The Changing Virtual Machine Environment: Interfaces to Real Hardware, Virtual Hardware, and Other Virtual Machines

a survey of changes to virtual machine interfaces, implementation, architecture, and simulation techniques as they affect IBM System1370 and 303X (3031, 3032, 3033) processors, the system control program to which virtual machines interface, and other virtual machines executing on the same real computing system or elsewhere. The paper seeks to summarize such changes and provide a perspective on the virtual machine environment. New uses of virtual machine subsystems are discussed as they relate to inter-virtual-machine communication. When IBM introduced virtual machine products with CP-67 on the System/360 Model 67, an early view of the uniqueness of virtual machines focused on the isolation of one virtual machine from another.lS2 CP-67 was able to provide Systed360 hardware systems with a variety of operating system environments, or virtual machines, all independent of each other. This was accomplished with dynamic address translation hardware and the use of multiple virtual address spaces, hypervision by the control program (cP), and CP'S handling of the real machine. Systed360 split instructions into privileged system control instructions and non-privileged computational instructions, enabling CP to achieve its objectives with virtual machines by exploiting the real hardware whenever possible. CP-67's ability to handle a variety of virtual machine software environments and the separation and isolation of virtual machines made the system attractive to users. Development of the Cam-bridge Monitor System (CMS) provided conversational computing to CP-67, which became known as CP-67/CMS.3 Since 1972, the use of virtual machines has become more widespread with the availability of IBM'S Virtual Machine Facility/37O (VM/370) for many models of Sy~tem/370.~ Virtual machine archi-Copyright 1 9 9 by International Business Machines Corporation. Copying is permitted without payment of royalty provided that (1) each reproduction is done without alteration and (2) the Journal reference and IBM copyright notice are included on the first page. The title and abstract may be used without further permission in computer-based and other information-service systems. Permission to republish other excerpts should be obtained from the Editor.