Abstract: A tampering-protected microprocessor system includes a microprocessor, an internal write/read memory integrated with the microprocessor into a common module, and a second memory in which at least a portion of an operating program to be executed by the microprocessor is stored. At least one procedure of the operating program which is indispensable for the function of the microprocessor system is stored in encrypted form in the external memory. The operating program includes a decryption procedure which decrypts each encrypted procedure of the operating program and stores it in the internal write/read memory. The indispensable procedure, in order to function properly, requires a successful execution of an integrity test procedure which tests the integrity of at least a portion of the operating program.