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Cache hierarchy, or multi-level cache, is a memory architecture that uses a hierarchy of memory stores based on varying access speeds to cache data. Highly requested data is cached in high-speed access memory stores, allowing swifter access by central processing unit (CPU) cores.
An 11-bit tag supports up to 512 MiB. Each cache line also has a valid bit and a dirty bit, stored in the cache controller. (16 Kbits, or 2 Kbytes, total size.) A 512K module contains twice as many cache lines, and so requires one fewer tag bit to support the same cacheable memory size.
A CPU cache is a hardware cache used by the central processing unit (CPU) of a computer to reduce the average cost (time or energy) to access data from the main memory. [1] A cache is a smaller, faster memory, located closer to a processor core, which stores copies of the data from frequently used main memory locations.
A block of memory cannot necessarily be placed at an arbitrary location in the cache; it may be restricted to a particular cache line or a set of cache lines [1] by the cache's placement policy. [2] [3] There are three different policies available for placement of a memory block in the cache: direct-mapped, fully associative, and set-associative.
A cache hit occurs when the requested data can be found in a cache, while a cache miss occurs when it cannot. Cache hits are served by reading data from the cache, which is faster than recomputing a result or reading from a slower data store; thus, the more requests that can be served from the cache, the faster the system performs. [2]
The gap between processor speed and main memory speed has grown exponentially. Until 2001–05, CPU speed, as measured by clock frequency, grew annually by 55%, whereas memory speed only grew by 7%. [1] This problem is known as the memory wall. The motivation for a cache and its hierarchy is to bridge this speed gap and overcome the memory wall.
The resulting load on memory use is known as pressure (respectively register pressure, cache pressure, and (main) memory pressure). Terms for data being missing from a higher level and needing to be fetched from a lower level are, respectively: register spilling (due to register pressure : register to cache), cache miss (cache to main memory ...
If the block is not found in the L1 cache, but present in the L2 cache, then the cache block is fetched from the L2 cache and placed in L1. If this causes a block to be evicted from L1, there is no involvement of L2. If the block is not found in either L1 or L2, then it is fetched from the main memory and placed in both L1 and L2.