Cache

a hiding place, especially one in the ground, for ammunition, food, treasures, etc.:
She hid her jewelry in a little cache in the cellar.
anything so hidden:
The enemy never found our cache of food.
Computers. a temporary storage space or memory that allows fast access to data: Web browser cache;
CPU cache.
Alaska and Northern Canada. a small shed elevated on poles above the reach of animals and used for storing food, equipment, etc.
to put in a cache; conceal; hide.
Contemporary Examples

Romney Edges Santorum in Ohio Cliffhanger on Super Tuesday Howard Kurtz March 6, 2012
Belfast in Chaos After Days of Protestant Rioting, Police Injuries Nico Hines July 15, 2013
The Week in Death: Irving Milchberg, the Teenage Gunrunner of the Warsaw Ghetto The Telegraph February 28, 2014
Iraq’s Long-Lost Mythical Temple Has Been Found…and Is In Danger of Disappearing Again Nina Strochlic July 23, 2014
French Shooting Suspect Mohamed Merah Under Siege in Raid Tracy McNicoll March 21, 2012

Historical Examples

Ten American Girls From History Kate Dickinson Sweetser
The Leech Phillips Barbee
Danger at the Drawbridge Mildred A. Wirt
Warrior Race Robert Sheckley
The Secret Cache E. C. [Ethel Claire] Brill

noun
a hidden store of provisions, weapons, treasure, etc
the place where such a store is hidden
(computing) a small high-speed memory that improves computer performance
verb
(transitive) to store in a cache
n.
cache
(kāsh)
An area of computer memory devoted to the high-speed retrieval of frequently used or requested data.
memory management
/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.
(1997-06-25)