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  Function 00H (decimal 0) ends a program and passes control back to DOS. It
  is functionally identical to DOS interrupt 20H, discussed on page 299.
  Either can be used interchangeably to exit a program.

  DOS versions 2.0 and later provide an enhanced terminate service through
  function 4CH, which leaves a return code (an error code) in register AL
  when a program ends. DOS batch files can act on the return codes with the
  DOS subcommand ERRORLEVEL. Use function 4CH instead of function 00H if
  you want to use a return code to record errors that occur when a program
  ends. (See page 377.)

  Like DOS interrupt 20H, function 00H does not close files opened with
  functions 0FH or 16H. To ensure that the proper length of such files is
  recorded in the file directory, use function 10H to close them before
  calling function 00H. Also, as with interrupt 20H, you must be sure the
  PSP segment address is in the CS register before exiting.