Forth programming language

The weirdest language ever?

• RPN - Reverse Polish Notation
• fourth generation programming language
  • the system used allow only five character names
• based on two stacks
• “words” as the only language construction
• lower level that assembly on one side
• higher level than some high level programming languages (C for sure)

Beginning of Forth

• 1970
• Charles “Chuck” Moore
• Minimal language on minimal hardware
  • “nucleus: 4kB
  • contains compiler
  • also editor
  • standard library
  • and the whole interactive environment!
• Initially not based on source files
• “screens” are used instead
  • screen 64x16 characters
  • mapped directly into sectors
  • explicit link to the next screen
  • no real OS needed!

microFORTH

• For all 8bit processors
  • 1kB “nucleus”
  • Intel 8080, Motorola 6800, MOS 6502, Zilog Z80
  • used in industry
  • usually based on 16kB RAM

How the Forth look like?

• Based on two stacks and words
  • data stack
  • return stack
  • words usually manipulate stack items

Data stack

10 20 + .
10 20 * 30 + .
1 2 swap . . cr
1 dup . . cr
1 2 drop . cr
1 2 over . . . cr

Words

• Basic building block for Forth programs
• Words are basically “compiled” on the fly
• No explicit parameters not return values
  • everything’s on stack!

: print . cr ;
: square dup * ;

10 square . cr
10 square print

Conditions and loops

condition
if
  commands
then

10 1 do loop

: my-loop
   10 1 do
     i . cr
   loop
;

Variables

1 one !
2 two !

1 1 + two !

: store ! ;

1 one store

one @ . cr
two @ . cr

a @ .
a ? // the same

Interested?

• There’s one very nice, powerful, and weird tool in Linux
• dc as desktop calculator
• Based on stack as well
• It’s older than even C!
• The classic unix tool
  • very powerful
  • cumbersome for new users
  • unforgiving

GCD computation

[a=]P?[b=]P?[dSarLa%d0<a]dsax+[GCD:]Pp

Factorial computation

?[q]sQ[d1=Qd1-lFx*]dsFxp

Printing all prime numbers

2p3p[dl!d2+s!%0=@l!l^!<#]s#[s/0ds^]s@[p]s&[ddvs^3s!l#x0<&2+l.x]ds.x