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The Pentium 4 brand refers to Intel's line of single-core mainstream desktop and laptop central processing units (CPUs) introduced on November 20, 2000 (August 8, 2008 was the date of last shipments of Pentium 4s). They had the 7th-generation microarchitecture, called NetBurst, which was the company's first all-new design since 1995, when the Intel P6 microarchitecture of the Pentium Pro CPUs had been introduced. NetBurst differed from the preceding Intel P6 - of Pentium III, II, etc. - by featuring a very deep instruction pipeline to achieve very high clock speeds (up to 4 GHz) limited only by maximum power consumption (TDP) reaching up to 115 W in 3.6–3.8 GHz Prescotts and Prescotts 2M (a high TDP requires additional cooling that can be noisy or expensive). In 2004, the initial 32-bit x86 instruction set of the Pentium 4 microprocessors was extended by the 64-bit x86-64 set.

The original Pentium 4, codenamed "Willamette", ran at 1.4 and 1.5 GHz and was released in November 2000 on the Socket 423 platform. Notable with the introduction of the Pentium 4 was the 400 MT/s FSB. It was actually based on a 100 MHz clock wave, but the bus was quad-pumped, meaning that the maximum transfer rate was four times that of a normal bus, so it was considered to run at 400 MT/s. The AMD Athlon was running at 266 MT/s (using a double-pumped bus) at that time.

Pentium 4 CPUs introduced the SSE2 and SSE3 instruction sets to accelerate calculations, transactions, media processing, 3D graphics, and games. They also integrated Hyper-threading (HT), a feature to make one physical CPU work as two logical and virtual CPUs. The Intel's flagship Pentium 4 also came in a low-end version branded Celeron (often referred to as Celeron 4), and a high-end derivative, Xeon, intended for multiprocessor servers and workstations. In 2005, the Pentium 4 was complemented by the Pentium D and Pentium Extreme Edition dual-core CPUs.

Architecture

In benchmark evaluations, the advantages of the NetBurst architecture were not clear. With carefully optimized application code, the first P4 did outperform Intel's fastest Pentium III, as expected. But in legacy applications with many branching or x87 floating-point instructions, the P4 would merely match or even fall behind its predecessor. Its main handicap was a shared uni-directional bus. Furthermore, the NetBurst architecture dissipated more heat than any previous Intel or AMD processor.

As a result, the Pentium 4's introduction was met with mixed reviews: Developers disliked the Pentium 4, as it posed a new set of code optimization rules. For example, in mathematical applications AMD's much lower-clocked Athlon easily outperformed the Pentium 4, which would only catch up if software were re-compiled with SSE2 support. Tom Yager of Infoworld magazine called it "the fastest CPU - for programs that fit entirely in cache". Computer-savvy buyers avoided Pentium 4 PCs due to their price-premium and questionable benefit. In terms of product marketing, the Pentium 4's singular emphasis on clock frequency (above all else) made it a marketer's dream. The result of this was that the NetBurst architecture was often referred to as a marchitecture by various computing websites and publications during the life of the Pentium 4.

The two classical metrics of CPU performance are IPC (instructions per cycle) and clock-frequency. While IPC is difficult to quantify (due to dependence on the benchmark application's instruction mix), clock-frequency is a simple measurement yielding a single absolute number. Unsophisticated buyers would simply associate the highest clock-rating with the best product, and the Pentium 4 was the undisputed megahertz champion. As AMD was unable to compete by these rules, it countered Intel's marketing advantage with the 'megahertz myth campaign.' AMD product marketing used a "PR-rating" system, which assigned a merit value based on relative-performance to a baseline machine.

A Pentium 4, 2.4 GHz

At the launch of the P4, Intel stated NetBurst was expected to scale to 10 GHz (over several fabrication process generations). However, the NetBurst architecture ultimately hit a frequency ceiling far below expectation—the fastest retail Pentium 4 never exceeded 4 GHz. Intel had not anticipated a rapid upward scaling of transistor power leakage that began to occur as the chip reached the 90 nm process node and smaller. This new power leakage phenomenon, along with the standard thermal output, created cooling and clock scaling problems as clock speeds increased. Reacting to these unexpected obstacles, Intel attempted several core redesigns ("Prescott" most notably) and explored new manufacturing technologies. Nothing solved their problems though and in 2005–06 Intel shifted development away from NetBurst to focus on the cooler-running Pentium M architecture. In March 2006, Intel announced the Intel Core microarchitecture, which puts greater emphasis on energy efficiency and performance per clock. The final NetBurst-derived products were released in 2006, with all subsequent product families switching exclusively to the Intel Core microarchitecture.