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PL/0 is a programming language, intended as an educational programming language, that is similar to but much simpler than Pascal, a general-purpose 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 1976. Read more on Wikipedia...
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.