Abstract: The invention is directed to a method for operating a computer system, as well as to a byte code verifier and to a computer system. The inventive method checks whether a computer program loaded onto a computer system exercises an illegal access to a variable, i.e. whether the variable is initialized before it is read. This test ensues before the execution of the program, so that such a test no longer need be performed upon execution of the program. The inventive method requires little memory space in the testing of the program and nonetheless carries out a complete test. Since the testing ensues before the execution of the program, the program execution itself is considerably speeded up since no further test is thereby required.