The IOCTL read function allows the device driver to pass information directly to the application program. This function is called only if bit 14 is set in the device attribute word. MS-DOS performs no error check on IOCTL I/O calls.
The IOCTL read function is called with
RH + 1 BYTE Unit code (block devices)
RH + 2 BYTE Command code = 3
RH + 13 BYTE Media descriptor byte
RH + 14 DWORD Transfer address
RH + 18 WORD Byte/sector count
RH + 20 WORD Starting sector number (block
devices)
It returns
RH + 3 WORD Status
RH + 18 WORD Actual bytes or sectors transferred