<software> Instructions for a computer in some programming
	language, often {machine language}.  The word "code" is often
	used to distinguish instructions from {data} (e.g. "The code
	is marked 'read-only'") whereas "{software}" is used in
	contrast with "{hardware}" and may consist of more than just
	code.

	(2000-04-08)

<programming> (See below for synonyms) A data structure for
	storing items which are to be accessed in last-in first-out
	order.

	The operations on a stack are to create a new stack, to "push"
	a new item onto the top of a stack and to "pop" the top item
	off.  Error conditions are raised by attempts to pop an empty
	stack or to push an item onto a stack which has no room for
	further items (because of its implementation).

	Most processors include support for stacks in their
	{instruction set architectures}.  Perhaps the most common use
	of stacks is to store {subroutine} arguments and return
	addresses.  This is usually supported at the {machine code}
	level either directly by "jump to subroutine" and "return from
	subroutine" instructions or by {auto-increment} and
	auto-decrement {addressing modes}, or both.  These allow a
	contiguous area of memory to be set aside for use as a stack
	and use either a special-purpose {register} or a general
	purpose register, chosen by the user, as a {stack pointer}.

	The use of a stack allows subroutines to be {recursive} since
	each call can have its own calling context, represented by a
	stack frame or {activation record}.  There are many other
	uses.  The programming language {Forth} uses a data stack in
	place of variables when possible.

	Although a stack may be considered an {object} by users,
	implementations of the object and its access details differ.
	For example, a stack may be either ascending (top of stack is
	at highest address) or descending.  It may also be "full" (the
	stack pointer points at the top of stack) or "empty" (the
	stack pointer points just past the top of stack, where the
	next element would be pushed).  The full/empty terminology is
	used in the {Acorn Risc Machine} and possibly elsewhere.

	In a list-based or {functional language}, a stack might be
	implemented as a {linked list} where a new stack is an empty
	list, push adds a new element to the head of the list and pop
	splits the list into its head (the popped element) and tail
	(the stack in its modified form).

	At {MIT}, {pdl} used to be a more common synonym for stack,
	and this may still be true.  {Knuth} ("The Art of Computer
	Programming", second edition, vol. 1, p. 236) says:

	  Many people who realised the importance of stacks and queues
	  independently have given other names to these structures:
	  stacks have been called push-down lists, reversion storages,
	  cellars, dumps, nesting stores, piles, last-in first-out
	  ("LIFO") lists, and even yo-yo lists!

	[{Jargon File}]

	(1995-04-10)

<programming> (Or "computer program", "program", "code") The
	instructions executed by a computer, as opposed to the
	physical device on which they run (the "{hardware}").

	The term was coined by the eminent statistician, {John Tukey}.

	Programs stored on {non-volatile storage} built from
	{integrated circuits} (e.g. {ROM} or {PROM}) are usually
	called {firmware}.

	Software can be split into two main types - {system software}
	and application software or {application programs}.  System
	software is any software required to support the production or
	execution of application programs but which is not specific to
	any particular application.  Examples of system software would
	include the {operating system}, {compilers}, editors and
	sorting programs.

	Examples of application programs would include an accounts
	package or a {CAD} program.  Other broad classes of
	application software include {real-time} software, {business
	software}, scientific and engineering software, {embedded
	software}, personal computer software and {artificial
	intelligence} software.

	Software includes both {source code} written by humans and
	executable {machine code} produced by {assemblers} or
	{compilers}.  It does not usually include the data processed
	by programs unless this is in a format such as {multimedia}
	which depends on the use of computers for its presentation.
	This distinction becomes unclear in cases such as {spread
	sheets} which can contain both instructions (formulae and
	{macros}) and data.  There are also various intermediate
	compiled or {semi-compiled}, forms of software such as
	{library} files and {byte-code}.

	Some claim that {documentation} (both paper and electronic) is
	also software.  Others go further and define software to be
	programs plus documentation though this does not correspond
	with common usage.

