1. Forth
A stack language

2. Free implementations For our purposes, pforth seems to be best
Another alternative is SwiftForth
Both are ANS compliant

3. Forth syntax
A word consists of a sequence of printable (non-whitespace) characters
A program consists of a sequence of words, separated by whitespace
Any questions?

4. Basic execution
Forth uses a dictionary to hold function definitions, and a stack to hold data
Words are evaluated left to right, top to bottom, in the usual order
Forth is case insensitive
Most documentation uses words in all caps, but lowercase is also ok
If a word is in the dictionary,
then the function is looked up and executed
else Forth converts the word to a number, if it can, and places it on the stack

5. Defining and calling functions
A function is defined with : functionName ( old -- new , words ) body ;
The function expects to find its parameters on the stack
The function places its return values on the stack
The ( old -- new , words ) tells how the stack is changed
For example, addition might be documented as ( n1 n2 -- n3 , adds top two items )
A function is called by mentioning its name
Parameters, if any, are taken from the stack
Return values, if any, are placed on the stack
Example:
: add3 ( n1 n2 n3 -- n4 , add 3 items) + + ; ok
add3 .s <-Top ok

6. Displaying the stack . ( N -- , print number on top of stack )
Removes (“pops”) the top number from the stack and prints it ok
.s ( -- , print the entire stack )
This does not modify the stack contents
s <-Top ok ok
.s <-Top ok
The top of the stack is on the right
The above examples are from SwiftForth, which prints <- Top
Most other Forth implementations just take this for granted

7. Manipulating the stack
0sp ( clears the stack ) (pforth only)
0sp 0sp ? .s <-Top ok
dup ( n -- n n , duplicate top of stack )
dup .s <-Top ok
swap ( a b -- b a , swap top two items on stack )
swap .s <-Top ok
over ( a b -- a b a , copy second item on stack )
over .s <-Top ok
pick ( ... v3 v2 v1 v0 N v3 v2 v1 v0 vN )
Makes a copy of the Nth stack item; 0 PICK is equivalent to DUP

8. More stack manipulations
DROP ( a -- , remove top item from the stack )
ROT ( a b c -- b c a , ROTate third item to top )
?DUP ( n -- n n | 0 , duplicate only if non-zero, '|' means OR )
-ROT ( a b c -- c a b , rotate top to third position )
2SWAP ( a b c d -- c d a b , swap pairs )
2OVER ( a b c d -- a b c d a b , leapfrog pair )
2DUP ( a b -- a b a b , duplicate pair )
2DROP ( a b -- , remove pair )
NIP ( a b -- b , remove second item from stack )
TUCK ( a b -- b a b , copy top item to third position )

9. Arithmetic Arithmetic is integer
+ - * / mod are add, subtract, multiply, integer divide, modulus, all ( n1 n2 -- n3 )
Some “shorthand” operators are * 2/
10 2/ . 5 ok
/MOD ( n1 n2 -- rem quot , remainder and quotient of n1/n2 )
MIN ( n1 n2 -- n3 , minimum of n1 and n2 )
MAX ( n1 n2 -- n3 , maximum of n1 and n2 )
ABS ( n -- abs(n) , absolute value of n )
NEGATE ( n -- -n , negate value, faster then -1 * )
LSHIFT ( n c -- n<<c , left shift of n )
RSHIFT ( n c -- n>>c , logical right shift of n )
ARSHIFT ( n c -- n>>c ) , arithmetic right shift of n )

10. Output ." ( -- , print string up to next " )
EMIT ( n -- , display character with ASCII value n )
dup emit emit emit emit emit Hello ok
CHAR ( <char> -- char , get ASCII value of a character )
Unusual: Uses the following text, not the stack!
char A . 65 ok
." ( -- , print string up to next " )
." Hello from pforth!" Hello from pforth! ok
Works in SwiftForth when in a function, but not by itself in REPL (?)
SPACE ( -- , output a space )
SPACES ( n -- , output n spaces )
CR ( -- , start new line , carriage return )

11. Strings ." will output a string, up to a "
." hello" hello ok Stack<10>
S" will put a string on the stack
Format: Character count on top, machine address below that
s" Hello from Forth!" ok Stack<10>
TYPE will remove a print a string from the stack
type Hello from Forth! ok Stack<10>

