See {tuple}.

1. <operating system> {device driver}.

	2. <programming> The {main loop} of an event-processing
	program; the code that gets commands and dispatches them for
	execution.

	3. <tool> In the {TeX} world and the computerised typesetting
	world in general, a program that translates some
	device-independent or other common format to something a real
	device can actually understand.

	[{Jargon File}]

<database, programming> (JDBC) Part of the {Java Development
	Kit} which defines an {application programming interface} for
	{Java} for standard {SQL} access to {databases} from Java
	{programs}.

	{Home
	(http://java.sun.com/products/jdk/1.1/docs/guide/jdbc/index.html)}.

	{FAQ (http://www.yoyoweb.com/Javanese/JDBC/FAQ.html)}.

	See also {Open Database Connectivity}.

	(1997-09-04)

1. {application program}.

	2. {function application}.

1. <database> One or more large structured sets of persistent
	data, usually associated with software to update and {query}
	the data.  A simple database might be a single file containing
	many {records}, each of which contains the same set of
	{fields} where each field is a certain fixed width.

	A database is one component of a {database management system}.

	See also {ANSI/SPARC Architecture}, {atomic}, {blob}, {data
	definition language}, {deductive database}, {distributed
	database}, {fourth generation language}, {functional
	database}, {object-oriented database}, {relational database}.

	{Carol E. Brown's tutorial
	(http://accounting.rutgers.edu/raw/aies/www.bus.orst.edu/faculty/brownc/lectures/db_tutor/db_tutor.htm)}.

	2. <hypertext> A collection of {nodes} managed and stored in
	one place and all accessible via the same {server}.  {Links}
	outside this are "external", and those inside are "internal".

	On the {World-Wide Web} this is called a {website}.

	3. All the facts and rules comprising a {logic programming}
	program.

	(2005-11-17)

<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)

<language, database, standard> /S Q L/ An industry-standard
	language for creating, updating and, querying {relational
	database management systems}.

	SQL was developed by {IBM} in the 1970s for use in {System R}.
	It is the {de facto standard} as well as being an {ISO} and
	{ANSI} {standard}.  It is often embedded in general purpose
	programming languages.

	The first SQL standard, in 1986, provided basic language
	constructs for defining and manipulating {tables} of data; a
	revision in 1989 added language extensions for {referential
	integrity} and generalised {integrity} {constraints}.  Another
	revision in 1992 provided facilities for {schema} manipulation
	and {data administration}, as well as substantial enhancements
	for data definition and data manipulation.

	Development is currently underway to enhance SQL into a
	computationally complete language for the definition and
	management of {persistent}, complex objects.  This includes:
	generalisation and specialisation hierarchies, {multiple
	inheritance}, user defined {data types}, {triggers} and
	{assertions}, support for {knowledge based systems},
	{recursive query expressions}, and additional data
	administration tools.  It also includes the specification of
	{abstract data types} (ADTs), object identifiers, {methods},
	{inheritance}, {polymorphism}, {encapsulation}, and all of the
	other facilities normally associated with object data
	management.

	The emerging {SQL3} standard is expected to be complete in
	1998.

	According to Allen G. Taylor, SQL does _not_ stand for
	"Structured Query Language".  That, like "SEQUEL" (and its
	pronunciation /see'kw*l/), was just another unofficial name
	for a precursor of SQL.  However, the IBM SQL Reference manual
	for DB2 and Craig Mullins's "DB2 Developer's Guide" say SQL
	_does_ stand for "Structured Query Language".

	{SQL Standards (http://www.jcc.com/sql_stnd.html)}.

	{An SQL parser
	(ftp://ftp.ora.com/published/oreilly/nutshell/lexyacc/)} is
	described in "Lex & Yacc", by Levine, Mason & Brown published
	by O'Reilly.

	{The 1995 SQL Reunion: People, Projects, and Politics
	(http://www.mcjones.org/System_R/SQL_Reunion_95/)}.

	["A Guide to the SQL Standard", C.J. Date, A-W 1987].

	["SQL for Dummies", Allen G. Taylor, IDG Books Worldwide].

	(2005-11-17)

See also {complete graph}, {complete inference system},
	{complete lattice}, {complete metric space}, {complete partial
	ordering}, {complete theory}.

	[1. or 2. or both?]

	(1996-04-24)

1. <data, graphics> Data representing a two-dimensional scene.
	A digital image is composed of {pixels} arranged in a
	rectangular array with a certain height and width.  Each pixel
	may consist of one or more {bits} of information, representing
	the brightness of the image at that point and possibly
	including colour information encoded as {RGB} triples.

	{Images} are usually taken from the real world via a {digital
	camera}, {frame grabber}, or {scanner}; or they may be
	generated by computer, e.g. by {ray tracing} software.

	See also {image formats}, {image processing}.

	(1994-10-21)

	2. <mathematics> The image (or range) of a {function} is the
	set of values obtained by applying the function to all
	elements of its {domain}.  So, if f : D -> C then the set f(D)
	= \{ f(d) | d in D \} is the image of D under f.  The image is
	a subset of C, the {codomain}.

	(2000-01-19)

<data processing, signal processing> Performing some
	predefined sequence of operations on an input to produce an
	output or change of internal state; activity specifically
	involving the computer's {CPU}.

	The term is often qualified: "{data processing}" treats
	{digital} data, "{signal processing}" treats {analog} data
	(possibly in digital form), "{word processing}" takes in typed
	human language input and produces digital documents, {image
	processing} transforms digital {images}.

	(2003-10-23)

1. See {multithreading}.

	2. See {threaded code}.

	3. {topic thread}.

	[{Jargon File}]