Location dependence for assuring the security of system-control

Abstract

A computer operating system security mechanism employing a non-penetrable hardware means for storing privileged instruction identification data for preventing direct unauthorized access to certain privileged instructions forming a part of the computer operating system; and further including memory address data boundary storage means for each said privileged instruction for preventing indirect access to such privileged instructions in memory by unauthorized branch instructions. The mechanism includes means associated with the computing system instruction register for recognizing privileged instruction and branch instructions. Storage means are included for each privileged instruction it is desired to protect for storing the address of the privileged instruction itself in memory, and both the upper and lower boundary address in memory relating to each such privileged instruction. Whenever a privileged instruction is recognized its address as specified in the running program is compared with the present security mechanism address for said privileged instruction and if there is 'no match' a security violation signal and interrupt will be produced. Similarly when any branch instruction is recognized, its address is checked with all privileged instruction boundaries and if found to lie within any stored boundary a security violation and interrupt will similarly be signaled.