Intel's Pentium 4 3.06GHz: Hyper-Threading on Desktopsby Anand Lal Shimpi on November 14, 2002 5:39 AM EST
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Hyper-Threading - How it Works (continued)
Operating System support for Hyper-Threading is necessary but it currently exists in two different forms. Windows 2000 Professional supports multiple processors but it does not properly support Hyper-Threading. This means that it will see a single HT enabled Pentium 4 as two CPUs, but the OS will think that it is running on two physical CPUs instead of one physical CPU split into two logical CPUs. Why is this a problem?
With a single Pentium 4 processor this isn't much of an issue, but things get much more complicated with multiprocessor Xeons with HT enabled under Windows 2000 Professional or Server. Windows 2000 Professional only supports a maximum of two processors, and 2000 Server supports a max of 4 processors. With two HT enabled CPUs under Windows 2000 Professional, enabling HT will not make a difference as the OS will only work with a maximum of two CPUs. Similarly, a quad HT system under Windows 2000 Server would appear to the OS as an 8 processor system and thus exceed its licensing limitations giving you the use of only 4 of the CPUs.
Luckily Windows XP was designed with Hyper-Threading support in mind and thus even Home Edition will support a single CPU with HT enabled. Keep in mind that Windows XP Home does not support multiple physical processors, but if you enable HT on a Pentium 4 XP Home will recognize it as two CPUs.
The same situation exists with Windows XP Professional where the OS supports a maximum of two physical processors but it will allow a configuration with 4 logical processors.
Microsoft's SQL Server also has an identical licensing scheme where you do not have to pay for more expensive licensing for the number of logical CPUs you have; you simply make sure you are properly licensed for the number of physical CPUs present in your system.