12. Strings, character by character
Given the machine address of a string, will return the first character
CHAR+ will advance to the next character
s" ABCDE" ok Stack<10>
drop dup ok Stack<10>
emit A ok Stack<10>
char+ dup emit B ok Stack<10>
char+ char+ char+ dup emit E ok Stack<10>

13. Input KEY ( -- char , input character )
: testkey ( -- ) ." Hit a key: " key cr ." That was " . cr ; ok
testkey Hit a key: A That was 65

14. Loading from a file In file sample.fth, in same directory as sf:
\ Sample Forth Code \ Author: Phil Burk ANEW \ forget what was loaded before : SQUARE ( n -- n*n , square number ) DUP * ; : TEST.SQUARE ( -- ) CR ." 7 squared = " SQUARE . CR ;
In SwiftForth:
include sample.fth ok test.square 7 squared = 49 ok
Very similar in pforth
forget square – forgets everything from square on down (pforth only)

15. More about loading
FORGET definition is supposed to forget the named definition and everything that follows
This works in pforth, but I don’t think it does in SwiftForth
ANEW should forget what was loaded before
INCLUDE? filename will only load code if it isn’t already in the dictionary

16. Constants and variables
Constants can be defined as value CONSTANT name
42 constant theAnswer ok
theAnswer ok
Variables can be defined with VARIABLE name
VARIABLE ( <name> -- , define a 4 byte memory storage location )
@ ( address -- value , FETCH value FROM address in memory )
! ( value address -- , STORE value TO address in memory )
variable ans ok 42 ans ! ok ans ? 42 ok . 42

17. Logic true and false are constants -1 and 0
Comparisons are < <= = >= > 0= 0> 0<
2 3 < ok > . 0 ok 2 3 <= ok 2 3 >= . 0 ok
Logical operators are and or not
true -1 and ok
true true and not . 0 ok

18. IF-ELSE-THEN statements
Forth has an “if” statement
: showIf ( flag -- , print logical value ) if ." true" else ." false" then ; ok
true showIf true ok
false showIf false ok
55 showIf true ok
The “else” part is optional
It helps to think of then as “end if

19. BEGIN … END Loops
BEGIN … END will loop until a given condition is true
5 begin 1- dup . dup not until ok Stack<10> 0
0 begin dup . 1+ dup 3 = until ok Stack<10> 3

20. DO … LOOP Loop end start DO … LOOP will loop a fixed number of times
5 1 do ." hi! " loop hi! hi! hi! hi! ok Stack<10>
You can access the loop counter with I
5 1 do ." hi#" i 48 + emit loop hi#1 hi#2 hi#3 hi#4 ok Stack<10>
70 65 do i . i emit space loop 65 A 66 B 67 C 68 D 69 E ok
You can leave the loop with LEAVE
50 1 do ." hi " i 5 > if leave then loop hi hi hi hi hi hi ok Stack<10>

21. BEGIN … WHILE … REPEAT Loop
BEGIN test WHILE body REPEAT acts like a while loop in more traditional languages
2 begin dup 1000 < while dup . 2* repeat ok Stack<10> 1024

22. “Case” statements On file sample.fth:
: day-of-week ( n -- str , return day of week, Sunday = 1 ) case of s" Sunday" endof of s" Monday" endof of s" Tuesday" endof of s" Wednesday" endof of s" Thursday" endof of s" Friday" endof of s" Saturday" endof endcase ;
include sample.fth Include sample.fth include added 768 bytes,39236 left. ok Stack<10> 4 day-of-week type Wednesday ok Stack<10>

23. References
Most of this lecture based on (or shamelessly stolen from) the excellent Forth Tutorial by Phil Burk:

24. The End
The first time I combined the ideas I had been developing into a single entity, I was working on an IBM 1130, a “third-generation” computer. The result seemed so powerful that I considered it a “fourth generation computer language.” I would have called it Fourth, except that the 1130 permitted only five-character identifiers. So Fourth became Forth, a nicer play on words anyway Charles “Chuck” Moore