A semiconductor device that functions as a key to control access to a software program resident in a computer. The device includes a continuously running pulse generator that produces an output representative of real time, a shift register permanently storing a unique number and circuitry for executing...http://www.google.com.tw/patents/US4599489?utm_source=gb-gplus-share專利 US4599489 - Solid state key for controlling access to computer software
