Editing Hypervisor (section)

==Mainframe origins==
The first hypervisors providing [[full virtualization]] were the test tool [[SIMMON]] and the one-off [[IBM CP-40]] research system, which began production use in January 1967 and became the first version of the IBM [[CP/CMS]] operating system. CP-40 ran on a [[System/360|S/360-40]] modified at the  [[Cambridge Scientific Center]] to support [[dynamic address translation]], a feature that enabled virtualization. Prior to this time, computer hardware had only been virtualized to the extent to allow multiple user applications to run concurrently, such as in [[Compatible Time-Sharing System|CTSS]] and [[IBM M44/44X]]. With CP-40, the hardware's ''supervisor state'' was virtualized as well, allowing multiple operating systems to run concurrently in separate [[virtual machine]] contexts.

Programmers soon implemented CP-40 (as [[CP-67]]) for the [[IBM System/360-67]], the first production computer system capable of full virtualization. IBM shipped this machine in 1966; it included [[page translation table|page-translation-table]] hardware for virtual memory and other techniques that allowed a full virtualization of all kernel tasks, including I/O and interrupt handling. (The  "official" operating system, the ill-fated [[TSS/360]], did not employ full virtualization.) Both CP-40 and CP-67 began production use in 1967. [[CP/CMS]] was available to IBM customers from 1968 to early 1970s, in source code form without support.

[[CP/CMS]] formed part of IBM's attempt to build robust [[time-sharing]] systems for its [[Mainframe computer|mainframe]] computers. By running multiple operating systems concurrently, the hypervisor increased system robustness and stability: Even if one operating system crashed, the others would continue working without interruption. Indeed, this even allowed [[Development stage#Beta|beta]] or experimental versions of operating systems{{mdashb}}or even of new hardware<ref>See [[History of CP/CMS]] for virtual-hardware simulation in the development of the [[System/370]]</ref>{{mdashb}}to be deployed and debugged, without jeopardizing the stable main production system, and without requiring costly additional development systems.

IBM announced its [[System/370]] series in 1970 without the [[virtual memory]] feature needed for virtualization, but added it in the August 1972 Advanced Function announcement. Virtualization has been featured in all successor systems, such that all modern-day IBM mainframes, including the [[zSeries]] line, retain backward compatibility with the 1960s-era IBM S/360 line. The 1972 announcement also included [[VM (operating system)|VM/370]], a reimplementation of [[CP/CMS]] for the S/370. Unlike [[CP/CMS]], IBM provided support for this version (though it was still distributed in source code form for several releases). ''VM'' stands for ''[[Virtual Machine]]'', emphasizing that all, not just some, of the hardware interfaces are virtualized. Both VM and CP/CMS enjoyed early acceptance and rapid development by universities, corporate users, and [[time-sharing]] vendors, as well as within IBM. Users played an active role in ongoing development, anticipating trends seen in modern [[Open-source model|open source]] projects. However, in a series of disputed and bitter battles{{Citation needed|date=June 2021}}, time-sharing lost out to [[batch processing]] through IBM political infighting, and VM remained IBM's "other" mainframe operating system for decades, losing to [[MVS]]. It enjoyed a resurgence of popularity and support from 2000 as the [[z/VM]] product, for example as the platform for [[Linux on IBM Z]].

As mentioned above, the VM control program includes a ''hypervisor-call'' handler that intercepts DIAG ("Diagnose", opcode x'83') instructions used within a virtual machine. This provides fast-path non-virtualized execution of file-system access and other operations (DIAG is a model-dependent privileged instruction, not used in normal programming, and thus is not virtualized. It is therefore available for use as a signal to the "host" operating system). When first implemented in [[CP/CMS]] release 3.1, this use of DIAG provided an operating system interface that was analogous to the [[System/360]] [[Supervisor Call instruction]] (SVC), but that did not require altering or extending the system's virtualization of SVC.

In 1985 IBM introduced the [[PR/SM]] hypervisor to manage [[Logical partition (virtual computing platform)|logical partitions]] (LPAR).