program Param (input, output);
var
  a, b: integer;
  procedure p (x, y : integer);
  begin
    x := x + 2 ;
    a := x * y ; (1)
    x := x + 1   (2)
  end;
begin
  a := 1 ;
  b := 2 ;
  p (a, b) ;
  writeln (a)
end.

procedure p(var x, y : Integer); // simulates call by value-result
var
  xtmp, ytmp : Integer;

  procedure p_hidden(var x, y : Integer); // nested "hidden" procedure
  begin
  ... YOUR ORIGINAL CODE HERE ...
  end;

begin
  xtmp := x; ytmp := y; // use auxiliary variables
  p_hidden(xtmp,ytmp);  // call by ref the nested procedure
  x := xtmp; y := ytmp; // copy values before return
end;

a := 1; b := 2;
p(a, b)
//// procedure p (var x, y : integer);
//// x is the same as a, y is the same as b
//// begin
  a := a + 2 ;  // a = 3
  a := a * b ;  // a = 3 * 2 = 6
  a := a + 1    // a = a + 1 = 7
//// end;
writeln (a)     // prints 7

a := 1; b := 2;
p(a, b)
//// procedure p (valres x, y : integer); // by value result
//// x is the same as a, y is the same as b like in a by reference call
  xt := a; yt := b;  // xt and yt are two "new" variables
  //// begin
  xt := xt + 2 ;  // xt = 3
  a := xt * yt ;  // a = 3 * 2 = 6
  xt := xt + 1    // xt = 3 + 1 = 4
//// end;
//// the values of xt and yt are copied back to a and b (x and y) before return
  a := xt;       // a = 4
  b := yt;       // b = 2
writeln (a)      // prints 4

//a := 1

|   1   | <-- Variable a, with holding the value 1
|_______|

procedure f(x: Integer)...

//f(a)

 a -> |   1   | -- copying -->  |   1   | <- x
      |_______|                 |_______|

//f(a)

 a -> |   1   | <- x -- for the duration of the call f(a)
      |_______|