An apparatus for protecting BIOS, including a BIOS ROM, a detector, a generator, JTAG control, a machine specific register, and a controller. The BIOS ROM stores plaintext and an encrypted digest that is an encrypted version of a first digest corresponding to BIOS contents. The detector generates an interrupt at a combination of prescribed intervals and event occurrences, accesses the BIOS contents and the encrypted digest upon the interrupt, and directs a microprocessor to generate a second digest of the BIOS contents and a decrypted digest corresponding to the encrypted digest, compares the second digest with the decrypted digest, and precludes operation of the microprocessor when the second digest and decrypted digest are unequal. A random number is generated at completion of a current BIOS check, and sets a following prescribed interval. JTAG control programs the combination of prescribed intervals and event occurrences.