Using kForth I

Using kForth

2.1 Basics

Type

kforth

to start the program. Upon startup, kForth will inform you that it is ready to accept input by displaying

Ready!

You may type commands, a sequence of words, and press Enter. kForth will respond with the prompt

ok

after it finishes executing each line of input. To illustrate, try typing the following

2 5 + .

and press Enter. kForth will respond with

7 ok

You may now enter another sequence of words. One particularly useful word to know is

bye

kForth will respond by saying

Goodbye

and exiting. Finally, note that kForth is not case sensitive --- you may enter words in lower case or upper case.

2.2 More Words

The word

words

will display a list of currently defined words in the dictionary. You may define your own words by typing them at the kForth prompt. For example, a word that counts from one to ten and displays each number counted may be defined by entering

: count_to_ten 10 0 do i 1+ . loop ;

The symbols ":" and ";" are very important --- they indicate to the kForth compiler the beginning and ending of the definition of the word, called count_to_ten in this example. kForth will display the prompt ok after the new word has been compiled into the dictionary.

You can verify that our newly defined word has been added to the dictionary by using words. Now, execute the word by typing

count_to_ten

and pressing Enter. kForth will display the output

1 2 3 4 5 6 7 8 9 10 ok

If you are entering a definition that requires several lines of typing, the ok prompt will not be displayed until the end of the definition has been entered, i.e. until the compiler encounters a semicolon.

Although you can write Forth programs this way, it is much easier to create the definitions in a separate source file and then load them into kForth by issuing the command

include filename

For example, the definition of count_to_ten could have been entered into a plain text file called prog1.4th. Once kForth has been started, you can simply issue the command

include prog1

kForth will read the input from the specified file as though it was being entered from the keyboard. You may have noticed that the full filename was not entered in the include command. If no extension is specified, the file is assumed to have an extension of .4th.

You may also load a source file upon startup of kForth by typing

kforth filename

2.3 Using the Stack

Forth provides a memory region called the stack in which data may be placed and operated upon by defined words. You may enter numbers onto the stack simply by typing them and pressing Enter. You can use the word .S to list the contents of the stack. For example, type the following and press Enter.

2 5

You have placed two numbers onto the stack. Now, type

.S

and press Enter. kForth will respond by listing the items on the stack:

5 2

Notice that 5 is on the top of the stack --- items are placed into the stack in a first-in, last-out order. kForth provides most of the stack operators (words) that are a part of the Forth language. Examples include the arithmetic operators

+ - * /

These operate on the top two items on the stack and replace them with the result. Other words change the order of items on the stack or copy or remove items from the stack:

SWAP ROT DUP OVER TUCK DROP NIP

Each stack cell holds a single integer number. You may also place real numbers, also known as floating point numbers, onto the stack. These must be entered in a special way known as exponential notation. For example, to enter the real number 3.14 onto the stack, type

3.14e0

and press Enter. The zero following the 'e' indicates the power of ten that is multiplied to the number (10 raised to the zero power is equal to 1). If the exponent is zero, as in this example, the entry can be shortened to simply

3.14e

Exponential format allows you to enter very small and very large numbers easily. To enter the fractional number representing one-billionth, 0.000000001, you may type

1e-9

and press Enter.

When you enter a floating point number onto the stack and list the stack using .S, you will see two integer numbers printed instead of one real number. A floating point number occupies two stack cells instead of one, and .S lists the contents of each cell as though it were a single integer. You may print the floating point number occupying the top two cells of the stack with the word

F.

Use the words

F+ F- F* F/

to perform arithmetic on floating point numbers which have been entered onto the stack. Example:

3.14e 6.28e f+ f.

will print the result 9.42. Words to manipulate floating point numbers on the stack include

FSWAP FROT FDUP FOVER FDROP