Pentium D

The advent of the multicore desktop processor is expected to end the clock rate race between Intel and AMD which has raged for the past several years, with further exponential gains in clock rate looking unlikely. Instead, as long as Moore’s Law continues to hold up, it is expected that the increasing number of transistors chipmakers manage to cram into their processors will increase CPU processing power in other ways, such as by adding cores, as the Pentium D does. Indeed, Intel’s expectation is that over 70% of its shipping desktop CPUs would be multicore by the end of 2006.

The first Pentium D chips introduced in May 2005 were built on Intel’s 90nm process technology and had 800-series model numbers. The fastest CPU released had a clock speed of 3.2GHz. Early 2006 saw the release of model 900-series Pentium D chips, codenamed Presler, manufactured on Intel’s 65nm process technology.

The Presler chips comprise a pair of Cedar Mill cores. However, unlike the previous Smithfield Pentium Ds, the pair of cores are physically separated. Implementing the chips as a single package comprising two discrete die allows production flexibility, allowing the same die can to be used either for a single core Cedar Mill or a dual core Presler processor. Moreover, manufacturing yields are improved as a consequence of only having to discard one die has in case of defects rather than an entire dual core package.

The new process technology has allowed an increase in not only the clock speeds, but also the number of transistors in a die. Consequently, Presler has 376 million transistors compared with Smithfield’s 230 million. At the same time the die size has been reduced from 206mm2 to 162mm2. As a result, it has been possible to increase Presler’s L2 cache. While its predecessor featured two L2 caches of 1MB each, the new Presler processors’ L2 caches have doubled in size, to 2MB each. Proximity of multiple CPU cores on the same die have the advantage that the cache coherency circuitry can operate at a much higher clock rate than is possible if the signals have to travel off-chip, so combining equivalent CPUs on a single die significantly improves the performance of cache snoop operations.

It should be noted that the Pentium D will realise significant performance improvements only with applications that have been written specifically for multiple CPUs or cores – such as most 3D rendering programs and video encoders – or in multitasking situations where the PC user is running several CPU-intensive applications, allowing each core to handle a different application. Since most business applications and games as of 2005 only use a single thread, for these the Pentium D will deliver similar performance as an older Pentium 4 CPU running at the same clock rate.

By the spring, the fastest mainstream Pentium D chip announced was the model 950, at 3.4GHz. However, the announcement of the Intel Core brand for their future processors at the Intel Development Forum in March 2006 indicates that the Pentium D will be the final processor to carry the Pentium brand name that has been at the forefront of Intel’s products since 1993.