Device driver function 03h      IOCTL Read              b   c

    IOCTL Read transfers data from a device driver into the specified
    buffer.

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

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    DOS calls this function only if bit 14 is set in the dhAttributes
    field of the DeviceHeader structure for the driver.

    The format of the returned data is device-specific and does not
    follow any standard.

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    irwrUnit
    Specifies the device from which data is to be read. This field is
    used for block device drivers only.


    irwrStatus
    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.


    irwrBuffer
    Contains the 32-bit address of the buffer that receives data read
    from the device.


    irwrBytes
    Contains the number of bytes to read and receives the number of
    bytes read.

        input   Specifies the number of bytes to read. This number
                must not exceed the size, in bytes, of the specified
                buffer.

        output  Specifies the number of bytes read. This number cannot
                exceed the requested number of bytes.