gistfile1.pas

program Project1;

{$APPTYPE CONSOLE}

{$R *.res}

uses
  System.Classes,
  System.SysUtils;

const
  CTestFileName = 'g:\test.bin';

procedure AllocBigFile;
begin
  with TFileStream.Create(CTestFileName, fmCreate) do
  try
    Size := 300 * Int64(1024*1024 *1024); // 300 GB
  finally
    Free;
  end;
end;

procedure DoTest;
var
  VBuffer: TBytes;
  VStream: TFileStream;
begin
  VStream := TFileStream.Create(CTestFileName, fmOpenReadWrite);
  try
    VStream.Position := 300 * Int64(1024*1024*1024);

    SetLength(VBuffer, 1);
    VBuffer[0] := 42;

    VStream.WriteBuffer(VBuffer, 0, 1);

    VStream.Position := VStream.Position - 1;

    VBuffer[0] := 0;
    VStream.ReadBuffer(VBuffer[0], 1);

    Assert(VBuffer[0] = 42);
  finally
    VStream.Free;
  end;
end;

begin
  try
    AllocBigFile;
    DoTest;
  except
    on E: Exception do
      Writeln(E.ClassName, ': ', E.Message);
  end;
end.