In computer science, a cache is a collection of data duplicating original values
stored elsewhere or computed earlier, where the original data is expensive to
fetch (owing to longer access time) or to compute, compared to the cost of reading the cache. In other
words, a cache is a temporary storage area where frequently accessed data can be
stored for rapid access. Once the data is stored in the cache, it can be used in
the future by accessing the cached copy rather than re-fetching or recomputing
the original data.
A cache has proven to be extremely effective in many areas of computing
because access patterns in typical computer applications have locality of
reference. There are several kinds of locality, but this article primarily deals
with data that are accessed close together in time (temporal locality). The data
might or might not be located physically close to each other (spatial locality).
Use of the word cache in the computer context originated in 1967
during preparation of an article for publication in the IBM Systems Journal. The
paper concerned an exciting memory improvement in Model 85, a latecomer in the
IBM System/360 product line. The Journal editor, Lyle R. Johnson, pleaded for a
more descriptive term than high-speed buffer. When none was forthcoming, he
suggested the noun cache, from the French noun meaning a safekeeping or storage
place . The paper was published in early 1968, the authors were honored by IBM, their work was
widely welcomed and subsequently improved upon, and cache soon became
standard usage in computer literature.
A cache is a block of memory for temporary storage of data likely to be used
hard drive frequently use a cache, as do web browsers and web servers.
A cache is made up of a pool of entries. Each entry has a datum (a nugget of
data) which is a copy of the datum in some backing store. Each entry also has a
tag, which specifies the identity of the datum in the backing store of which the
entry is a copy.
When the cache client (a CPU, web browser, operating system) wishes to access
a datum presumably in the backing store, it first checks the cache. If an entry
can be found with a tag matching that of the desired datum, the datum in the
entry is used instead. This situation is known as a cache hit. So, for example,
a web browser program might check its local cache on disk to see if it has a
local copy of the contents of a web page at a particular URL. In this example,
the URL is the tag, and the contents of the web page is the datum. The
percentage of accesses that result in cache hits is known as the hit rate or hit
ratio of the cache.
The alternative situation, when the cache is consulted and found not to
contain a datum with the desired tag, is known as a cache miss. The previously
uncached datum fetched from the backing store during miss handling is usually
copied into the cache, ready for the next access.
During a cache miss, the CPU usually ejects some other entry in order to make
room for the previously uncached datum. The heuristic used to select the entry
to eject is known as the replacement policy. One popular replacement policy,
least recently used (LRU), replaces the least recently used entry (see cache
algorithms). More efficient caches compute use frequency against the size of the
stored contents, as well as the latencies and throughputs for both the cache and
the backing store. While this works well for larger amounts of data, long
latencies, and slow throughputs, such as experienced with a hard drive and the
Internet, it's not efficient to use this for cached main memory (RAM).
When a datum is written to the cache, it must at some
point be written to the backing store as well. The timing of this write is
controlled by what is known as the write policy.
In a write-through cache, every write to the cache causes a synchronous write
to the backing store.
Alternatively, in a write-back (or write-behind) cache, writes are not
immediately mirrored to the store. Instead, the cache tracks which of its
locations have been written over (these locations are marked dirty). The data in
these locations is written back to the backing store when those data are evicted
from the cache, an effect referred to as a lazy write. For this reason, a read
miss in a write-back cache (which requires a block to be replaced by another)
will often require two memory accesses to service: one to retrieve the needed
datum, and one to write replaced data from the cache to the store.
Data write-back may be triggered by other policies as well. The client may
make many changes to a datum in the cache, and then explicitly notify the cache
to write back the datum.
No-write allocation is a cache policy where only processor reads are cached,
thus avoiding the need for write-back or write-through when the old value of the
datum was absent from the cache prior to the write.
The data in the backing store may be changed by entities other than the
cache, in which case the copy in the cache may become out-of-date or stale.
