Designing Embedded Hardware, 2nd Edition

Data Structures

The fact that Forth uses a stack for parameter passing between words makes it very powerful. It is because of the stack that the language is re-entrant and supports recursion. However, the dynamic nature of the stack makes it unsuitable for global data. Fortunately, Forth is not limited to using the stack, as the language provides for both named constants and variables.

A constant is declared by placing the value for the constant onto the stack, then the word constant followed by the constant name. For example, the following code declares a constant called kilobyte (representing the number of bytes in a K) to have a value of 1024:

1024 constant kilobyte

Using the constant in your code is trivial. Simply use the constant name to place the value onto the stack:

kilobyte

For example, here is a word definition that takes a value from the stack for a number of kilobytes, converts this into bytes, and prints out the result:

: kilobytes  (N --  , convert N to bytes)
   kilobyte  *  .
   ." bytes"  cr
;

So, typing:

32 kilobytes

gives a result on the console of:

32768 bytes

There is an alternative way to declare a constant. Simply define a word that places that value onto the stack:

: myconstant
    908612
;

This is not as efficient as using the constant word, but it works just the same.

Variables are declared in a similar way to constants, but the way they are used is quite different. Rather than placing a value onto the stack, the variable name will place a pointer to the variable onto the stack. ...