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Device driver function 0Dh      Open Device             b   c

    Open Device informs the device driver that a file device or a
    character device is being opened or created.

    This function can be used for both block and character device
    drivers.

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    DOS calls this function only if bit 11 is set in the dhAttributes
    field of the DeviceHeader structure for the driver.

    DOS calls this function whenever an application opens or creates a
    file or opens a device. This function can be used in conjunction
    with Close Device (device driver function 0Eh) to manage internal
    buffers and device initialization. To manage internal buffers,
    this function, when used in a block device driver, should
    increment the count of open files on the specified drive; Close
    Device decrements this count and flushes internal buffers when all
    files are closed. Keeping this count can also help the driver
    determine whether the medium in the drive has been removed before
    all files have been closed.
    To help manage device initialization, this function, when used
    in a character device driver, can reset the device and send it
    control strings to prepare it for subsequent input. If a character
    device driver offers this feature, it should also provide IOCTL
    Read and IOCTL Write (device driver functions 03h and 0Ch) to let
    programs get and set the current control strings.

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    ocrUnit
    Specifies which device contains the file being opened or created.
    This field is used with block device drivers only.

    
    ocrStatus
    Specifies the status of the completed function. If the function
    was successful, the driver must set the done bit (bit 8).
    Otherwise, the driver must set both the error and done bits (bits
    15 and 8) and copy an error value to the low-order byte.