Forth, a river of Scotland, formed by the junction of Duchray Water and the Avondhu, streams which rise one on Ben Lomond and the other on Ben Venue, and which, after 14 and 9 m., unite at Aberfoyle; the river thence flows with many windings, called Links, through some of the fairest country of the eastern lowlands to Alloa (51½ m.), where begins the Firth, which stretches 51 m. to the German Ocean, and which at Queensferry is spanned by a massive railway bridge known as the Forth Bridge (1882-1890).

Forth (?), v.[AS. forð, fr. for akin to D. voort, G. fort √78. See Fore, For, and cf. Afford, Further, adv.] 1. Forward; onward in time, place, or order; in advance from a given point; on to end; as, from that day forth; one, two, three, and so forth.

Lucas was Paul's companion, at the leastway from the sixteenth of the Acts forth.

Tyndale.

From this time forth, I never will speak word.

Shak.

I repeated the Ave Maria; the inquisitor bad me say forth; I said I was taught no more.

Strype.

2. Out, as from a state of concealment, retirement, confinement, nondevelopment, or the like; out into notice or view; as, the plants in spring put forth leaves.

When winter past, and summer scarce begun,
Invites them forth to labor in the sun.

Dryden.

3. Beyond a (certain) boundary; away; abroad; out.

I have no mind of feasting forth to- night.

Shak.

4. Throughly; from beginning to end. [Obs.] Shak.

And so forth, Back and forth, From forth. See under And, Back, and From. -- Forth of, Forth from, out of. [Obs.] Shak. -- To bring forth. See under Bring.

Forth, prep. Forth from; out of. [Archaic]

Some forth their cabins peep.

Donne.

Forth, n. [OE., a ford. &?; 78. See Frith.] A way; a passage or ford. [Obs.] Todd.

1. <language> An interactive extensible language using
postfix syntax and a data stack, developed by Charles
H. Moore in the 1960s. FORTH is highly user-configurable and
there are many different implementations, the following
description is of a typical default configuration.

Forth programs are structured as lists of "words" - FORTH's
term which encompasses language keywords, primitives and
user-defined subroutines. Forth takes the idea of
subroutines to an extreme - nearly everything is a subroutine.
A word is any string of characters except the separator which
defaults to space. Numbers are treated specially. Words are
read one at a time from the input stream and either executed
immediately ("interpretive execution") or compiled as part of
the definition of a new word.

The sequential nature of list execution and the implicit use
of the data stack (numbers appearing in the lists are pushed
to the stack as they are encountered) imply postfix syntax.
Although postfix notation is initially difficult, experienced
users find it simple and efficient.

Words appearing in executable lists may be "primitives"
(simple assembly language operations), names of previously
compiled procedures or other special words. A procedure
definition is introduced by ":" and ended with ";" and is
compiled as it is read.

Most Forth dialects include the source language structures
BEGIN-AGAIN, BEGIN-WHILE-REPEAT, BEGIN-UNTIL, DO-LOOP, and
IF-ELSE-THEN, and others can be added by the user. These are
"compiling structures" which may only occur in a procedure
definition.

FORTH can include in-line assembly language between "CODE"
and "ENDCODE" or similar constructs. Forth primitives are
written entirely in assembly language, secondaries contain a
mixture. In fact code in-lining is the basis of compilation
in some implementations.

Once assembled, primitives are used exactly like other words.
A significant difference in behaviour can arise, however, from
the fact that primitives end with a jump to "NEXT", the entry
point of some code called the sequencer, whereas
non-primitives end with the address of the "EXIT" primitive.
The EXIT code includes the scheduler in some multi-tasking
systems so a process can be descheduled after executing a
non-primitive, but not after a primitive.

Forth implementations differ widely. Implementation
techniques include threaded code, dedicated Forth
processors, macros at various levels, or interpreters
written in another language such as C. Some implementations
provide real-time response, user-defined data structures,
multitasking, floating-point arithmetic, and/or virtual memory.

Some Forth systems support virtual memory without specific
hardware support like MMUs. However, Forth virtual memory
is usually only a sort of extended data space and does not
usually support executable code.

FORTH does not distinguish between operating system calls
and the language. Commands relating to I/O, file systems
and virtual memory are part of the same language as the
words for arithmetic, memory access, loops, IF statements, and
the user's application.

Many Forth systems provide user-declared "vocabularies" which
allow the same word to have different meanings in different
contexts. Within one vocabulary, re-defining a word causes
the previous definition to be hidden from the interpreter (and
therefore the compiler), but not from previous definitions.

FORTH was first used to guide the telescope at NRAO, Kitt
Peak. Moore considered it to be a fourth-generation language but his operating system wouldn't let him use six
letters in a program name, so FOURTH became FORTH.

Versions include fig-FORTH, FORTH 79 and FORTH 83.

FAQs.
ANS Forth standard, dpANS6.

FORTH Interest Group, Box 1105, San Carlos CA 94070.

See also 51forth, F68K, cforth, E-Forth, FORML,
TILE Forth.

[Leo Brodie, "Starting Forth"].

[Leo Brodie, "Thinking Forth"].

[Jack Woehr, "Forth, the New Model"].

[R.G. Loeliger, "Threaded Interpretive Languages"].

2. FOundation for Research and Technology - Hellas.

(1997-04-16)