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  DOS executes interrupt 28H (decimal 40) within interrupt 21H services that
  loop while waiting for an expected event, such as a keystroke. For
  example, if you execute the DOS keyboard-input service (interrupt 21H,
  service 01H), DOS executes interrupt 28H within an idle loop that waits
  for the next keystroke.

  The default DOS handler for interrupt 28H is merely an IRET instruction;
  that is, executing interrupt 28H normally does nothing at all. You can
  substitute your own interrupt 28H handler, however, that does something
  useful while DOS is otherwise idle. In particular, a memory-resident
  program can contain an interrupt 28H handler that is executed repeatedly
  whenever DOS is waiting for keyboard output.

  The biggest problem with installing your own interrupt 28H handler is that
  the handler can execute interrupt 21H to access DOS services only under
  very specific circumstances. Unfortunately, you must know many details
  about the way DOS internally processes interrupt 21H requests in order to
  use these safely within an interrupt 28H handler.