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Unformatted text preview: cess will fall in the same line. (This does not catch every access within the same line, but it catches nearly all of them and is very simple to implement.) Where an access will be in the same line, bypassing the tag look-up increases the access speed and saves power. Potentially, sequential accesses could use slower sense-amplifiers (possibly using standard logic rather than analogue circuits) and save considerable power, though care must be taken to ensure that an increased voltage swing on the bit lines does not cost more energy than is saved in the sense-amplifiers. The cache designer must remember that the goal is to minimize the overall system power, not just the cache power. Off-chip accesses cost a lot more energy than on-chip accesses, so the first priority must be to find a cache organization which gives a good hit rate. Deciding between a highly associative CAM--RAM organization or a set-associative RAM--RAM organization requires a detailed investigation of all of the design issues, and may be strongly influenced by low-level circuit issues such as novel ways to build power-efficient sense-amplifiers or CAM hit detectors. Cache power Sequential accesses Power optimization The ARM710T, ARM720T and ARM740T 321 Exploiting sequential accesses to save power and to increase performance is always a good idea. Typical dynamic execution statistics from the ARM show that 75% of all accesses are sequential, and since sequential accesses are fundamentally easier to deal with, this seems to be a statistic that should not be overlooked. Where power-efficiency is paramount, it may be worth sacrificing performance by making nonsequential accesses take two clock cycles; this will only reduce performance by about 25% and may reduce the cache power requirements by a factor of two or three. An interesting question for research into low power is to ask whether the best cache organization for power-efficiency is necessarily the same as the best organization for high performance...
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This document was uploaded on 10/30/2011 for the course CSE 378 380 at SUNY Buffalo.
- Spring '09