up receiving the calls directed towards the device drivers 108 e g ulds such as sd sr st sg by swapping the detect attach finish and detach routines of the uld with its own detect attach finish and detect routines so the filter driver 114 is called instead of device drivers 108 for u201c claiming u201d a device 106 which in a preferred embodiment may be a logical unit represented by a lun the mid level scsi 102 calls the device drivers 39 108 ulds attach routine through the scsi_device_template structure instead of the ulds attach routine being called the filter driver 39 s 114 attach routines gets called as discussed above the filter driver 114 maintains a local copy of each sdp that represents or pertains to a newly discovered scsi device 106 each newly discovered scsi device 106 may be embodied as a lun of a logical unit for example the sdp is passed as a parameter to the attach routine to create a uid the filter driver 114 takes each sdp which refers to a lun logical unit by the logical unit 39 s h b t l address and calculates a uid all calculated uids are stored in uid db 116 this is done so the filter driver 114 knows which h b t l addresses point.