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Explanation of the L1 and L2 Cache

CPU cache is something more complicated than other kind of caches and thus, CPU caches are divided into 2 groups, the Level 1 and Level 2 caches, usually called L1 and L2. A L1 cache is some kind of memory which is built into the same CPU and it is the place where the CPU first try to access. The L2 cache is another memory but instead of feeding the CPU, this one feeds the L1 cache and this way the L2 cache can be understood as a cache of the L1 cache.

L2 caches may be built the same way as the L1 caches, into the CPU but sometimes it can also be located in another chip or in a MCP (Multichip Package Module), it can also be a completely separate chip. With some exceptions, L1 and L2 caches are considered SRAM (static RAM) while the memory of the computer is considered DRAM (Dynamic Ram) or any kind of variation of DRAM. Some processors use another cache level named L3.

The difference between L1 and L2 (and L3 in some cases) is the size. L1 is smaller than L2 and L3. This way the data is easier to be found in the L1 than L2, making the access much faster, if the data is not found in the L1 the data will be looked in the L2 bigger cache and if it is not there, an access to memory will be needed making the access much slower than either to L1 or L2.

The way the caches are managed depends on the architecture of the processors, but there are 2 main methods, inclusive and exclusive. In some processors the data which is stored on the L1 cache will also be present in the L2, this is called inclusive or more technically “strictly inclusive”. The AMD Athlon for example uses an exclusive cache pattern so the data will either be available in the L1 or the L2 but will never be stored in both. Intel Pentium II, III and 4 use a mixed pattern where the data must not be in both of them but usually it is. This is called mainly inclusive policy.

Which method is better is a very complicated question. The exclusive cache method can store more data because the data is not repeated on both of them. The advantage is even greater depending on the size of both caches. The major advantage of inclusive policy is that when other devices or processors in a system with several processors want to delete some data, they only need to check the L2 cache because the data will be also stored in the L1 cache for sure, while the exclusive cache policy will have to check on both the L1 and the L2 cache, making the operation slower.

There are more advantages / disadvantages of each policy but it can be greatly discussed because each company will think their pattern is better than their competitor’s.

 

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