Microarchitecture
In benchmark evaluations, the advantages of the NetBurst microarchitecture were not clear. With carefully optimized application code, the first Pentium 4s did outperform Intel's fastest Pentium III (clocked at 1.13 GHz at the time), as expected. But in legacy applications with many branching or x87 floating-point instructions, the Pentium 4 would merely match or even fall behind its predecessor. Its main handicap was a shared unidirectional bus. Furthermore, the NetBurst microarchitecture consumed more power and emitted more heat than any previous Intel or AMD microarchitectures.
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 lower-clocked Athlon (the fastest-clocked model was clocked at 1.2 GHz at the time) 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, questionable benefit, and initial restriction to Rambus RAM. 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 microarchitecture was often referred to as a marchitecture by various computing websites and publications during the life of the Pentium 4. A synonym for "marchitecture" was also in use: "NetBust", popular with reviewers who reflected negatively upon the processor performance.
The two classical metrics of CPU performance are IPC (instructions per cycle) and clock speed. While IPC is difficult to quantify (due to dependence on the benchmark application's instruction mix), clock speed is a simple measurement yielding a single absolute number. Unsophisticated buyers would simply consider the processor with the highest clock speed to be 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 Pentium 4, Intel stated NetBurst-based processors were expected to scale to 10 GHz (which should be achieved over several fabrication process generations). However, the NetBurst microarchitecture ultimately hit a frequency ceiling far below that expectation – the fastest clocked NetBurst-based models reached a peak clock speed of 3.8 GHz. Intel had not anticipated a rapid upward scaling of transistor power leakage that began to occur as the die reached the 90 nm lithography 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, such as using multiple cores, increasing FSB speeds, increasing the cache size, and using a longer instruction pipeline along with higher clock speeds. Nothing solved their problems though and in 2003–05 Intel shifted development away from NetBurst to focus on the cooler-running Pentium M microarchitecture. On January 5, 2006, Intel launched the Core processors, which put greater emphasis on energy efficiency and performance per clock. The final NetBurst-derived products were released in 2007, with all subsequent product families switching exclusively to the Core microarchitecture.
Read more about this topic: Pentium 4