Back to the Future

Back to the Future

By Tom R. HalfhilltomH.jpg
Intel’s next-generation microarchitecture for future x86 processors resembles an old-generation microarchitecture—but that’s OK. As expected, Intel and other companies are retreating from their reckless pursuit of high clock frequencies, and in some ways are reverting to designs reminiscent of the 1990s.

Note that a microarchitecture is a design for a microprocessor chip or processor core. Before Intel announced the next-generation microarchitecture, Internet rumors suggested that Intel would reveal a new architecture, which is nuts. Intel’s crown jewel is the x86 architecture, which was born in 1978 and might outlive us all. Specific implementations of the x86—such as Intel’s Pentium 4 or AMD’s Opteron—have their own microarchitectures. Intel’s next-generation microarchitecture will debut in processors code-named Conroe (desktops), Merom (notebooks), and Woodcrest (servers) in the second half of 2006.

An important microarchitectural feature is the number of stages in the instruction pipeline. Generally, the deeper the pipeline, the higher the maximum clock frequency, because each stage has fewer logic-gate delays. In addition, the pipeline depth influences other design points, such as branch-prediction accuracy, misprediction penalties, overall complexity, and power consumption.

The Pentium 4’s NetBurst “hyperpipeline” has grown from 22 stages at introduction in 2000 to 31 stages today. In contrast, Intel’s next-generation microarchitecture has only 14 stages. The Pentium M has 12 stages, the Pentium III (1999) had 11 stages, and the original Pentium (1993) had five stages. Clearly, the days of pipelines on Viagra have ended. But just because the new microarchitecture has only a 14-stage pipeline doesn’t mean it isn’t potent.

Although the shorter pipeline won’t reach the orgasmic 10GHz clock speeds once predicted for 2011, it’s simpler and wastes less power, so the chip runs cooler. Those characteristics are crucial for multicore processors, and multicore is the future. Furthermore, the new microarchitecture is better in ways that conventional benchmarks don’t measure.

It supports 64-bit extensions, Vanderpoole virtualization technology, LaGrande security, and new manageability features. Unfortunately, Intel jettisoned Hyper-Threading, which isn’t incompatible with multicore design but does add complexity.

The 14-stage pipeline might seem like a step backward, but it’s still deep by most standards. Intel’s next-generation microarchitecture will bring us highly efficient chips with two, four, or more processor cores.

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