Node:Bit; Byte and Memory Manipulation, Next:User-defined Operators in GPC, Previous:Typed Constants, Up:Borland Pascal

### Bit, Byte and Memory Manipulation

The bitwise operators `shl`, `shr`, `and`, `or`, `xor` and `not` work in GNU Pascal like in Borland Pascal. As an extension, you can use them as procedures, for example

    program AndProcedureDemo;
    var x: Integer;
    begin
      and (x, $0000ffff);
    end.

as an alternative to

    program AndOperatorDemo;
    var x: Integer;
    begin
      x := x and $0000ffff;
    end.

GPC accepts the BP style notation `$abcd` for hexadecimal numbers, but you also can use Extended Pascal notation:

    program EPBaseDemo;
    const
      Binary =  2#11111111;
      Octal  =  8#177;
      Hex    = 16#ff;
    begin
    end.

and so on up to a basis of 36. Of course, you can mix the notations as you like, e.g.:

    program BPEPBaseDemo;
    begin
      WriteLn ($cafe = 2#1100101011111110)
    end.

`Inc` and `Dec` are implemented like in Borland Pascal. `Pred` and `Succ` are generalized according to Extended Pascal and can have a second (optional) parameter:

    procedure SuccDemo;
    var a: Integer = 42;
    begin
      a := Succ (a, 5);
      WriteLn (a) { 47 }
    end.

BP style `absolute` variables work in the context of overloading other variables as well as in the context of specifying an absolute address, but the latter is highly unportable and not very useful even in Dos protected mode.

    program BPAbsoluteDemo;

    type
      TString = String (80);
      TTypeChoice = (t_Integer, t_Char, t_String);

    { @@ WARNING: BAD STYLE! }
    procedure ReadVar (var x: Void; TypeChoice: TTypeChoice);
    var
      xInt: Integer absolute x;
      xChar: Char absolute x;
      xStr: TString absolute x;
    begin
      case TypeChoice of
        t_Integer: ReadLn (xInt);
        t_Char   : ReadLn (xChar);
        t_String : ReadLn (xStr);
      end
    end;

    var
      i: Integer;
      c: Char;
      s: TString;

    begin
      ReadVar (i, t_Integer);
      ReadVar (c, t_Char);
      ReadVar (s, t_String);
      WriteLn (i, ' ', c, ' ', s)
    end.

GNU Pascal knows Borland Pascal's procedures `FillChar` and `Move`. However, their use can be dangerous because it often makes implicit unportable assumptions about type sizes, endianness, internal structures or similar things. Therefore, avoid them whenever possible. E.g., if you want to clear an array of strings, don't `FillChar` the whole array with zeros (this would overwrite the Schema discriminants, see Strings), but rather use a `for` loop to assign the empty string to each string. In fact, this is also more efficient than `FillChar`, since it only has to set the length field of each string to zero.