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Smaller transistors are faster than larger ones. In the past, the speed increase of a transistor was roughly equivalent to the square of the larger gating channel length divided by the length of the smaller channel. In other words, moving from a 200 nm channel to a 100 nm channel would make the transistor about 4 times as fast as the original. With refined technologies, the speed increment has decreased to almost a linear function, meaning that a 70 nm channel transistor is only about twice as fast as a 140 nm channel. There are several reasons for that, for once, it is difficult to shrink the oxide layer serving as insulator proportionally to the channel length.

Increased L2 cache

Where the Willamette core uses 256 kB L2 cache that is 8 way set associative, the Northwood core has doubled the unified L2 cache size to 512 kB that are still 8 way set associative. Each byte comprises 8 bits and for every bit, 6 transistors are necessary, meaning that a minimum of 12.2 million additional transistors have been added. Since the transistor count has increased by 13 million (from 42 -55 million), it is reasonable to assume that there have not been too many other additional features that have been added to the core.

The Willamette core (left) compared to the Northwood (right), the floor plan of the Willamette CPU is in the center. The layout has not changed except for the shrink and the relatively larger size of the L2 cache. The pictures have been adjusted to the exact same scale.
- 1: Rapid Execution Engine - 2: 400 MHz bus interface - 3: Pipeline stages - 4: FPU unit - 5: two blocks of 256 kB L2 cache around the central I/O logic

Thermal Dissipation

Above, I mentioned that thermal issues have been a major consideration for the Pentium 4 with an estimated 70 W heat dissipation at 2.0 GHz. The Northwood looks much better in this respect, according to Intel, the thermal power has been reduced to 49.8W. The caveat is that there is no statement about the frequency that this number relates to, so we assume the worst possible case, which is 2.0 GHz. If this is correct, the thermal power at 2.2 GHz will be around 45 W. In real live, it comes down to the fact that in idle, the fan of the heat sink often shut off completely, one thing we never found with the older Willamette design.

Supply Issues

Lately, there have been supply bottlenecks for all different Intel CPUs, so what is going to happen with the new Northwood core-based P4s? Intel has been mass producing the new core since May 2001 and the goal is to have 6 factories running full steam ahead sometime in 2002. Considering that there has to be a substantial inventory of the new CPU, it does not appear as if there will be any shortage in the near future but we will see in the next months to come.

The Competition

While the big machinery was in place to get going with the launch of both the i845D (DDR) chipset and the Northwood, a smaller launch of another high-end processor happened, that is, AMD released its Athlon XP2000+. Granted that the number refers to the QuantiSpeed architecture and the actual clock speed is only 1667 MHz, it is still the first time that AMD has an official offering in the bit 2.0 clock speed range. Aside from the rated frequency achieved by a 12.5 x multiplier, nothing has changed from the original XP reviewed here and here. Nonetheless, having three 2.0 or higher clock speed rated CPUs really calls for pitting them against each other in a variety of benchmarks.

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