An apparatus including a ROM, an event detector, and a tamper detector. The ROM has BIOS contents stored as plaintext, and an encrypted digest. The encrypted digest is an encrypted version of a first digest corresponding to the BIOS contents. The event detector generates an interrupt that interrupts operation of the system upon occurrence of an event. The tamper detector is operatively coupled to the ROM and accesses the BIOS contents and the encrypted digest upon assertion of the interrupt, and directs a microprocessor to generate a second digest corresponding to the BIOS contents and a decrypted digest corresponding to the encrypted digest using the same algorithms and key that were employed to generate the first digest and the encrypted digest, and compares the second message digest with the decrypted message digest, and precludes the operation of the microprocessor if the second digest and the decrypted digest are not equal.