Partition (mainframe)

A logical partition, commonly called an LPAR, is a subset of a computer's hardware resources, virtualized as a separate computer. In effect, a physical machine can be partitioned into multiple logical partitions, each hosting a separate operating system.

IBM developed the concept of hypervisors (virtual machines in CP-40 and CP-67) and in 1972 provided it for the S/370 as Virtual Machine Facility/370. IBM introduced the Start Interpretive Execution (SIE) instruction (designed specifically for the execution of virtual machines) as part of 370-XA architecture on the 3081, as well as VM/XA versions of VM to exploit it. PR/SM is a type-1 Hypervisor based on the CP component of VM/XA that runs directly on the machine level and allocates system resources across LPARs to share physical resources. It is a standard feature on IBM System z only. An IBM POWER system uses PHYP (the POWER Hypervisor) to enable its LPAR functionalities for System p and System i since approximately 2000 in POWER4 systems.

The terms PR/SM and LPAR are often used interchangeably in IBM System z, including in IBM documentation. Formally, LPAR designates the logical partitioning function and mode of operation, whereas PR/SM is the commercial designation of the feature.

Amdahl's MDF (multiple domain facility) was introduced in 1984. IBM began marketing its functionally similar PR/SM in 1988, implemented on its ESA/390 architecture released that year. MDF-based LPAR technology continued to be developed separately by Amdahl, and Hitachi Data Systems in part for their implementations of the new architecture, which featured the introduction of access registers that allowed use of multiple data spaces addressable by a single address space. IBM subsequently continued its LPAR development with its 64-bit System z and System i architectures. LPAR and PR/SM reconfigurations can be made without rebooting the computer, i.e., while some LPARs remain active. Reconfigurations can include changing channel path definitions and device definitions.

...
Wikipedia