Online Computer Terms Dictionary - L

{log}

["log: A Logic Programming Language with Finite Sets", A Dovier et al, Proc 8th Intl Conf Logic Prog, June 1991, pp.111-124].

logarithmus dualis

<mathematics> (ld) Latin for logarithm base two. More commonly written as "log" with a subscript "2".

Roughly the number of bits required to represent an integer.

(1999-03-19)

LogC

A C extension incorporating rule-oriented programming, for AI application programs. Production rules are encapsulated into functional components called rulesets. LogC uses a search network algorithm similar to RETE.

Version 1.6.

["LogC: A Language and Environment for Embedded Rule Based Systems", F. Yulin et al, SIGPLAN Notices 27(11):27-32 (Nov 1992)].

logic

1. <philosophy, mathematics> A branch of philosophy and mathematics that deals with the formal principles, methods and criteria of validity of inference, reasoning and knowledge.

Logic is concerned with what is true and how we can know whether something is true. This involves the formalisation of logical arguments and proofs in terms of symbols representing propositions and logical connectives. The meanings of these logical connectives are expressed by a set of rules which are assumed to be self-evident.

Boolean algebra deals with the basic operations of truth values: AND, OR, NOT and combinations thereof. Predicate logic extends this with existential and universal quantifiers and symbols standing for predicates which may depend on variables. The rules of natural deduction describe how we may proceed from valid premises to valid conclusions, where the premises and conclusions are expressions in predicate logic.

Symbolic logic uses a meta-language concerned with truth, which may or may not have a corresponding expression in the world of objects called existance. In symbolic logic, arguments and proofs are made in terms of symbols representing propositions and logical connectives. The meanings of these begin with a set of rules or primitives which are assumed to be self-evident. Fortunately, even from vague primitives, functions can be defined with precise meaning.

Boolean logic deals with the basic operations of truth values: AND, OR, NOT and combinations thereof. Predicate logic extends this with existential quantifiers and universal quantifiers which introduce bound variables ranging over finite sets; the predicate itself takes on only the values true and false. Deduction describes how we may proceed from valid premises to valid conclusions, where these are expressions in predicate logic.

Carnap used the phrase "rational reconstruction" to describe the logical analysis of thought. Thus logic is less concerned with how thought does proceed, which is considered the realm of psychology, and more with how it should proceed to discover truth. It is the touchstone of the results of thinking, but neither its regulator nor a motive for its practice.

See also fuzzy logic, logic programming, arithmetic and logic unit, first-order logic,

See also Boolean logic, fuzzy logic, logic programming, first-order logic, logic bomb, combinatory logic, higher-order logic, intuitionistic logic, equational logic, modal logic, linear logic, paradox.

2. <electronics> Boolean logic circuits.

See also arithmetic and logic unit, asynchronous logic, TTL.

(1995-03-17)

logical

(From the technical term "logical device", wherein a physical device is referred to by an arbitrary "logical" name) Having the role of. If a person (say, Les Earnest at SAIL) who had long held a certain post left and were replaced, the replacement would for a while be known as the "logical" Les Earnest. (This does not imply any judgment on the replacement).

Compare virtual.

At Stanford, "logical" compass directions denote a coordinate system in which "logical north" is toward San Francisco, "logical west" is toward the ocean, etc., even though logical north varies between physical (true) north near San Francisco and physical west near San Jose. (The best rule of thumb here is that, by definition, El Camino Real always runs logical north-and-south.) In giving directions, one might say: "To get to Rincon Tarasco restaurant, get onto El Camino Bignum going logical north." Using the word "logical" helps to prevent the recipient from worrying about that the fact that the sun is setting almost directly in front of him. The concept is reinforced by North American highways which are almost, but not quite, consistently labelled with logical rather than physical directions.

