Abstract: An output bit sequences is derived from an initial bit sequence and this output bit sequence is used to encrypt an input bit sequence in a first mode of operation or not to so encrypt the input bit sequence in a second mode of operation. The mode of operation is switched automatically whenever the output bit sequence contains a predetermined trap bit sequence. As a result of this automatic switching between such encryption and no such encryption, unauthorized determination of secret codes is thwarted.

Abstract: A means for preserve cache consistency is provided for a system comprising a central processing unit, a first physical memory, a second physical memory for which the address is common to the first physical memory in at least some duplicated address range of the entire physical address, a cache memory, and a memory controller, wherein the first or second physical memory is selected depending on the operation mode. Flag bits are provided in a tag memory of the cache for information identifying the data source. The cache then does not determine a cache hit/miss only based on whether data related to a CPU requested address exists in the cache, but determines whether the source of data requested by the CPU is consistent with the source of data stored in the cache by taking into account information on the operation mode simultaneously sent from the CPU. A cache hit is determined only when such two conditions are met.

Abstract: The computer system comprises a processor 1, a cache memory 2 of this processor, a main memory 4, an I/O device 6 directly accessible to this main memory, an I/O controller 7, and the like. It is made so that the I/O controller can execute processing to retain data consistency between the cache memory and the main memory during access to the main memory by the I/O device. The I/O controller has an address buffer 9 that retains the cache line address of the previous access by the I/O device and it is made so that the consistency maintenance operation is not executed in cases other than the first access when accesses to the main memory by the I/O device are made continuously in the same data block.