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  Function 4CH (decimal 76) ends a program and passes back the return code
  you specify in AL. If the program was invoked as a child program, the
  parent program can retrieve the return code through function 4DH. If the
  program was invoked as a DOS command, then the return code can be tested
  in a batch file using the DOS ERRORLEVEL option.

  When this function is performed, DOS does some cleanup work in case your
  program neglected to do so: It restores the interrupt 22H, 23H, and 24H
  vectors to default values, flushes the file buffers and closes all open
  files, and frees all memory allocated to the program.

  Because function 4CH is more flexible and easier to use than interrupt 20H
  or interrupt 21H, function 00H, you should normally use function 4CH to
  terminate your programs. The only exception to this rule is if you need to
  maintain compatibility with DOS version 1, which does not support function
  4CH. In that case, you should use either interrupt 20H or function 00H
  of interrupt 21H.