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cache

Temporary computer storage used for quick retrieval of data in order to increase processing speed. The cached data can be stored in a reserved area of RAM, a special cache chip (separate from the CPU) that provides faster access than RAM, or on the disk drive. By keeping frequently accessed data in a rapidly accessible place, the computer can respond quickly to requests for those data without having to perform time-consuming searches of RAM or hard drives. Since a “stale” cache will contain data that have been superseded by later information, the cached data must be refreshed periodically.

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Pronounced "cash." A cache is used to speed up data transfer and may be either temporary or permanent. Memory and disk caches are in every computer to speed up instruction execution and data retrieval and updating. These temporary caches serve as staging areas, and their contents are constantly changing.

Browser caches and Internet caches store copies of Web pages retrieved by the user for some period of time in order to speed up retrieval the next time the same page is requested (see Web cache and browser cache). See also router cache.

Following are descriptions of the traditional memory and disk caches that are common in all computers.

Memory Caches
A memory cache, or "CPU cache," is a memory bank that bridges main memory and the CPU. It is faster than main memory and allows instructions to be executed and data to be read and written at higher speed. Instructions and data are transferred from main memory to the cache in fixed blocks, known as cache "lines," using some kind of look-ahead algorithm. See cache line.

Temporal and Spatial (Time and Space)
Caches take advantage of "temporal locality," which means the same data item is often reused many times. They also benefit from "spatial locality," wherein the next instruction to be executed or the next data item to be processed is likely to be the next in line. The more often the same data item is processed or the more sequential the instructions or data, the greater the chance for a "cache hit." If the next item is not in the cache, a "cache miss" occurs, and the CPU has to go to main memory to retrieve it.

Level 1 and Level 2
A level 1 (L1) cache is a memory bank built into the CPU chip. A level 2 cache (L2) is a secondary staging area that feeds the L1 cache. Increasing the size of the L2 cache may speed up some applications but have no effect on others. L2 may be built into the CPU chip, reside on a separate chip in a multichip package module (see MCP) or be a separate bank of chips on the motherboard. Caches are typically static RAM (SRAM), while main memory is generally some variety of dynamic RAM (DRAM). See SRAM and DRAM.

L1 and L2 Caches

The whole idea is to keep staging more instructions and data in a high-speed memory closer to the CPU.

Disk Caches
A disk cache is a section of main memory or memory on the disk controller board that bridges the disk and the CPU. When the disk is read, a larger block of data is copied into the cache than is immediately required. If subsequent reads find the data already stored in the cache, there is no need to retrieve it from the disk, which is slower to access.

If the cache is used for writing, data are queued up at high speed and then written to disk during idle machine cycles by the caching program. If the cache is built into the hardware, the disk controller figures out when to do it. See cache coherency, write back cache, write through cache, pipeline burst cache, lookaside cache, inline cache, backside cache and NV cache.

Disk Cache

Disk caches are usually just a part of main memory made up of common dynamic RAM (DRAM) chips, whereas memory caches (CPU caches) use higher-speed static RAM (SRAM) chips.

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(memory management)

cache - /kash/ A small fast memory holding recently accessed data, designed to speed up subsequent access to the same data. Most often applied to processor-memory access but also used for a local copy of data accessible over a network etc. When data is read from, or written to, main memory a copy is also saved in the cache, along with the associated main memory address. The cache monitors addresses of subsequent reads to see if the required data is already in the cache. If it is (a cache hit) then it is returned immediately and the main memory read is aborted (or not started). If the data is not cached (a cache miss) then it is fetched from main memory and also saved in the cache. The cache is built from faster memory chips than main memory so a cache hit takes much less time to complete than a normal memory access. The cache may be located on the same integrated circuit as the CPU, in order to further reduce the access time. In this case it is often known as primary cache since there may be a larger, slower secondary cache outside the CPU chip. The most important characteristic of a cache is its hit rate - the fraction of all memory accesses which are satisfied from the cache. This in turn depends on the cache design but mostly on its size relative to the main memory. The size is limited by the cost of fast memory chips. The hit rate also depends on the access pattern of the particular program being run (the sequence of addresses being read and written). Caches rely on two properties of the access patterns of most programs: temporal locality - if something is accessed once, it is likely to be accessed again soon, and spatial locality - if one memory location is accessed then nearby memory locations are also likely to be accessed. In order to exploit spatial locality, caches often operate on several words at a time, a "cache line" or "cache block". Main memory reads and writes are whole cache lines. When the processor wants to write to main memory, the data is first written to the cache on the assumption that the processor will probably read it again soon. Various different policies are used. In a write-through cache, data is written to main memory at the same time as it is cached. In a write-back cache it is only written to main memory when it is forced out of the cache. If all accesses were writes then, with a write-through policy, every write to the cache would necessitate a main memory write, thus slowing the system down to main memory speed. However, statistically, most accesses are reads and most of these will be satisfied from the cache. Write-through is simpler than write-back because an entry that is to be replaced can just be overwritten in the cache as it will already have been copied to main memory whereas write-back requires the cache to initiate a main memory write of the flushed entry followed (for a processor read) by a main memory read. However, write-back is more efficient because an entry may be written many times in the cache without a main memory access. When the cache is full and it is desired to cache another line of data then a cache entry is selected to be written back to main memory or "flushed". The new line is then put in its place. Which entry is chosen to be flushed is determined by a "replacement algorithm". Some processors have separate instruction and data caches. Both can be active at the same time, allowing an instruction fetch to overlap with a data read or write. This separation also avoids the possibility of bad cache conflict between say the instructions in a loop and some data in an array which is accessed by that loop. See also direct mapped cache, fully associative cache, sector mapping, set associative cache.