Forth Programming Beginner Guide

Forth Programming Beginner Guide (Using SwiftForth)

How Forth programming works

Forth is a stack based language. You can think of the “stack” as something similar to a “stack” of books.

Example image

If book number 5 is the “top” of the stack. Then in the Forth programming language we would use the word “drop” to take this book off the stack.

This would leave us with books 1-4.

Words used in the video:

Word Description
DUP Duplicates the top of stack
SWAP Swaps first item with 2nd item on the data stack
ROT Rotates 3 items on the stack
DROP Drops the first item off of the stack
PAD Scratch pad of memory
DUMP Dump a piece of memory
-TRAILING Elimnates trailing spaces in memory
ZCOUNT Counts until you get to a zero byte


PAD is a volatile piece of memory. If you create a new word, lets say
: test ." hello world" ;

Your memory in PAD will be overwritten. It’s recommended to move whatever you put in there to somewhere else.

PAD exists about 512 bytes above the dictionary. So thats why it’s recommended to treat it as a temporary sctrach pad of memory.

Useful SwiftForth words to figure out what a word does

Say you odn’t know what DUP does. Just type into the SwiftForth Terminal:

Locate dup

LOCATE will show you the file directoy its located in and the source code. Also it will show you the usage:
( x -- xx ) means if you do this for example:

1 DUP <press enter key on keyboard>

Then the output on the data stack will be:

1 1 <-TOP

So we know that DUP will duplicate whatever number is currently at the top of the data stack.

Another handy word is WHERE. This word will show all *.f files that have used this word:

Doing this will generate a lot of output to the screen and show you how its used in different programs and applications. win

These two words LOCATE and WHERE are what I use the most to figure out what a word does.

LOCATE DUP Displays the word DUP, its location in file directory and source code
WHERE DUP Displays ALL *.f files that have used the word DUP