A digital to analog converter is provided. The wafer polishing apparatus comprises a head body and a carrier which are connected with each other by a connecting bar which is provided with a strain gage, and determines strain in a horizontal direction of the connecting bar by the strain gage so as to determine a rotation torque of the carrier, whereby accurately detects the polishing end. If the file backup software detects a modification to a file, the file backup software determines whether a file modification policy associated with the file has been violated. The torsional region is defined by first and second torsional members initially formed in a relaxed state. A passing module is included to pass the dynamic service call to a predefined SCA implementation handler associated with the new implementation type. The message storage system accepts to perform the operations on the message during transmission only after expiry of a minimum delay as from the beginning of the transmission of the message to the message restoring device.