program t67_strparam;

{ t67 -- a STRING VALUE parameter.  CALC.INC:17 is

      procedure Center(S : AnyString);

  TP3 puts a value parameter's whole SIZE on the stack (TPSRC7 tstscal), so a
  string[20] parameter is 21 bytes: the caller copies <len><chars> into the
  callee's slot, with the length byte at the lowest address (BP+4).  That is
  what makes it a VALUE: the callee has its own copy, and mutating S must not
  touch the caller's variable.

  The fixture proves both halves:
    - `Show (t)' then `writeln (t)' prints `hello' again, so S[1] := 'X'
      changed only the copy;
    - `Show ('world')' shows a LITERAL argument being copied too.
  A by-address implementation would print `Xello' after the first call.

  The bytes come from a DS-relative byte loop, not REP MOVSB: MOVS addresses
  through ES, and nothing here establishes ES = DS. }

var
   t : string[20];
procedure Show (S : string[20]);
begin
   writeln (S);
   S[1] := 'X'
end;
begin
   t := 'hello';
   Show (t);
   writeln (t);
   Show ('world');
   writeln (t)
end.