Abstract: Microcontrollers generally comprise, in addition to the actual processor, a program memory which is constructed as a read-only memory. In order to enable testing of the contents of the program memory without making these contents also available to the environment of the microcontroller, the reference data of the program are externally supplied and the comparison is performed inside the microcontroller. In accordance with the invention this test is preferably performed as a small test program which is preferably stored in an additional read-only memory in the microcontroller which operates in the test mode and which performs this test. Consequently, except for the memory for the test program, this test program requires hardly any additional hardware.