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  When you call an interrupt 21H function in DOS versions 2.0 and later, the
  function returns an error status code in the AX register. These functions
  also set the carry flag to signal that an error has occurred. You should
  generally follow each call to these interrupt 21H functions with a test of
  the carry flag; if the flag is set, the value in AX describes what caused
  the error.

  In DOS versions 3.0 and later, you can also use interrupt 21H, function
  59H, to obtain extended error information from DOS. You can call function
  59H after any interrupt 21H function reports an error; you can also use it
  inside a critical-error (interrupt 24H) handler to determine the nature of
  a DOS critical error. In both situations, function 59H returns an extended
  error code and also suggests possible actions to alleviate the problem.

  For a complete list of extended error codes and how to use them, see the
  discussion of function 59H on page 381.