System Architecture and Concepts
GCOS is a multithreading, multiprogramming operating system originally oriented towards batch processing, although later versions incorporated enhancements for timesharing and online transaction processing environments. Systems running GCOS today use it mainly for batch and OLTP, or as a backend enterprise server.
Although GCOS has a basic architecture similar to that of the IBM 360 and subsequent operating systems with which it competed, it was also heavily influenced by projects such as MEDINET, Multics, and WWMCCS, and has inherited a strong security structure in consequence. Hardware and software features combine to render the operating system unusually secure for an operating system of its generation and class. Multics influenced the design of the hardware, with gate-oriented secure transfer-of-control instructions and a hardware-enforced system of security levels very similar to that of the famous Multics rings. Operational environments such as WWMCCS drove development of special security features to allow secure hosting of classified information and compartmentalization. For a time separate versions of the GCOS system with special security features turned on were maintained specifically for government customers.
GCOS is a process-oriented OS, in which each process hosts one or more execution threads and executes in its own virtual memory space. Virtual memory is divided into segments of arbitrary size reminiscent of Multics segments, and a second level of address translation converts pure virtual addresses to pageable addresses, which are then converted to real addresses in main memory or backing store. Segments and pages and other constructs include hardware-enforced security parameters. The top-level virtual memory architecture also simplifies sharing of code and data in a secure fashion, again in a way reminiscent of Multics.
GCOS requires specific hardware designed for the operating system, although the most recent machines capable of running the OS do so through emulation. The hardware originally had much in common with Multics hardware, so much so that some mainframe equipment could be switched from "GCOS mode" to "Multics mode" with the turn of a dial. Much of the peripheral equipment used with GCOS shared a great deal with Multics, although front-end network processors were very different between the two systems.
Read more about this topic: General Comprehensive Operating System
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