A similar situation exists at MIT: Route 128 (famous for the electronics industry that has grown up along it) is a 3-quarters circle surrounding Boston at a radius of 10 miles, terminating near the coastline at each end. It would be most precise to describe the two directions along this highway as "clockwise" and "counterclockwise", but the road signs all say "north" and "south", respectively. A hacker might describe these directions as "logical north" and "logical south", to indicate that they are conventional directions not corresponding to the usual denotation for those words. (If you went logical south along the entire length of route 128, you would start out going northwest, curve around to the south, and finish headed due east, passing along one infamous stretch of pavement that is simultaneously route 128 south and Interstate 93 north, and is signed as such!)

[Jargon File]

(1995-01-24)

logical address

virtual address

Logical Block Addressing

<storage> (LBA) A hard disk sector addressing scheme used on all SCSI hard disks, and on ATA-2 conforming IDE hard disks. The addressing conversion is performed by the hard disk firmware.

Prior to LBA, combined limitations of IBM PC BIOS and ATA restricted the useful capacity of IDE hard disks on IBM PCs and compatibles to 1024 cylinders * 63 sectors per track * 16 heads * 512 bytes per sector = 528 million bytes = 504 megabytes. Modern BIOSes select LBA mode automatically, and work around the 1024-cylinder BIOS limit by representing a hard disk to the OS as having e.g. half as many cylinders and twice as many heads. However, there is still an unbreakable BIOS disk size limit of 1024 cylinders * 63 sectors per track * 256 heads * 512 bytes per sector = 8 gigabytes, but modern OSes (including Windows 9x, Windows NT and Linux) are not affected by it, since they issue direct LBA-based calls, bypassing the BIOS hard disk services completely.

(2000-04-30)

logical complement

<logic> In Boolean algebra, the logical complement or negation of a Boolean value is the opposite value, given by the following truth table:

	A | -A
	--+---
	T | F
	F | T

(1995-01-24)

Logical Interchange Format

<file format, file system> (LIF) A Hewlett-Packard simple file system format used to boot HP-PA machines and to interchange files between older HP machines. A LIF file system is a header, containing a single directory, with 10-character case sensitive filenames and 2-byte file types, followed by the files.

LIF Utilities for linux.

(2003-10-09)

Logical Link Control

<networking> (LLC) The upper portion of the data link layer, as defined in IEEE 802.2. The LLC sublayer presents a uniform interface to the user of the data link service, usually the network layer. Beneath the LLC sublayer is the Media Access Control (MAC) sublayer.

(1995-02-14)

Logical Link Control and Adaptation Protocol

<protocol> (L2CAP) A Bluetooth protocol in the Core Protocol Stack providing data services to higher layer Bluetooth protocols.

L2CAP Layer Tutorial.

(2002-06-28)

logical relation

A relation R satisfying

f R g <=> For all a, b, a R b => f a R g b

This definition, by Plotkin, can be used to extend the definition of a relation on the types of a and b to a relation on functions.

logical shift

<programming> (Either shift left logical or shift right logical) Machine-level operations available on nearly all processors which move each bit in a word one or more bit positions in the given direction. A left shift moves the bits to more significant positions (like multiplying by two), a right shift moves them to less significant positions (like dividing by two). The comparison with multiplication and division breaks down in certain circumstances - a logical shift may discard bits that are shifted off either end of the word and does not preserve the sign of the word (positive or negative).

Logical shift is approriate when treating the word as a bit string or a sequence of bit fields, whereas arithmetic shift is appropriate when treating it as a binary number. The word to be shifted is usually stored in a register, or possibly in memory.

(1996-07-02)

logical shift left

logical shift

logical shift right

logical shift

Logical Unit

<networking> (LU) A primary component of SNA, an LU is a type of NAU that enables end users to communicate with each other and gain access to SNA network resources.

(1997-04-30)

Logical Unit 6.2

<networking> (LU6.2) A type of logical unit that governs peer-to-peer SNA communications. LU6.2 supports general communication between programs in a distributed processing environment.

LU6.2 is characterised by a peer relationship between session partners, efficient use of a session for multiple transactions, comprehensive end-to-end error processing and a generic application program interface consisting of structured verbs that are mapped into a product inplementation.

