Computers with two or more physical processors have long had a performance advantage over single-processor computers when the operating system supported multiple processors, as is the case with Windows NT 4.0, 2000, XP Professional, and Linux. However, dual-processor motherboards and systems have always been more expensive than otherwise-comparable single processor systems, and upgrading a dual-processor-capable system to dual-processor status can be difficult with only one processor because of the need to match processor speeds and specifications. However, Intel's Hyper-Threading (HT) Technology allows a single processor to handle two independent sets of instructions at the same time. In essence, HT Technology converts a single physical processor into two virtual processors.

Intel originally introduced HT Technology in its line of Xeon processors for servers in March 2002. HT Technology enables multiprocessor servers to act as if they had twice as many processors installed. HT Technology was introduced on Xeon workstation-class processors with a 533MHz system bus and later found its way into PC processors, with the Pentium 4 3.06GHz processor in November 2002. HT Technology is also present in all Pentium 4 processors with 800MHz CPU bus speed (2.4GHz up through 3.8GHz) as well as the Pentium 4 Extreme Edition and the dual-core Pentium Extreme Edition. However, the dual-core Pentium D does not include HT Technology.

How Hyper-Threading Works

Internally, an HT-enabled processor has two sets of general-purpose registers, control registers, and other architecture components, but both logical processors share the same cache, execution units, and buses. During operations, each logical processor handles a single thread.

Although the sharing of some processor components means that the overall speed of an HT-enabled system isn't as high as a true dual-processor system would be, speed increases of 25% or more are possible when multiple applications or a single multithreaded application is being run.

Hyper-Threading Requirements

The first HT-enabled processor was the Intel Pentium 4 3.06GHz. All 3.06GHz and faster Pentium 4 models support HT Technology, as do all processors 2.4GHz and faster that use the 800MHz bus. However, an HT-enabled P4 processor by itself can't bring the benefits of HT Technology to your system. You also need the following:

• A compatible motherboard (chipset). It might need a BIOS upgrade.

• BIOS support to enable/disable HT Technology. If your operating system doesn't support HT Technology, you should disable this feature. Application performance varies (some faster, some slower) when HT Technology is enabled. If this is a matter of concern, you should perform application-based benchmarks with HT Technology enabled and disabled to determine whether your application mix will benefit from using HT Technology.

• A compatible operating system such as Windows XP. When hyper-threading is enabled, the Device Manager shows two processors.

Intel's newer chipsets for the Pentium 4 support HT Technology. However, if your motherboard or computer was released before HT Technology was introduced, you will need a BIOS upgrade from the motherboard or system vendor to be able to use HT Technology. Although Windows NT 4.0 and Windows 2000 are designed to use multiple physical processors, HT Technology requires specific operating system optimizations to work correctly. Linux distributions based on kernel 2.4.18 and higher also support HT Technology.