A numerical controller executes a system program. While executing the system program, the numerical controller executes a sub-program. On the basis of the execution of the sub-program, the numerical controller controls position-controlled shafts of a machine tool controlled by the numerical controller. The sub-program contains instruction sets which are retrieved sequentially one after the other by the numerical controller. The numerical controller only executes the retrieved instruction sets when the instruction sets comply with permitted boundary conditions. Otherwise, the instruction sets are not executed. Before executing the sub-program and while executing the system program, the numerical controller receives information defining the permitted boundary conditions via an interface protected from unauthorized access.