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[ The PC Guide | Systems and Components Reference Guide | Motherboard and System Devices | System Cache | Cache Characteristics ]

Cache Size

The size of the cache normally refers actually to the size of the data store, where the memory elements are actually stored. A typical PC level 2 cache is either 256 KB or 512 KB, but can be as small as 64 KB on older machines, or as high as 1 MB or even 2 MB. Within processors, level 1 cache usually ranges in size from 8 KB to 64 KB.

The more cache the system has, the more likely it is to register a hit on a memory access, because fewer memory locations are forced to share the same cache line. Let's use an example to illustrate (the same one we used when we discussed cache operation in detail.). We have a system with 64 MB of memory and 512 KB of direct-mapped cache, arranged into 32-byte cache lines. This means that we have 16,384 cache lines (512 K divided by 32). Each line is shared by 4,096 memory addresses (64 MB divided by 16,384). Now if we increase the amount of cache to 1 MB, we will have 32,768 cache lines, and each will only be shared by 2,048 addresses. Conversely, if we leave the cache at 512 KB but increase the system memory to 256 MB, each of the 16,384 cache lines will be shared by 16,384 addresses.

There are many areas in the computer world where Pareto's Law applies, and cache size is definitely one of them. If you have a 256 KB cache on a system using 32 MB, increasing the cache by 100% to 512 KB will probably result in an increase in the hit ratio of less than 10%. Doubling it again will likely result in an increase of less than 5%. In the real world, this differential is not noticeable to most people. However, if you greatly increase the amount of system memory you use, you will probably want to up your cache total as well to prevent a degradation in performance. Just make sure you watch closely the system RAM cacheability issue.

Next: System RAM Cacheability


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