Alternatively, when the client updates the data in the cache, copies of that
data in other caches will become stale. Communication protocols between the
cache managers which keep the data consistent are known as
Small memories on or close to the
can be made faster than the much larger main memory. Most CPUs since the 1980s
have used one or more caches, and modern microprocessors inside personal
computers may have as many as half a dozen, each specialized to a
different part of the task of executing programs.
While CPU caches are generally managed entirely by hardware, other caches are
managed by a variety of software. The
main memory, which is an example of disk cache, is usually managed by the
While the hard drive's hardware
buffer is sometimes misleadingly referred to as "disk cache", its main
functions are write sequencing and read prefetching. Repeated cache hits are
relatively rare, due to the small size of the buffer in comparison to HDD's
In turn, fast local hard disk can be used to cache information held on even
slower data storage devices, such as remote servers (web cache) or local tape
drives or optical jukeboxes. Such a scheme is the main concept of hierarchical
Web caches are employed byweb browsers and web proxy servers to store
previous responses from web servers, such as web pages. Web caches reduce the
amount of information that needs to be transmitted across the network, as
information previously stored in the cache can often be re-used. This reduces
bandwidth and processing requirements of the web server, and helps to improve
responsiveness for users of the web.
Modern web browsers employ a built-in web cache, but some
internet service providers or organizations also use a caching proxy server,
which is a web cache that is shared between all users of that network.
Another form of cache isP2P caching, where the files most sought for by
Peer-to-peer applications are stored in an ISP
cache to accelerate P2P transfers.
DNS daemon caches a mapping of domain names to
addresses, as does a resolver library.
Write-through operation is common when operating over unreliable networks
(like an Ethernet LAN), because of the enormous complexity of the
coherency protocol required between multiple write-back caches when
communication is unreliable. For instance, web page caches and client-side
network file system caches (like those in NFS or SMB) are typically read-only or write-through specifically to keep the
network protocol simple and reliable.
Search engines also frequently make web pages they have indexed available from
their cache. For example, Google provides a "Cached" link next to each search
result. This is useful when web pages are temporarily inaccessible from a web
Another type of caching is storing computed results that will likely be
needed again, or
memoization. An example of this type of caching is
program that caches the output of the compilation to speed up the second-time
Database caching can substantially improve the throughput of database
applications, for example in the processing of indexes, data dictionaries, and
frequently used subsets of data.
The difference between buffer and cache
The terms are not mutually exclusive and the functions are frequently
combined; however, there is a difference in intent. A
buffer is a temporary memory location, that is traditionally used because
instructions cannot directly address data stored in peripheral devices.
Thus, addressable memory is used as intermediate stage.
Additionally such a buffer may be feasible when a large block of data is
assembled or disassembled (as required by a storage device), or when data may be
delivered in a different order than that in which it is produced. Also a whole
buffer of data is usually transferred sequentially (for example to hard disk),
so buffering itself sometimes increases transfer performance. These benefits are
present even if the buffered data are written to the
buffer once and
read from the buffer once.
A cache also increases transfer performance. A part of the increase similarly
comes from the possibility that multiple small transfers will combine into one
large block. But the main performance gain occurs because there is a good chance
that the same datum will be read from cache multiple times, or that written data
will soon be read. A cache's sole purpose is to reduce accesses to the
underlying slower storage. Cache is also usually an
abstraction layer that is designed to be invisible from the perspective of
Dealers of North America conference. ComputerLand dealers
placed orders for nearly 250,000 computers that day. On
August 12, 1981 IBM took orders for almost 250,000 more Personal
Computers. IBM's planners have not been correct since.
At the same meeting the target environment
for the PC was described. Here are some of the assumptions
Small business would buy most PCs.
Large business would stick with mainframes and dumb
A few departments in large businesses would use PCs for
local, non-connected work.
The PC would be used for one task only. Not just one task at
a time, but a single task all day long. This might be a
spreadsheet, or word processing, or accounting, but no more
than one task would be performed all day