Many differences exist between Windows XP Home and Windows XP Professional – many of them well-documented. One of the most significant is XP's support for multiple physical processors. As two- and four-processor workstations (not just servers) become more common, whether or not they're supported by the operating system running on them in the first place is critical.

While Windows XP Home does not support more than one physical processor, there's a lot of confusion about whether or not it supports multiple processor cores in a single physical processor. This is implemented in several ways, of which the most widely used is Intel's

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Hyper-Threading technology. Also, AMD is currently rolling out a series of 64-bit processors called the X2 dual core series, which features two processor cores per die.

How much of this is supported? The answer is all of it -- in XP Professional, and, surprisingly enough, in XP Home as well. If you install XP Home on a system that has a single processor with dual cores or hyper-threading (what is referred to as two logical processors—XP Home will recognize both of them, and will install the appropriate Hardware Abstraction Layer (HAL) to work with it.

However, if you have more than one physical processor (such as two dual-core or Hyper-Threading processors), XP Home will only recognize the first one. Note: If you enable hyper-threading on a machine on which Windows is already installed, the HAL will be automatically upgraded (if it hasn't been already) and you'll be prompted to reboot a second time to finalize those changes.

Microsoft's official word about multiple processors across all its products is that they are licensed by physical processor socket, not by the number of cores on each processor. For instance, if you buy a single-CPU license for SQL Server 2005, that license is valid no matter how many cores are in that one CPU. Likewise, Windows XP Home will only work with one socket at a time regardless of its cores or threading potential, and XP Professional will recognize up to two sockets.

About the author: Serdar Yegulalp is editor of the Windows Power Users Newsletter. Check it out for the latest advice and musings on the world of Windows network administrators. He is also the author of the book Windows Server Undocumented Solutions.

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This was first published in May 2006

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