INT 21 - DOS 2+ - IOCTL - READ FROM BLOCK DEVICE CONTROL CHANNEL
        AX = 4404h
        BL = drive number (00h = default, 01h = A:, etc.)
        CX = number of bytes to read
        DS:DX -> buffer
Return: CF clear if successful
            AX = number of bytes actually read
        CF set on error
            AX = error code (01h,05h,06h,0Dh) (see AH=59h)
Note:   format of data is driver-specific
SeeAlso: AX=4402h,AX=4405h,INT 2F/AX=122Bh

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