var
  f: hFile;
  st: _OfStruct;
  count: Dword;
  buf: array[0..255] of char;

begin
  fillchar(st,sizeof(st),0);
  f := OpenFile('c:\1.txt',st,OF_READ);
    repeat
      fillchar(buf,sizeof(buf),0);
      ReadFile(f,buf,sizeof(buf),count,nil);
      {...операции с блоком данных buf...}
    until
      count = 0;
  CloseHandle(f);