What supports Hyper-Threading?
Only specific combinations of hardware and software will fully support Hyper-Threading. I'll try to lead you through the maze the best I can.

First and foremost, you'll need a Pentium 4 processor 3.06GHz , which is the only desktop processor to support HT. Interestingly enough, the logic necessary for Hyper-Threading has been present in Pentium 4 silicon since the first, original Pentium 4 "Willamette" chips arrived on the scene. However, Intel didn't enable Hyper-Threading in those early P4 chips, preferring instead to introduce HT—pardon the abbreviation, but HT will have to do from here out—with its Xeon server chips.

Before you current P4 owners get too excited, Intel says it can disable access to Hyper-Threading "in the factory," so your current P4 chips aren't likely to sprout a second head any time soon. Also, Intel has taken the P4 through a number of revisions (or steppings) over time, and HT logic has been improved as that's happened. So even if you can hack an older P4 into enabling HT, performance may not be all that great.

For now, Intel plans to keep Hyper-Threading exclusive to P4 chips at 3GHz and above, although I get the sense the company might change its plans if screaming hordes of Taiwanese mobo makers were to show up at its doors. Or if Michael Dell made a phone call.


The WinXP Device Manager sees double

Next, you'll need a Hyper-Threading-aware operating system. To date, only Windows XP and Linux (kernel versions 2.4.18 and higher) are HT-aware. Some multithreaded operating systems, like Windows 2000, will run fine with multiple logical processors, but they don't offer the performance benefits of an HT-aware OS. Microsoft has produced an interesting little white paper on HT support in WinXP, which explains the kernel tweaks needed for best performance. For instance, WinXP more aggressively executes the HLT command on an unused logical CPU. (The HLT command exists to tell processors to take a break for a while.) Doing so frees up shared resources for the other logical processor. In case you were wondering, while the Home edition of WinXP supports only one physical processor, it will support HT on that processor.

Finally, you'll need a motherboard with Hyper-Threading support. Realistically, that means you need a mobo capable of running a P4 3.06GHz chip, so it will require a 533MHz front-side bus and the right voltage regulator. These requirements will eliminate many existing P4 mobos. Beyond that, you'll need a BIOS capable of supporting Hyper-Threading. With BIOS level support, users can turn HT on and off at will. Without BIOS-level support, HT won't work. And you'll need a chipset capable of supporting Hyper-Threading.

All of Intel's current P4 chipsets and most of its past ones, with the exception of one stepping of the 845, support HT. With Taiwanese chipset makers VIA and SiS, the situtation is somewhat murky. I asked VIA about it, and I got this response:

Hyperthreading support is enabled in BIOS. VIA P4 mainboards will support hyperthreading. There will be more official info about this coming out of VIA before the end of this week.
That sounds promising, and I expect we'll know more soon. SiS's story seems similar. I wouldn't count on HT support with mobos based on current Taiwanese chipsets, but I wouldn't count it out, either.