Assembly Language Help (alang.hlp) (Table of Contents; Topic list)
Int 21h Function 44h Subfunction 04h
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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 provided 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 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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