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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documentation, particularly for programming, you should not rely on the information found here, as it will be woefully out of date.

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▀ If supported by the driver, this subfunction can be used to request
  hardware-dependent operations (such as setting baud rate for a serial
  port) that are not supported by other MS-DOS function calls.
 
▀ Character device drivers are not required to support IOCTL Subfunction
  03H. A program can test bit 14 of the device information word returned
  by IOCTL Subfunction 00H to determine whether the driver supports this
  subfunction. If Subfunction 03H is requested and the driver does not
  have the ability to process control data, control returns to the program
  with the carry flag set and error code 0001H (invalid function) in
  register AX.
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