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Unformatted text preview: correct way to design a cache for this sort of application, the designer can be guided by the following observations on the examples presented in these ARM chips. 320 ARM CPU Cores Cache speed High-associativity caches give the best hit rate, but require sequential CAM then RAM accesses which limits how fast the cycle time can become. Caches with a lower associativity can perform parallel tag and data accesses to give faster cycle times, and although a direct mapped cache has a significantly lower hit rate than a fully associative one, most of the associativity benefits accrue going from direct-mapped to 2- or 4-way associative; beyond 4-way the benefits of increased associativity are small. However, a fully associative CAM-RAM cache is much simpler than a 4-way associative RAM-RAM cache. CAM is somewhat power-hungry, requiring a parallel comparison with every entry on each cycle. Segmenting the cache by reducing the associativity a little and activating only a subsection of the CAM reduces the power cost significantly for a small increase in complexity. In a static RAM the main users of power are the analogue sense-amplifiers. A 4-way cache must activate four times as many sense-amplifiers in the tag store as a direct-mapped cache; if it exploits the speed advantage offered by parallel tag and data accesses, it will also uses four times as many sense-amplifiers in the data store. (RAM-RAM caches can, alternatively, perform serial tag and data accesses to save power, only activating a particular data RAM when a hit is detected in the corresponding tag store.) Waste power can be minimized by using self-timed power-down circuits to turn off the sense-amplifiers as soon as the data is valid, but the power used in the sense-amplifiers is still significant. Where the processor is accessing memory locations which fall within the same cache line it should be possible to bypass the tag look-up for all but the first access. The ARM generates a signal which indicates when the next memory access will be sequential to the current one, and this can be used, with the current address, to deduce that the ac...
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This document was uploaded on 10/30/2011 for the course CSE 378 380 at SUNY Buffalo.
- Spring '09