Dual core processors may be the future of desktop computing, but until multi-threaded games and applications become the rule, rather than the exception, then high-end single core designs will still have their place. This type of dichotomy is quite interesting, as dual core processors may be the best option for most desktop users today, assuming no need for ultra high-end gaming performance. Add to that the presence of multi-threaded support in most media encoders, and you have a very compelling multimedia case for dual core.
On the flip side, single core processor designs feature higher clock speeds for the same price, and where Intel is concerned, Hyper-Threading still offers dual core emulation and increased multi-threaded performance. The above scenarios are illustrated quite well with the Pentium 4 670 and Pentium D 820 processors, which truly exemplify the lower-clocked dual core vs. higher-speed single core question.
The Pentium 4 670 is a 3.8 GHz single core processor, and it is the highest clocked revision of the 90nm Prescott 2MB core, ahead of even the Pentium 4-3.73 GHz Extreme Edition. Intel has previously hit the 3.8 GHz mark, but this was with the Pentium 4 570J, which featured the older Prescott 1MB core. The Pentium 4 670 is an LGA775 processor and runs on the 800 MHz bus, and includes support for EM64T 64-bit extensions, Execute Disable Bit, and SSE3. The thermal guidelines call for a rating of 115W, and the Pentium 4 670 runs at a max core voltage of 1.4V, although this is variable and can drop as low as 1.25V. This small bump in core voltage (from 1.388V) is the only real change, and otherwise the 3.8 GHz Pentium 4 670 is simply a 200 MHz speed upgrade from the 3.6 GHz Pentium 4 660.
The Pentium D 820 is the second phase of Intel dual core strategy, following the Pentium Extreme Edition 840 to market. The two processors are really quite similar in terms of physical design, and both feature Smithfield cores, which is really two 90nm Prescott 1-MB cores a single processor die. As we noted in the Pentium EE 840 review, this architecture is a bit different than incorporating the two cores at the silicon level, and rather than the two processor cores communicating internally, they pass data and interact across the processor bus using an arbiter chip. Utilizing dual Prescott 1MB cores also means that the physical CPUs are totally duplicated, and the Pentium D is a true dual core design, with 2x1MB L2, and double the registers and execution units. It also means doubling the physical size of the processor, with none of the potential savings from potentially redesigning at the silicon stage.
The Pentium D comes in a few different flavors, and includes the Pentium D 840 (3.2 GHz), Pentium D 830 (3.0 GHz), and Pentium D 820 (2.8 GHz). These are based on 2xPrescott 1MB cores, and like the Pentium EE 840, it would have been nice to see Intel doubling up its top core, the Prescott 2MB. The Pentium D 820 is based on the latest Prescott 1-MB revision, so it has support for EM64T 64-bit extensions, Execute Disable Bit, and SSE3. The Pentium D processors are all LGA775 models and run on the 800 MHz bus, but since these are dual core models, then it means a new platform like the Intel 945P/G Express.
Where the Pentium D differs from the Pentium Extreme Edition is with Hyper-Threading, and there is no support for this technology in the Pentium D line. This is easy to understand, as both the Pentium D 840 and Pentium EE 840 sport Smithfield cores and 3.2 GHz clock speeds, so HT support is the only real difference between the two. This means that the Pentium D offers true dual processing, which is an improvement over the dual core emulation of Hyper-Threading. The Pentium EE takes a bit from both camps, and has the option of quad processing through further HT segmentation of the dual physical cores.
These Pentium D clock speeds are quite a bit lower than the high-end of the single-core Pentium 4 line, but with the 3.2 GHz Pentium D 840 having a thermal guideline of 130W, it's easy to see why. The 2.8 GHz Pentium D 820 actually drops down quite a bit from that level, with a thermal guideline of only 95W and a thermal spec of 64 degrees C, down from 70 degrees C for the Pentium D 840. This puts the Pentium D 820 more in line with the thermals of a 3.2 GHz Pentium 4 640 and makes it a compelling option for standard desktop systems, where you really don't want to worry that much about thermal or power requirements.