This paper describes the evolution of facilities for virtual machines on IBM System/370 computers, and presents the elements of a new architectural facility designed for the virtual-machine environment. Assists that have been added to various System/370 models to support the use of virtual machines are summarized, and a general facility for this purpose which was introduced with the System/370 Extended Architecture (370-XA) is described. A new instruction of the 370-XA architecture places the machine in a specific mode in which several special capabilities are enabled. These allow the machine to provide execution in the virtual-machine environment of most of the instructions (including many privileged instructions) and most of the facilities (such as dynamic address translation) of both the System/370 and the 370-XA architectures. The major features of this new facility are individually discussed and summarized.
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