Program with factorial procedure that uses recursion to calculate the
factorial:

	program recurse;
	var
   	i, j: integer;
	procedure fact(var n, f: integer);
    		var
			m, g: integer;
    		begin
    		writeln(' fact in: ', n); {debugging output}
    		if n <= 1 then
			begin
			f := 1;
			end
    		else
			begin
			m := n - 1;
			fact(m, g);
			f := n * g;
			end;
    		writeln(' fact out: ', f); {debugging output}
    		end;
	begin
	write('n? ');
	readln(i);
	fact(i, j);
	writeln(i:5, ' factorial is ',j:6);
	readln;
	end.

Note that the statements flagged with  {debugging output} would normally
not be included but are here to give some clues as to how it all works.

Sample output:

>n?
<4
> fact in: 4
> fact in: 3
> fact in: 2
> fact in: 1
> fact out: 1
> fact out: 2
> fact out: 6
>fact out: 24
>    4 factorial is     24