var Stream:TMemoryStream;
begin
  Stream := TMemoryStream.Create(  );
  (Stream as TMemoryStream).LoadFromFile( FileName);

  TCustomMemoryStream = class(TStream)
  private
    FMemory: Pointer;
    FSize, FPosition: NativeInt;
//...
procedure TMemoryStream.SetSize(NewSize: Longint);
var
  OldPosition: Longint;
begin
  OldPosition := FPosition;
  SetCapacity(NewSize);
  FSize := NewSize;
  if OldPosition > NewSize then Seek(0, soFromEnd);
end;

  TCustomMemoryStream = class(TStream)
    function Seek(const Offset: Int64; Origin: TSeekOrigin): Int64; override;
    function Read(var Buffer; Count: Longint): Longint; override;

 TStream = class(TObject)
    function Read(var Buffer; Count: Longint): Longint; virtual; abstract;

 Function GetFileData(Const AFileName : String) : TBytes;
 Var

  Index : NativeInt;
  Count : Integer;

 Begin

  With TFileStream.Create(AFileName, fmOpenRead Or fmShareDenyWrite) Do
   Try

    SetLength(Result, Size);

    Index := 0;

    Repeat

     Count := Read(Result[Index], Min(High(Longint), Length(Result) - Index - 1));
     Inc(Index, Count);

    Until Count < High(Longint);

   Finally

    Free;

   End;

 End;

   var Buffer : TBytes;


    SetLength(Buffer, 3000000000);

    Index := 3000000000 -1;

    Buffer[Index] := 0;

   var B : array [1..3000000000] of byte;

 Index : NativeInt;
  Count : Integer;
  Stream : TFileStream;
  Portion: Integer;
  Current : TBytes;

  var Rest : NativeInt;
 Begin
  Portion := 1000000000;
  Stream:=TFileStream.Create(FileName, fmOpenRead Or fmShareDenyWrite);
   Try

    SetLength(Buffer, Stream.Size);
    Current := Buffer;

    Repeat
    Rest := Length(Buffer) - Index - 1;
    Count := Stream.Read(Current[0], Min(Portion, Rest));
    Inc(NativeUint(Current),Count);
    Inc(Index, Count);

    Until Count < Portion;

   Finally

    Stream.Free;

   End;

PD>    var Buffer : TBytes;

PD>     SetLength(Buffer, 3000000000);

PD>     Index := 3000000000 -1;

PD>     Buffer[Index] := 0;

PD>

program Project1;

{$APPTYPE CONSOLE}

Uses SysUtils;

Const

 Size3Gb = Int64(1024) * Int64(1024) * Int64(1024) * Int64(3);

Var

 Bytes : TBytes;

Begin

 SetLength(Bytes, Size3Gb);

 WriteLn(Length(Bytes));
 Bytes[Length(Bytes) - 1] := 255;
 WriteLn('Last byte has been modified!');
 ReadLn;

End.