LU6.2 is used by IBM's TPF operating system.

[IBM Dictionary of Computing, McGraw-Hill 1993].

(1996-08-26)

Logical Unit Number

<storage> (LUN) A 3-bit identifier used on a SCSI bus to distinguish between up to eight devices (logical units) with the same SCSI ID.

(1999-02-11)

logic bomb

<programming, security> Code surreptitiously inserted into an application or operating system that causes it to perform some destructive or security-compromising activity whenever specified conditions are met.

Compare back door.

[Jargon File]

(1996-07-02)

Logic Design Language

<language> A language for computer design.

["A System Description Language Using Parametric Text Generation", R.H. Williams, TR 02.487, IBM San Jose, Aug 1970].

(1994-11-29)

logic emulator

A system of FPGAs, programmable interconnect and software which automatically configures itself into an operating prototype of a large-scale logic design, such as a microprocessor. An emulated design can be connected into the target system and really operated and tested before the design is made into an integrated circuit.

Quickturn is the leading logic emulation system.

(1994-11-29)

Logic for Computable Functions

<language> (LCF) Part of the Edinburgh proof assistant.

[What is it? Address?]

(1995-01-06)

logic gate

An integrated circuit or other device whose inputs and outputs represent Boolean or binary values as voltages (TTL uses 0V for False or 0, +5V for True or 1). Different gates implement different Boolean functions: AND, OR, NAND, NOR (these may take two or more inputs) NOT (one input), XOR (two inputs). NOT, NAND and NOR are often constructed from single transistors and the other gates made from combinations of these basic ones. These functions are all combinatorial logic functions, i.e. their outputs depend only on their inputs and there is no internal state. Gates with state, such as latches and flip-flops, are constructed by feeding some of their outputs back to their inputs.

(1995-02-08)

logic programming

<artificial intelligence, programming, language> A declarative, relational style of programming based on first-order logic. The original logic programming language was Prolog. The concept is based on Horn clauses.

The programmer writes a "database" of "facts", e.g.

	wet(water).

	mortal(X) :- human(X).

The user supplies a "goal" which the system attempts to prove using "resolution" or "backward chaining". This involves matching the current goal against each fact or the left hand side of each rule using "unification". If the goal matches a fact, the goal succeeds; if it matches a rule then the process recurses, taking each sub-goal on the right hand side of the rule as the current goal. If all sub-goals succeed then the rule succeeds.

Each time a possible clause is chosen, a "choice point" is created on a stack. If subsequent resolution fails then control eventually returns to the choice point and subsequent clauses are tried. This is known as "backtracking".

Clauses may contain logic variables which take on any value necessary to make the fact or the left hand side of the rule match a goal. Unification binds these variables to the corresponding subterms of the goal. Such bindings are associated with the choice point at which the clause was chosen and are undone when backtracking reaches that choice point.

The user is informed of the success or failure of his first goal and if it succeeds and contains variables he is told what values of those variables caused it to succeed. He can then ask for alternative solutions.

(1997-07-14)

Logic Replacement Technology

(LRT) Reading, BERKS. Tel: (0734) 751087. Marketing Director Bob Barrett. Manufacturers of the Ethernet hardware including the Filtabyte Ethernet controller card and EtherGate open access gateway.

logic variable

<programming> A variable in a logic programming language which is initially undefined ("unbound") but may get bound to a value or another logic variable during unification of the containing clause with the current goal. The value to which it is bound may contain other variables which may themselves be bound or unbound.

For example, when unifying the clause

	sad(X) :- computer(X, ibmpc).

	sad(billgates).

(1995-03-14)

LOGIN

1. An object-oriented deductive language and database system integrating logic programming and inheritance.

["LOGIN: A Logic Programming Language with Built-In Inheritance", H. Ait-Kaci et al, J Logic Programming 3(3):185-215 (1986)].

log in

<security> (Or "login", "log on", "logon") To start a session with a system, usually by giving a user name and password as a means of user authentication. The term is also used to mean the ability to access a service (also called an account), e.g. "Have you been given a login yet?"

