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A Level 3 (L3) cache is a specialized cache that that is used by the CPU and is usually built onto the motherboard and, in certain special processors, within the CPU module itself. It works together with the L1 and L2 cache to improve computer performance by preventing bottlenecks due to the fetch and execute cycle taking too long. The L3 cache feeds information to the L2 cache, which then forwards information to the L1 cache. Typically, its memory performance is slower compared to L2 cache, but is still faster than the main memory (RAM).
The L3 cache is usually built onto the motherboard between the main memory (RAM) and the L1 and L2 caches of the processor module. This serves as another bridge to park information like processor commands and frequently used data in order to prevent bottlenecks resulting from the fetching of these data from the main memory. In short, the L3 cache of today is what the L2 cache was before it got built-in within the processor module itself.
The CPU checks for information it needs from L1 to the L3 cache. If it does not find this info in L1 it looks to L2 then to L3, the biggest yet slowest in the group. The purpose of the L3 differs depending on the design of the CPU. In some cases the L3 holds copies of instructions frequently used by multiple cores that share it. Most modern CPUs have built-in L1 and L2 caches per core and share a single L3 cache on the motherboard, while other designs have the L3 on the CPU die itself.