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documentation, particularly for programming, you should not rely on the information found here, as it will be woefully out of date.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