1. Apparatus for protecting a program stored on a storage medium such as a magnetic disc, for enabling trial use of said program in connection with a computer processor, said apparatus comprising: first means for recording a predetermined indication on said medium indicative of the number of times said program can be used, second means for recording a predetermined code on said medium associated with said predetermined indication, means for accessing said indication when said medium is being used by said computer processor, means for modifying said indication on said medium during use to reduce the number of times represented that said program can be used, further means responsive to said indication to detect a state indicating a specified number of uses indicative of an end of a period of said trial use to provide an output signal indicative of said end of said period of trial use, means responsive to said output signal for preventing further use of said program stored on said medium by said computer processor, logic means responsive to said output signal and said predetermined code to permit said computer processor to receive an enable code from an input means and means for comparing said enable code with said predetermined code to allow said program to be further used without further interruption upon a favorable comparison.