The Outer Interpreter(s)

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“… the outer interpreter, is the Forth system’s default SHELL. It is absolutely essential that the reader understand it completely before proceeding to other sections. Many of the properties of Forth language, such as compactness, execution efficiency and ease in programming and utilisation, are embedded in the outer interpreter. When the Forth computer is booted up, it immediately enters into the outer interpreter. In the default interpretive state, the Forth computer waits for the operator to type a line of commands on his console terminal. The command text string he types on the terminal, after a carriage return being entered, is then parsed by the text interpreter and appropriate actions will be performed accordingly…“     Based upon:   (Ting 2013, pp. 19-24) (My Emphasis)

Forth’s default outer interpreter is implemented by a word called QUIT which is an “infinite loop” ¹

QUIT also defines the Forth system’s default parsing behaviour

QUIT is really just a container for two other very important words:

QUERY & INTERPRET which do the real work !!

Although QUIT is Forth’s default shell, it is possible to have other shells which may be written in such as way as to determine different Forth System behaviour, etc. These other shells can co-exist with QUIT if desired…

References:

Haydon, G. B., 1982. All About Forth - An Annotated Forth Glossary [online]. Available from: Http://figforth.org.uk/library/All.About.Forth.pdf.

Loeliger, R. G., 1981. Threaded Interpretive Languages - Their Design and Implementation [online]. Available from: Http://figforth.org.uk/library/R. G. Loeliger__Threaded.Interpretive.Languages.pdf.

Ting, C. H., 2013. Systems Guide to fig-Forth [online]. 3rd ed. San Mateo, CA 94402, USA: Offete Enterprises, Inc. Available from: http://figforth.org.uk/library/Systems.Guide.to.figForth.pdf.