At the beginning of February engineers and designers came together for the International Solid-State Circuit Conference (ISSCC) at the Marriott Hotel in San Francisco to present papers on the latest advances in solid-state circuits. One of the highlights of this year's conference were the sessions on high-frequency microprocessors. Three companies, Compaq, IBM and Intel introduced papers on CPUs that cross the 1 GHz barrier; AMD did not give a talk but actually showed a working system with a 1.1 GHz Athlon in one of the hotel suites.
The landmark of 1 GHz is reached only 8 years after achieving 100 MHz. Microprocessor designs are now sufficiently complex that it often takes the second- or third-generation technology before all critical paths in a design are optimized. This explains why the highest frequency processors presented at the conference are all incremental improvements to existing micro-architectures. The other keys to achieving higher frequencies are the advancements in process technology and optimization of interconnects.
Compaq introduced a 6-way out-of-order issue custom VLSI implementation of the Alpha architecture that runs at 1 GHz. With a size of 13.1x14.7 mmІ, the die contains 15.2 million transistors and is manufactured in 0.18 µ CMOS with 7 aluminum interconnect layers and flip-chip packaging. The processor features two on-chip cache arrays: a 64 KByte 2-way set-associative instruction cache and a 64 KByte 2-way associative dual-ported data cache. The microprocessor runs with a nominal internal voltage of 1.65 V and supports two voltage levels at the chip interface. For the packaging Compaq chose a 587-pin ceramic PGA using flip-chip technology, but also supports wirebond packaging.
Even though the Alpha processors have always been known for their speed, they never gained a serious market share. Compaq is the only significant OEM using the chips in their servers. Some analysts believe that this might change, however, predicting that Compaq will switch to Intel's 64-bit architecture as soon as the processors become available.
IBM presented a paper on a 1 GHz, single-issue, 64-bit PowerPC processor. The chip contains 19 million transistors and utilizes a 4-stage design in a 0.22 µ process with a six-layer copper interconnect. Usually very long pipelines are necessary to generate high frequencies. IBM managed to achieve 1 GHz with a very short pipeline, which is the more efficient way to do it because longer pipelines add to the complexity of the chip.
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