'm telling you anything you don't know, here. Things have always been like this for the mobile-on-desktop market. But given that almost all of the PC market is very price-conscious, there's really not much manufacturer interest in this little niche.

AusPC also have a neat little AOpen bare-bones "miniPC", the MP945, that comes with a FCPGA6-socket motherboard and takes laptop components. It's even got a slot for a MiniPCI card, into which you can plug a WiFi adapter or... a different model of WiFi adapter.

Aus PC have a couple of system combos using the MP945. They cost around the $AU1000 mark, including delivery.

You could shoehorn the little AOpen box into being useful for HTPC duties, but it's obviously not ideal. And, of course, the AOpen boxes seem to have arrived from a parallel universe where the Mac Mini does not exist.

Current Mac Minis have faster Socket M Core 2 Duos in them than AusPC are bundling with the AOpen boxes. And they're about the same price, if not cheaper. And they come with Mac OS X; the AOpen boxes are a bring-your-own-OS affair.

The T5600-based HTPC system package is reasonable value for money, because it uses desktop memory and drives. The MP945 doesn't cut it.

If you're buying something with laptop components in it, mind you, you might as well get a bloody laptop.

Dell will sell you an Inspiron 1501 with a 1.8GHz, 25-watt Keene-core Mobile Sempron 3500+ in it, 2Gb of RAM and all of the other stuff you expect in a basic laptop these days, including a copy of WinXP-not-bloody-Vista, for less than nine hundred Australian dollars. (A 1.7GHz Athlon 64 X2, that weird "TK-53" one that's also labelled as a Turion, currently costs only twelve dollars and ten cents more.)

Getting back to the fruitless search for other desktop options: A couple of the 17-watt-TDP low-voltage Merom Core 2 Duos (the LV L7200 and LV L7400) are officially available as Socket M chips as well. But I have found no evidence to suggest that they actually exist in the retail market. They're only 1.33 and 1.5GHz, anyway, which isn't that thrilling.

So what would I do, if I were you?

Well, the HTPC combo looks like your best bet if you really must use a laptop CPU.

But I think the recent low-end "Pentium Dual-Core" chips are also a definite option.

They've officially got a 65 watt TDP just like the mainstream Core 2 Duos, but that's obviously an overestimate, since their manufacturing process isn't any coarser and they've got smaller L2 cache and therefore far fewer transistors.

(Core 2s with 4Mb of level 2 cache have 291 million transistors; Core 2s with 2Mb L2 have 167 million. All Pentium Dual-Cores have only one megabyte of L2 cache, and the desktop Dual-Cores are based on the original Core Duo chips, which only had 152 million transistors even with 2Mb of L2. Pay attention, I'll be asking questions later.)

Quite a lot of the world's Pentium Dual-Cores are probably running at more than 65 watts, since they overclock like crazy (in the great tradition of cheap reduced-cache chips), which makes them a great super-bargain option for that purpose. But at stock speed I'd be surprised if they actually consumed much more power than a standard Socket M Core 2 Duo.

The Pentium Dual-Cores also, of course, have the great advantage that they're available in LGA775, so you have lots of motherboard options.

And they're really, really cheap. The Pentium Dual-Core E2140 and E2160, at 1.6 and 1.8GHz stock speed, are as I write this only $AU118.80 and $AU143 delivered from Aus PC Market.

Most of the people who buy E2140s and E2160s from Aus PC Market are more interested in their repeatedly-demonstrated ability to run at 2.6GHz without even increasing the core voltage, and 3GHz or more without much further effort. Aus PC have an overclockers' combo with a E2160, Gigabyte motherboard and 2Gb of RAM for only $AU554.40 (Australians who'd like to order it can click here to do so!). It's only slightly cheaper than a similar combo with a Core 2 Duo E4400, but it's quite possible that the cheaper chip will end up giving you a faster system.

But Pentium Dual Cores also ought to be pretty miserly at stock speed. Or you could even underclock one a bit, and get quite close to laptop-chip heat output.

Underclocking is a bit of a blunt instrument, though, in this modern age. Power management features provide a better option.

All desktop chips today have laptop-style power saving features. Intel CPUs support SpeedStep, and AMDs support Cool'n'Quiet. SpeedStep should work automatically via Power Management options in Windows XP and above (or you can fiddle with it manually with a utility; if it doesn't work, check your BIOS setup program to make sure power management is enabled), and Cool'n'Quiet is apparently also automatic in Vista, or with add-on software in earlier Windows versions (and with the same caveat about BIOS setup).

The real-world difference between a "60 watt" CPU and a "30 watt" one may not actually be all that much for normal use, even without this. A home theatre PC may redline its CPU when it's encoding video, but other functions don't need nearly as much power. The faster desktop chips also get more work done per second, so jobs like encoding are over faster.

But an ordinary low-end desktop chip, plus power management, with your PC built into an enclosure with a couple of large and lazy cooling fans, really ought to get you very close to the performance of the thirty-something-watt laptop chips.

If you're building a solar-powered house in the bush or have to radiate your waste heat into hard vacuum, nothing but the most miserly will do, and the HTPC system package awaits you.

But I reckon standard motherboards and super-cheap dual-core CPUs will get you close enough for government work.

Australian shoppers who'd like to order Aus PC Market's mobile-on-desktop HTPC system package (and avoid the long and painful hunt for compatible motherboards and processors) can click here to do so.

Australian shoppers who'd rather just buy a Pentium Dual-Core E2140 and have done with it can do so here.