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Function 4404h (68-4)    IOCTL: Read from Block Device

    Reads a control string from a block device.

       On entry:      AH         44h
                      AL         04h
                      BL         Drive number (0=default, 1=A, etc.)
                      CX         Number of bytes to read
                      DS:DX      Data or buffer

       Returns:       AX         Number of bytes read
                                 Error code, if CF is set

       Error codes:   1          Device doesn't support control strings
                      5          Drive is invalid

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    This subfunction reads CX bytes of data from the block device's
    control channel into either DS:DX or the memory pointed to by DS:DX.
    How data is written is defined by the device driver.  Data can be read
    only if the device can process control strings.