"Log in/on" is occasionally misused to refer to starting a session where no authorisation is involved, or to access where there is no session involved. E.g. "Log on to our Web site!"

"login" is also the Unix program which reads and verifies a user's user name and password and starts an interactive session.

The noun forms are usually written as a single word whereas the verb forms are often written as two words.

To end a session is to "log out" or "off".

(2006-07-10)

LOGISCOPE

Software quality analysis tools from Verilog SA, used to evaluate the quality of software, both statically (based on software metrics) and dynamically.

Loglan

<human language> (Later "Lojban" /lozh'bahn/) An artificial human language designed by James Cooke Brown in the late 1950s.

Most artificial human languages devised in the 19th and 20th centuries (e.g. Esperanto) were designed to be easy to learn. Loglan, however, is unique in that its chief design goal was to avoid synactic ambiguity -- the kind that arises when trying to parse sentences like "The blind man picked up the hammer and saw".

Loglan is thus the only human language unambiguously parseable by a formal grammar (assuming you count Loglan as a human language; its grammar is not at all like that of any natural human language).

Most later development on Loglan continued under the name "Lojban".

The Loglan Institute, Inc. is a non-profit research corporation.

Loglan is apparently unrelated to the programming languages Loglan'82 or Loglan-88.

Halcyon Loglan.

Helsinki Lojban.

Address: The Loglan Institute, Inc., 3009 Peters Way, San Diego, CA, 92117-4313 U.S.A.

E-mail: loglan@compuserve.com

Telephone: +1 (619) 270 1691.

["Scientific American", June 1960].

(1999-01-14)

Loglan'82

<language> A teaching language including all the programming tools used in object-oriented programming, modular programming, and structured programming as well as programming by rules and functional programming.

Supported object-oriented programming features include classes, objects, coroutines, processes (in Loglan'82 processes are objects which are able to act in parallel), inheritance, exception handling, and dynamic arrays.

Loglan'82 is apparently unrelated to Loglan.

Home.

A cross-compiler to C is here.

[Related to Loglan-88?]

(1999-07-02)

Loglan-88

<language> An object-oriented language from the Institute of Informatics at Warsaw University.

Loglan-88 is apparently unrelated to Loglan.

[Loglan-88, "Report on the Programming Language, LNCS 414, Springer-Verlag, 1990, ISBN 3-540-52325-1].

[Related to Loglan'82?]

(1997-08-01)

LOGLISP

A version of Prolog implemented by Robinson in Lisp which allows Prolog programs to call Lisp and vice versa.

["LOGLISP: An Alternative to Prolog", J. Alan Robinson et al in Machine Intelligence 10, D. Michie ed, Ellis Horwood 1982].

LOGO

<language, education> A Lisp-like language for teaching programming, noted for its "turtle graphics" used to draw geometric shapes. LOGO was developed in 1966-1968 by a group at Bolt, Beranek & Newman (now "BBN Technologies") headed by Wally Fuerzeig <fuerzeig@bbn.com> (who still works there in 2003) and including Seymour Papert <seymour@media.mit.edu>.

There are Logo interpreters for Macintosh, Unix, IBM PC, X Window System, and many PCs. Implmentations include Berkeley Logo, MswLogo.

(2000-03-28)

log off

log out

LOGOL

Strings are stored on cyclic lists or 'tapes', which are operated upon by finite automata. J. Mysior et al, "LOGOL, A String manipulation Language", in Symbol Manipulations Languages and Techniques, D.G. Bobrow ed, N-H 1968, pp.166-177.

logon

1. <jargon> login.

2. <networking> In ACF/VTAM, an unformatted session-initiation request for a session between two logical units.

(1996-03-07)

log out

<security> (Or "log off") To end an authenticated session, undoing what happens when you log in. This is primarily to prevent other users gaining access to your logged in session, e.g. at an unattended computer, but typically also terminates any processes and network connections started as part of your session.

(2004-11-16)