Each System z® model has several elements that constitute the hardware control system for the mainframe.

Figure 1, while highly conceptual, shows several of the functions of the internal system controls on current mainframes. The internal controllers are microprocessors but use a much simpler organization and instruction set than zSeries® processors. They are usually known as controllers to avoid confusion with zSeries processors.

Figure 1. System control and partitioning

Among the system control functions is the capability to partition the system into several logical partitions (LPARs). An LPAR is a subset of the processor hardware that is defined to support an operating system. An LPAR contains resources (processors, memory, and input/output devices) and operates as an independent system. Multiple logical partitions can exist within a mainframe hardware system.

For many years there was a limit of 15 LPARs in a mainframe; more recent machines have 30 (and potentially more). Practical limitations of memory size, I/O availability, and available processing power usually limit the number of LPARs to less than these maximums.

The hardware and firmware that provides partitioning is known as PR/SM™ (Processor Resource/System Manager). It is the PR/SM functions that are used to create and run LPARs. This difference between PR/SM (a built-in facility) and LPARs (the result of using PR/SM) is often ignored and the term LPAR is used collectively for the facility and its results.

System administrators assign portions of memory to each LPAR; memory cannot be shared among LPARs. The administrators can assign processors (noted as CPs in Figure 1) to specific LPARs or they can allow the system controllers to dispatch any or all the processors to all the LPARs using an internal load-balancing algorithm. Channels (CHPIDs) can be assigned to specific LPARs or can be shared by multiple LPARs, depending on the nature of the devices on each channel.

A system with a single processor (CP processor) can have multiple LPARs. PR/SM has an internal dispatcher that can allocate a portion of the processor to each LPAR, much as an operating system dispatcher allocates a portion of its processor time to each process, thread, or task.

Partitioning control specifications are partly contained in the IOCDS and are partly contained in a system profile. The IOCDS and profile both reside in the Support Element (SE) which is simply a notebook computer inside the system. The SE can be connected to one or more Hardware Management Consoles (HMCs), which are desktop personal computers used to monitor and control hardware such as the mainframe microprocessors An HMC is more convenient to use than an SE and can control several different mainframes.

Working from an HMC (or from an SE, in unusual circumstances), an operator prepares a mainframe for use by selecting and loading a profile and an IOCDS. These create LPARs and configure the channels with device numbers, LPAR assignments, multiple path information, and so forth. This is known as a Power-on Reset (POR). By loading a different profile and IOCDS, the operator can completely change the number and nature of LPARs and the appearance of the I/O configuration. However, doing this is usually disruptive to any running operating systems and applications and is therefore seldom done without advance planning.