The Pentium 4 is a seventh-generation x86 architecture microprocessor produced by Intel and was their first all-new CPU design, called the NetBurst architecture, since the Pentium Pro of 1995. Unlike the Pentium II, Pentium III, and various Celerons, the architecture owed little to the Pentium Pro/P6 design, and was new from the ground up. The microarchitecture of Netburst featured a very deep instruction pipeline, with the intention of scaling to very high frequencies. It also introduced the SSE2 instruction set for faster SIMD integer, and 64-bit floating-point computation.
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 comparatively fast 400 MHz FSB, delivered with high-latency RDRAM. It was actually a 100 MHz quad-pumped bus, but the theoretical bandwidth was 4x that of a normal bus, so it was considered to run at 400 MHz. The AMD Athlon was running at 266 MHz (133 MHz double-pumped) at that time.
The next Pentium 4, codenamed "Northwood", was released at speeds from 2.0 Ghz to 3.06 Ghz. Northwood also included HyperThreading, but this was disabled on the core in all models but the 3.06 Ghz model.
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. 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. 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 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.
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 3.8 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 combined with the standard thermal output to create 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 2005-6 Intel shifted development away from Netburst to focus on the cooler running Pentium M architecture. In March 2006, Intel announced the Intel Core architectures, which puts greater emphasis on energy efficiency and performance per clock. The final NetBurst-derived products will be released in 2006, with all subsequent product families switching exclusively to the Intel Core architecture.
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