Device driver function 18h      Set Logical Device      b

    Set Logical Device sets the active drive number to the drive
    specified by the ldrUnit field.

    This function is used for block device drivers only.

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    DOS calls this function only if bit 6 is set in the dhAttributes
    field of the DeviceHeader structure for the driver.

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    ldrUnit
    Specifies the device to make active. This field contains a zero-
    based drive number.


    ldrStatus
    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.