uses crt, dos;
var 
  f:file;
  x:array[0..40] of byte;
begin
  assign(f, 'testfile.dat');
  rewrite(f, 1);
  blockwrite(f, x, sizeof(x));
  close(f);
  reset(f, 1);
  readkey;
  seek(f, 1023);
  blockwrite(f, x, 1);
  close(f);
end.