	The noun "program" describes a single, complete and
	more-or-less self-contained list of instructions, often stored
	in a single {file}, whereas "code" and "software" are
	uncountable nouns describing some number of instructions which
	may constitute one or more programs or part thereof.  Most
	programs, however, rely heavily on various kinds of {operating
	system} software for their execution.  The nounds "code" and
	"software" both refer to the same thing but "code" tends to
	suggest an interest in the implementation details whereas
	"software" is more of a user's term.

	(2002-07-21)

1. See {multithreading}.

	2. See {threaded code}.

	3. {topic thread}.

	[{Jargon File}]

<language, architecture> (JVM) A specification for software
	which interprets {Java} programs that have been compiled into
	{byte-codes}, and usually stored in a ".class" file.  The JVM
	{instruction set} is {stack}-oriented, with variable
	instruction length.  Unlike some other instruction sets, the
	JVM's supports {object-oriented} programming directly by
	including instructions for object {method} invocation (similar
	to {subroutine} call in other instruction sets).

	The JVM itself is written in {C} and so can be {ported} to run
	on most {platforms}.  It needs {thread} support and {I/O} (for
	{dynamic class loading}).  The Java byte-code is independent
	of the platform.

	There are also some hardware implementations of the JVM.

	{Specification
	(http://www.javasoft.com/docs/books/vmspec/html/VMSpecTOC.doc.html)}.

	{Sun's Java chip
	(http://news.com/News/Item/0,4,9328,00.html)}.

	[Documentation?  Versions?]

	(2000-01-03)

1. Not simulated.  Often used as a specific antonym to
	{virtual} in any of its jargon senses.

	2. <mathematics> {real number}.

	[{Jargon File}]

	(1997-03-12)

<operating system> To move a program from fast-access memory
	to a slow-access memory ("swap out"), or vice versa ("swap
	in").  The term often refers specifically to the use of a
	{hard disk} (or a {swap file}) as {virtual memory} or "swap
	space".

	When a program is to be executed, possibly as determined by a
	{scheduler}, it is swapped into {core} for processing; when it
	can no longer continue executing for some reason, or the
	scheduler decides its {time slice} has expired, it is swapped
	out again.

	This contrasts with "paging" systems in which only parts of a
	program's memory is transfered.

	[{Jargon File}]

	(1996-11-22)

<unit> (b) {binary} digit.

	The unit of information; the amount of information obtained by
	asking a yes-or-no question; a computational quantity that can
	take on one of two values, such as false and true or 0 and 1;
	the smallest unit of storage - sufficient to hold one bit.

	A bit is said to be "set" if its value is true or 1, and
	"reset" or "clear" if its value is false or 0.  One speaks of
	setting and clearing bits.  To {toggle} or "invert" a bit is
	to change it, either from 0 to 1 or from 1 to 0.

	The term "bit" first appeared in print in the computer-science
	sense in 1949, and seems to have been coined by the eminent
	statistician, {John Tukey}.  Tukey records that it evolved
	over a lunch table as a handier alternative to "bigit" or
	"binit".

	See also {flag}, {trit}, {mode bit}, {byte}, {word}.

	[{Jargon File}]

	(2002-01-22)

1. <networking> A {data link layer} "packet" which contains the
	header and trailer information required by the physical
	medium.  That is, {network layer} {packets} are encapsulated
	to become frames.

	See also {datagram}, {encapsulation}, {packet}, {Maximum
	Transmission Unit}.

	2. <programming> (language implementation) See {activation
	record}.

	3. <hardware> One complete scan of the active area of a
	{display screen}.  Each frame consists of a number N of
	horizontal {scan lines}, each of which, on a computer display,
	consists of a number M of {pixels}.  N is the {vertical
	resolution} of the display and M is the {horizontal
	resolution}.  The rate at which the displayed image is updated
	is the {refresh rate} in frames per second.

	(2000-10-07)