Architecture
VP/CSS shared the basic architecture and concepts of CP/CMS, which were revolutionary for their time. A control program (called CP in CP/CMS, VP in VP/CSS) created multiple independent virtual machines (VMs), implementing a full virtualization of the underlying hardware – meaning that each time-sharing user was provided with a private virtual machine. Each appeared to be an entire, stand-alone computer, capable of running any software that could run on the bare machine, including other operating systems. (This concept was pioneered with IBM's research system CP-40 in the first version of CP/CMS.)
This design was a departure from IBM's other monolithic operating systems. Isolating users from each other improved system stability: a bug in one user's software could not crash another user's virtual machine, nor the underlying control program. This approach made CP/CMS a superior choice for commercial time-sharing, and thus a strong foundation for National CSS as it broke new ground in what would become an important new industry.
Each VM created by VP ran a simple, single-user operating system called CSS, derived from IBM's Cambridge Monitor System. CSS allowed users to run programs, manipulate a file system, and manage virtual devices.
Since VP and CSS began life as CP and CMS, respectively, they closely resembled these systems, particularly in their early days. Continuous development and introduction of new features in both VP/CSS and IBM's VM led to significant differences over time; but the systems retained an obvious family resemblance.
IBM's decision to add virtualization and virtual memory features to the S/370 reflects the success of the virtual machine approach to time-sharing. Some credit for this belated decision has been attributed to IBM's awareness of the commercial success of National CSS.
Read more about this topic: VP/CSS
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