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From: steve@jplgodo.UUCP (Steve Schlaifer x43171 156/224)
Newsgroups: net.lang
Subject: Re: RRe: What's so good about FORTH?
Message-ID: <813@jplgodo.UUCP>
Date: Tue, 17-Jun-86 13:23:57 EDT
Article-I.D.: jplgodo.813
Posted: Tue Jun 17 13:23:57 1986
Date-Received: Thu, 19-Jun-86 19:47:38 EDT
References: <201@pyuxv.UUCP> <3700003@uiucdcsp> <132@vaxb.calgary.UUCP> <634@ucbcad.BERKELEY.EDU>
Organization: Jet Propulsion Labs, Pasadena, CA
Lines: 33
Keywords: FORTH, threaded-code

In article <634@ucbcad.BERKELEY.EDU>, keppel@pavepaws.berkeley.edu (David Keppel) writes:
> [ Summary: What is threaded code? ]

Essentially, threaded code consists of a long series of subroutine calls.
In a threaded code implementation, the implementor writes some primitive words
that actually use the machine instructions to perform the kernal operations
needed.  On top of this layer, the remaining (high level) words required to
complete the language implementation are written as a series of calls to
other words (either high level or primitive).  In either direct or indirect
threading, these calls are represented as just the address of the start of
the code for the word to be executed (primitive word) or code needed to
nest the interpretor down a level (high level word).  A part of the threaded
language system called the inner interpretor picks up each address in turn
and then jumps to the code to be executed.  The last operation performed in
a word is usually a branch back to the inner interpretor.

The compactness of threaded code comes from only needing 2 bytes to cause
any word in the dictionary to be executed no matter how complex the operation
actually is.  Further, if the language is extensible like Forth, then any
application specific sequence of operations that is going to be
used frequently, can be added to the language and thereafter represented
in the dictionary as just a 2 byte code address.

A good book on threaded code is Threaded Interpretive Languages published
by Byte Books.  Also, Leo Brodie's book on Forth (I think it's called 
Starting Forth) has some good discussion of threading.
-- 

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