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Cache Mapping and Associativity

A very important factor in determining the effectiveness of the level 2 cache relates to how the cache is mapped to the system memory. What this means in brief is that there are many different ways to allocate the storage in our cache to the memory addresses it serves. Let's take as an example a system with 512 KB of L2 cache and 64 MB of main memory. The burning question is: how do we decide how to divvy up the 16,384 address lines in our cache amongst the "huge" 64 MB of memory?

There are three different ways that this mapping can generally be done. The choice of mapping technique is so critical to the design that the cache is often named after this choice:

• Direct Mapped Cache: The simplest way to allocate the cache to the system memory is to determine how many cache lines there are (16,384 in our example) and just chop the system memory into the same number of chunks. Then each chunk gets the use of one cache line. This is called direct mapping. So if we have 64 MB of main memory addresses, each cache line would be shared by 4,096 memory addresses (64 M divided by 16 K).
• Fully Associative Cache: Instead of hard-allocating cache lines to particular memory locations, it is possible to design the cache so that any line can store the contents of any memory location. This is called fully associative mapping.
• N-Way Set Associative Cache: "N" here is a number, typically 2, 4, 8 etc. This is a compromise between the direct mapped and fully associative designs. In this case the cache is broken into sets where each set contains "N" cache lines, let's say 4. Then, each memory address is assigned a set, and can be cached in any one of those 4 locations within the set that it is assigned to. In other words, within each set the cache is associative, and thus the name.
  This design means that there are "N" possible places that a given memory location may be in the cache. The tradeoff is that there are "N" times as many memory locations competing for the same "N" lines in the set. Let's suppose in our example that we are using a 4-way set associative cache. So instead of a single block of 16,384 lines, we have 4,096 sets with 4 lines in each. Each of these sets is shared by 16,384 memory addresses (64 M divided by 4 K) instead of 4,096 addresses as in the case of the direct mapped cache. So there is more to share (4 lines instead of 1) but more addresses sharing it (16,384 instead of 4,096).

Conceptually, the direct mapped and fully associative caches are just "special cases" of the N-way set associative cache. You can set "N" to 1 to make a "1-way" set associative cache. If you do this, then there is only one line per set, which is the same as a direct mapped cache because each memory address is back to pointing to only one possible cache location. On the other hand, suppose you make "N" really large; say, you set "N" to be equal to the number of lines in the cache (16,384 in our example). If you do this, then you only have one set, containing all of the cache lines, and every memory location points to that huge set. This means that any memory address can be in any line, and you are back to a fully associative cache.

Next: Comparison of Cache Mapping Techniques

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