Microprocessors march on

Computerworld - For more than three decades, microprocessors have doubled in power every 18 to 24 months. That progress will continue for another 10 years or so, chip makers say; then some new technology may have to be found to replace the silicon semiconductor (see story).
Unfortunately, the companies that make microprocessors and use them to build computer systems can't just catch a free ride on the back of Moore's Law. As silicon transistors grow smaller -- there will be a billion on a single chip in five years -- chips become exponentially more expensive to design, manufacture and test. And the laws of physics intrude: In the mysterious realm called "deep submicron," for example, power dissipation gets nearly impossible to control, and cosmic rays cause random processing errors.
"The power-dissipation problem will prevent the further scaling after 10 years. Improvements will come about from system-level integration rather than transistor-level enhancements," says Bijan Davari, technology vice president at IBM's microelectronics division.
About 60% of the total performance gains in microprocessors have come from higher clock frequencies resulting from smaller and faster transistors. The balance have come from processing architectures that allow the execution of more than one instruction per clock tick. A microprocessor can do that by predicting the flow of a program through several branches of program logic or by executing instructions "speculatively" -- before they are needed. But pushing those tricks further is becoming difficult and expensive.

Dies like this one for an Intel Pentium 4 are used to fabricate microprocessors.

"We've gone from being able to execute two instructions at a time to eight or more," says James Hoe, a professor of electrical and computer engineering at Carnegie Mellon University in Pittsburgh. "But we are at the limit. The architecture is not scalable." Hoe says microprocessor developers will increasingly rely on the following ambitious schemes to find "parallelism" in programs and job streams:
• Multithreading: Breaking a single program into multiple instruction streams, or threads, to be processed simultaneously. Each thread could handle a data packet or transaction, for example.
• Simultaneous multithreading: A technique that makes a single physical processor appear to software as two processors, so two programs can execute simultaneously, boosting total throughput. Intel Corp. calls it "hyperthreading."
• Chip multiprocessing: The placement of two or more physical processor "cores" on one chip. The cores can run independently but share some resources. IBM is shipping a dual-core Power4 processor, and Sun Microsystems Inc. is expected to unveil one later this year in its UltraSPARC IV. Intel will introduce a dual-core Itanium chip in 2005.
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