memory protection

the hardware and software methods for the prevention of the writing or reading of information at an unauthorized address in the memory of a computer or computing system.

Memory protection is based on the division of the memory of a digital computer, either by software or by hardware, into a sequence of segments and the providing of each segment or group of segments with a code key, which is stored in the same or in a special memory. During a memory cycle the segment and the corresponding key are determined; then the key is compared with the authorized key of the memory protection, which is specified in the same instruction or in a supervisory program. A discrepancy in the keys is regarded as a memory protection violation, and the execution of the program is interrupted. The interruption of the program is organized so that the protected region of memory is left un-changed. Memory protection functions during every memory cycle, either in the cycle for writing information or in the information reading cycle, or in both.

Memory protection preserves the contents of specified regions of memory from loss of information during program execution as a result of erroneous transmission of information caused by failures or breakdowns in the equipment or in the supervisory program of the digital computer, or those caused by mistakes of the programmer or user; it also prevents information from falling into the hands of an unauthorized user through accidental or intentional interference.

Hardware methods of memory protection include storage devices for the protection keys; the capacity of these devices corresponds to the number of protected segments, and the speed is greater by a factor of 10 than that of the digital computer’s working memory. In addition, there are circuits for the comparison of the protection keys and for the interruption and indication of memory protection violations. Software means of memory protection include programs for monitoring the memory segments, encoding them, and compiling tables of correspondence; programs for the dynamic redistribution of memory protection according to user’s instructions and according to the parameters of the simultaneously solved tasks; and programs for the analysis of the causes of memory protection violations and the adoption of solutions for their elimination.

Memory protection increases the efficiency of digital computer operation, reducing time losses caused by the search for errors and repeated computation resulting from loss of information. Memory protection is necessary for the simultaneous solution of several problems on one digital computer in a time-sharing system, for the simultaneous service of several users, for the use of program libraries and archives belonging to several users, and for the simultaneous operation of several on-line units.