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2/12/2007
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IBM Eyes Denser Memory As Way To Boost Microprocessor Performance

IBM researchers found that tripling the amount of cache memory on a microprocessor could double its performance.

SAN FRANCISCO, Calif. — IBM Corp. will detail a method for tripling the amount of memory on a microprocessor, potentially doubling its performance. By combining techniques in process and circuit design, IBM believes it can put as much as 48 Mbytes of fast DRAM on a reasonably sized CPU when its 45nm technology becomes available in 2008.

IBM's upcoming Power6 CPUs use 8 Mbytes SRAM cache. Rival Intel Corp.'s Itanium processors use as much as 18 Mbytes.

"Processors are definitely cache starved, and as you go more towards multi-core processors, the need for memory integration becomes more acute," said Subramanian Iyer, a distinguished engineer and director of 45nm technology development at IBM. "There are some server chips that could not be made without this technology," he added.

In a paper at the International Solid State Circuits Conference here Wednesday (Feb. 14) IBM will describe a 65nm prototype embedded DRAM with a latency of just 1.5 ns and a cycle time of 2 ns. That's an order of magnitude faster than today's DRAMs and competitive with SRAM that is typically used for microprocessor cache memory.

"To put 24-36 Mbytes of memory on a chip, you would need a 600mm-squared die today. Using this technology you could put that much memory on a 300-350mm-squared die," Iyer said.

IBM expects to use the technique on its future Power and Cell processors as well as have it available for its ASIC customers. "It's being defined in a way that it can be part of our standard 45nm process technology," Iyer said.

IBM combined two advances to enable the new memory integration. The company found a way to migrate its deep trench technology used for DRAMs from CMOS to its silicon-on-insulator (SOI) logic process. In a paper last December, IBM described that work that involved suppressing the floating-body effect in SOI.

"Our entire processor road map is based on SOI," said Iyer.

New circuit designs use short bit lines to eliminate the need for sense amps that detect voltage differences between the bit lines and a capacitor, a process that makes DRAMs relatively slow. The new design uses a three-transistor micro-sense amp that lets voltage current directly drive transistor gates.

IBM used embedded DRAM in a custom processor designed for its high-end Blue Gene/L supercomputers, but has not been able to use the technology in mass market computer chips to date. "This is 100 percent mainstream and we expect to get it in products in 2008," Iyer said.

Intel and other chip makers are investigating using the floating body cells to store charge as one alternative for embedded memory. Other chip makers are researching stacking memory and processor dice in multi-chip modules.

Intel archrival Advanced Micro Devices co-develops process technology with IBM and could use the embedded DRAM technology as a way to compete with Intel.

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