The most common Forth interface is a command line interpreter. Forth recognises a large number of instructions (called 'words'). Here we will look at a small selection of them.
The word + adds two integers. It is used like this;
(Words in Forth are delimited by spaces.)
123 456 + . [cr] 579 ok
Where [cr] means press 'enter', and the emphasised text is the computers response.
The period . is another Forth word (called 'dot'), that
prints a number.
Forth has an exceptionally rich set of integer operators, as it is often used
on systems that do not support floating point very well. An unusual one is
*/, which is a scaling operator. It's
stack comment is
( n1 n2 n3--n4), where the result n4 is
n1 multiplied by n2 and divided by n3.
To test this we might try
10000 355 113 */ . [cr] 31415 ok
and thus discover that 355/113 is a good approximation to pi.
More than one word can be issued at a time, so as an example, to convert from degrees Celsius to degrees Fahrenheit, we take nine fifths, and add thirty two. This can be tested thus.
100 9 5 */ 32 + . [cr] 212 ok
The converse operation, converting from degrees Fahrenheight to degrees Celsius is left as an exercise for the reader.
(But as a hint, the word to subtract one number from another is
-. It's stack comment is ( n1 n2--n3),
where n3 is n1-n2)
Now that you know almost all there is to know about the Forth command line interpreter, and have learned a few maths operators as well, you can return to the contents page, or go on to the next section.