PL/0

The following is the syntax rules of the model language defined in EBNF:

program = block "." .

block = [ "const" ident "=" number {"," ident "=" number} ";"]
        [ "var" ident {"," ident} ";"]
        { "procedure" ident ";" block ";" } statement .

statement = [ ident ":=" expression | "call" ident 
              | "?" ident | "!" expression 
              | "begin" statement {";" statement } "end" 
              | "if" condition "then" statement 
              | "while" condition "do" statement ].

condition = "odd" expression |
            expression ("="|"#"|"<"|"<="|">"|">=") expression .

expression = [ "+"|"-"] term { ("+"|"-") term}.

term = factor {("*"|"/") factor}.

factor = ident | number | "(" expression ")".

It is rather easy for students to write a recursive descent parser for such a simple syntax. Therefore, the PL/0 compiler is still widely used in courses on compiler construction throughout the world. Due to the lack of features in the original specification, students usually spend most of their time with extending the language and their compiler. They usually start with introducing REPEAT .. UNTIL and continue with more advanced features like parameter passing to procedures or data structures like arrays, strings or floating point numbers.

The main article on compilers honours PL/0 for introducing several influential concepts (stepwise refinement, recursive descent parsing, EBNF, P-code, T-diagrams) to the field by educating students to use these concepts. Over the last 3 decades, most university courses on compiler construction that used PL/0 have followed Wirth strictly in employing these techniques (see references below). Some years ago university courses deviated from the course set by Wirth with the replacement of the classical recursive descent parsing technique by a (nonetheless classical) Unix-like approach of employing lex and yacc. Only recently an implementation (PL/0 Language Tools) along this way has also combined modern concepts like object-orientation and design patterns with a modern scripting language (Python), allowing students to consume the source text of the implementation in a contemporary programming style.

In December 1976, Wirth wrote a small booklet about compiler construction, containing the full source code of the PL/0 compiler. The syntax rules above were taken from this first edition of Wirth's book Compilerbau.[1] In later editions of this book (under the influence of his ongoing research) Wirth changed the syntax of PL/0. He changed the spelling of keywords like const and procedure to uppercase. This change made PL/0 resemble Modula-2 more closely. At the same time, Wirth's friend and collaborator C. A. R. Hoare was working on his influential communicating sequential processes concept, which used the exclamation mark ! and the question mark ? to denote communication primitives. Wirth added both symbols to the PL/0 language, but he did not mention their semantics in the book.

The following example is taken from such an extended language called PL/0E.[2] This program outputs the squares of numbers from 1 to 10. Most courses in compiler construction today have replaced the exclamation mark with the WriteLn procedure.

VAR x, squ;

PROCEDURE square;
BEGIN
   squ:= x * x
END;

BEGIN
   x := 1;
   WHILE x <= 10 DO
   BEGIN
      CALL square;
      ! squ;
      x := x + 1
   END
END.

This following program prints the prime numbers from 1 to 100. The write statement corresponds to '!' statement in the EBNF syntax above.

const max = 100;
var arg, ret;

procedure isprime;
var i;
begin
	ret := 1;
	i := 2;
	while i < arg do
	begin
		if arg / i * i = arg then
		begin
			ret := 0;
			i := arg
		end;
		i := i + 1
	end
end;

procedure primes;
begin
	arg := 2;
	while arg < max do
	begin
		call isprime;
		if ret = 1 then write arg;
		arg := arg + 1
	end
end;

call primes
.

The following example was taken from the second edition of Wirth's book Compilerbau,[1] which appeared in 1986 in Germany.

VAR x, y, z, q, r, n, f;

PROCEDURE multiply;
VAR a, b;
BEGIN
  a := x;
  b := y;
  z := 0;
  WHILE b > 0 DO
  BEGIN
    IF ODD b THEN z := z + a;
    a := 2 * a;
    b := b / 2
  END
END;

PROCEDURE divide;
VAR w;
BEGIN
  r := x;
  q := 0;
  w := y;
  WHILE w <= r DO w := 2 * w;
  WHILE w > y DO
  BEGIN
    q := 2 * q;
    w := w / 2;
    IF w <= r THEN
    BEGIN
      r := r - w;
      q := q + 1
    END
  END
END;

PROCEDURE gcd;
VAR f, g;
BEGIN
  f := x;
  g := y;
  WHILE f # g DO
  BEGIN
    IF f < g THEN g := g - f;
    IF g < f THEN f := f - g
  END;
  z := f
END;

PROCEDURE fact;
BEGIN
  IF n > 1 THEN
  BEGIN
    f := n * f;
    n := n - 1;
    CALL fact
  END
END;

BEGIN
  ?x; ?y; CALL multiply; !z;
  ?x; ?y; CALL divide; !q; !r;
  ?x; ?y; CALL gcd; !z;
  ?n; f := 1; CALL fact; !f
END.

In the third and last edition of his book on compiler construction, Wirth replaced PL/0 with Oberon-0. The language Oberon-0 is much more complex than PL/0. For example, Oberon-0 offers arrays, records, type declarations and procedure parameters. The publisher of Wirth's books (Addison-Wesley) has decided to phase out all his books, but Wirth has published revised editions of his book beginning in 2004.[3] As of August 2017, the most recent revision available is from May 2017.[4][5]

• P-code machine

• The compiler from the first edition of the Compilerbau book, written in Pascal
• The interpreter from "Algorithms + Data Structures = Programs" book, written in Pascal
• Development of a PL/0 style compiler based on 'Compiler construction' written in Mocka (Modula-2 for Linux)
• A paper explaining the use of PL/0 at the University of Rochester
• The homepage of the PL/0 reference book, "Algorithms + Data Structures = Programs"
• http://sourceforge.net/projects/pl0-compiler (written in C/C++, uses QT framework)
• https://modernc.org/pl0 (written in Go, runs in terminal, cross platform)
• https://github.com/dodobyte/plzero (a very small compiler produces windows executable)
• https://github.com/MarcRochkind/pl0compiler (compiler for IBM 701 written in C; generates 701 assembler)