PL/0
There are at least two programming languages known as PL/0. One is a subset of IBM's general purpose programming language PL/I.
The other PL/0, covered here, is a simplified version of the general-purpose programming language Pascal, intended as an educational programming language. It serves as an example of how to construct a compiler. It was originally introduced in the book, Algorithms + Data Structures = Programs, by Niklaus Wirth in 1975. It features quite limited language constructs: there are no real numbers, very few basic arithmetic operations and no control-flow constructs other than "if" and "while" blocks. While these limitations make writing real applications in this language impractical, it helps the compiler remain compact and simple.
Grammar
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 |
"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.
Compiler construction
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 Modula2 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.
Examples
The following example is taken from such an extended language called PL/0E.[2]
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 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.
The following example was taken from the second edition of Wirth's book Compilerbau,[1] which appeared in 1986 in Germany.
CONST
m = 7,
n = 85;
VAR
x, y, z, q, r;
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;
BEGIN
x := m;
y := n;
CALL multiply;
x := 25;
y := 3;
CALL divide;
x := 84;
y := 36;
CALL gcd
END.
Oberon-0
In the third and last edition of his book on compiler construction, Wirth replaced PL/0 with Oberon-0. The compiler is still presented in its entirety, although 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 revised the third edition of his book in 2005 and the result is now available online.[3]
See also
Notes
References
- Liffick, Blaise W., Ed (1979), The Byte Book of Pascal, ISBN 0-07-037823-1
- Wirth, Niklaus (1975), Algorithms + Data Structures = Programs, ISBN 0-13-022418-9
- Wirth, Niklaus (1986), Compilerbau, B.G. Teubner, Stuttgart ISBN 3-519-32338-9
External links
- The compiler from the first edition of the Compilerbau 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
- Course notes from CS Dept., Univ. of Canterbury, Christchurch, New Zealand containing compilers written in C, Pascal and Java
- Course notes from Appalachian State University explaining the addition of arrays, value parameters and functions to PL/0
- The homepage of the PL/0 reference book, "Algorithms + Data Structures = Programs" [1]
- http://sourceforge.net/projects/pl0-compiler
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