Interactive Use in the Forth Terminal

• How Forth prompts for input
• How Forth looks for words and numbers in your input
• How to do simple actions
• How to build simple new definitions
• Bye

• One peculiarity of Forth is that it doesn't prompt you for input. Instead it praises you for entering a good line of input with the ok prompt. An empty line is a good line. So if you want to check if your Forth system is alive and well, simply type [return]. If it prints -- one the same line-- one space and ok then it is read for you. The next input is expected on the next line.

• When reading your input, Forth looks for tokens seperated by whitespace. Almost no further syntax analysis takes place. Basically such a token can be one of two things: a word or a number.

  Forth-Word is the term for a definition of any kind. Words are stored in the Forth-Dictionary where they can be looked up by the Forth system.

  Having read a token Forth tries to look it up in the dictionary. Not the whole dictionary is search but only a subset defined by the active search order. The first occurence of the token in the search order of the dictionary is taken as the definition of the entered word.

  If the token cannot be found in the active search order of the dictionary then it is not considered a word. Next Forth tries to convert it to a number. If the token can be converted to a number then this number is placed on the data stack.

  If the token is neither a word nor a number then it is an error and reported as such.

• When numbers are put on the stack, what happens to the words found in the input stream? Unless you are about defining a new word (see below) they executed.

  Each word is associated a meaning, i.e. an action it performs when executed, the word's execution semantics.

  When the only syntax analysis a Forth system performs is seperating space-delimited tokens then a word can contain any character with the exception of white-space. For instance + is a good name for a word. The word + in fact exists and its execution semantics is to add the two topmost items in the data stack and replace them by their result.

  Let's put two numbers on the data stack and add them.

  1234 8765 + ok
  

  Where is the result? On top of the data stack. To print the topmost number from the data stack and remove it use the word .

  . 9999 ok
  

  Let's see if the operands are still on the stack:

  . 0
  Error: "." stack underflow
  

  Obviously not. Forth tried however to print and remove a number from the stack (the 0 above) and ran into the error condition that the stack contained -1 elements. After an error is reported the stack is reset to empty.

  A better way to see what's on the stack is the word .S

  .s  ok
  1 2 3 ok
  .s
             3 [00000003]
             2 [00000002]
             1 [00000001] ok
  + + . 6 ok
  .s  ok
  

  .S nondestructively prints all numbers on the stack. In the PFE numbers are printed both in decimal and in hex. The topmost stack element is printed first.

• Programming in Forth means adding words to the dictionary. When adding a new word every word already in the dictionary can be used by adding it's execution semantics to the execution semantics of the newly created word.

  Programming an application in Forth means building bottom up a hierarchy of words until a single word or a small set of application specific words solves the problem.

  When Forth is about build a new word it's in compiling state. The usual way to build a new word and enter compiling state is using the word : ('colon') like this:

  : PRINT-SUM + . ;
  

  In this example, the colon parses away the following white-space delimited token and creates a new definition then named PRINT-SUM. Then compilation state is entered. From now on each word is not executed but compiled. This means not the execution semantics is performed but an alternative compilation semantics.

  The compilation semantics of + and . is to add their respective execution semantics to the word in creation. The final ;-semicolon has the compilation semantics to finish compilation of the word defined by the preceding colon.

  The compilation semantics of most words is to add their execution semantics to the compiled word. There must be some special words too with other compilation semantics like ;-semicolon in order to control the compilation.

  Now we have new word named PRINT-SUM which has both the execution semantics of + and .. Instead of the sequence + . we can furtheron say PRINT-SUM:

  1234 8765 + . 9999 ok
  1234 8765 PRINT-SUM 9999 ok
  

• If you started the pfe inside another Operating System, you can quit the task by typing bye .

  Using quit will reset the current interpreter. If that is not sufficient try abort .