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The pages on this site contain documentation for very old MS-DOS software, purely for historical purposes. If you're looking for up-to-date documentation, particularly for programming, you should not rely on the information found here, as it will be woefully out of date.

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▀ When supported by the driver, this subfunction can be used to obtain
  hardware-dependent status and availability information that is not pro-
  vided by other MS-DOS function calls.
 
▀ Block device drivers are not required to support IOCTL Subfunction 04H.
  If this subfunction is requested and the driver does not have the abil-
  ity to process control data, control returns to the program with the
  carry flag set and error code 0001H (invalid function) in register AX.
                                    -♦-