figForth WORDs - Stack Requirements

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Forth is a “Stack-Based” Language

A Stack is a common data-structure

Forth has two stacks; a Parameter Stack (for passing values before and after execution of Forth Words) and a Return Stack as the Forth virtual machine equivalent of the low-level microprocessor “machine stack”…

The Parameter Stack

Virtual Machine Register SP

The Return Stack

Virtual Machine Register RP

Stack Rules

The granularity of the stacks is 16-bits; one word

However, that does not limit the data in any way; it just means Forth looks at data in 16-bit “chunks” ¹

The Forth Parser rules place numerical values on the stack implicitly, but they have to be taken off explicitly

Forth has a way of treating “double-number” (32-bit) values on the stack, as shown below…

eZ80 figForth Development Software R01
By David Husband  (c) Friday 30th July 2021

    ALL RIGHTS RESERVED

HEX ok
 ok

1234 ok
P-Stack Base> 1234

12.34 ok
P-Stack Base> 1234 0

 DPL ? 2 ok

Some Examples…

All based upon (Haydon 1982, pp. 6-214)

]

(1) - No Stack effect at all

(2) - But it IS taking text from the "Input Stream"

[’]

(1) - No Stack effect During Compilation

(2) - But there is a Stack effect upon Execution

(3) - This is an "IMMEDIATE" Word

(4) - But it IS taking text from the "Input Stream"

(5) - "How you say: ['] ?

(FIND)

(1) - No Stack effect During Compilation

(2) - But there is a Stack effect upon Execution

[COMPILE]

(1) - No Stack effect During Compilation

(2) - But there is a Stack effect upon Execution

(3) - This is an "IMMEDIATE" Word

(4) - But it IS taking text from the "Input Stream"

(5) - "How you say: "[']" ?

DIGIT

(1) - No Stack effect During Compilation

(2) - But there is a Stack effect upon Execution

WORD

(1) - No Stack effect During Compilation

(2) - But there is a Stack effect upon Execution

ENCLOSE

(1) - No Stack effect During Compilation

-FIND

(1) - No Stack effect During Compilation

(2) - But there is a Stack effect upon Execution

?DUP

(1) - No Stack effect During Compilation

TRAVERSE

(1) - No Stack effect During Compilation

U<

(1) - No Stack effect During Compilation

UNTIL

(1) - No Stack effect During Compilation

(2) - But there is a Stack effect upon Execution

(3) - This is an "IMMEDIATE" Word

(4) - But it IS taking text from the "Input Stream"

U/MOD

(1) - No Stack effect During Compilation

(2) - But there is a Stack effect upon Execution

WHILE

(1) - No Stack effect During Compilation

(2) - But there is a Stack effect upon Execution

(3) - This is an "IMMEDIATE" Word

(4) - But it IS taking text from the "Input Stream"

(5) - "How you say: ['] ?

References:

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