The paper outlines the structure of the operating system for the ILLIAC IV, a large array computer being built by the University of Illinois. The system is unique in that it resides primarily on a second control computer and distributes operating system functions among independent programs in a multi-programming, multi-processing mix, where inter-program communication is accomplished by buffer files. The job control language interpreter and hardware interrupt dispatcher are independent modules which may be user defined, invoked dynamically, and co-exist with the standard system modules.