Cache design an example 279 table 101 summary of cache

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Unformatted text preview: sumptions about the cache and external memory speeds (which were 20 MHz and 8 MHz respectively): caches which hold either just instructions, mixed instructions and data, or just data. The results are shown in Table 10.2 on page 280, normalized to the performance of a system with no cache. They show that instructions are the most important values to hold in the cache, but including data values as well can give a further 25% performance increase. 280 Memory Hierarchy Table 10.2 Cache form 'Perfect' cache performance. Performance 1 1.95 2.5 1.13 No cache Instruction-only cache Instruction and data cache Data-only cache Although a decision was taken early on that the cache write strategy would be write-through (principally on the grounds of simplicity), it is still possible for the cache to detect a write miss and load a line of data from the write address. This 'allocate on write miss' strategy was investigated briefly, but proved to offer a negligible benefit in exchange for a significant increase in complexity, so it was rapidly abandoned. The problem was reduced to rinding the best organization, consistent with chip area and power constraints, for a unified instruction and data cache with allocation on a read miss. Various different cache organizations and sizes were investigated, with the results show in Figure 10.6. The simplest cache organization is the direct-mapped cache, but even with a size of 16 Kbytes, the cache is significantly worse than the 'perfect' case. The next step up in complexity is the dual set associative cache; now the performance Figure 10.6 Unified cache performance as a function of size and organization. Cache design - an example 281 of the 16 Kbyte cache is within a per cent or so of the perfect cache. But at the time of the design of the ARMS (1989) a 16 Kbyte cache required a large chip area, and the 4 Kbyte cache does not perform so well. (The results depend strongly on the program used to generate the address traces used, but these are typical.) Going to the other...
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This document was uploaded on 10/30/2011 for the course CSE 378 380 at SUNY Buffalo.

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