It is possible to purchase a current mainframe with a single processor (CP), but this is not a typical system. The term multiprocessor means several processors (CP processors) and implies that several processors are used by a copy of z/OS®.

All operating systems today, from PCs to mainframes, can work in a multiprocessor environment. However, the degree of integration of the multiple processors varies considerably. For example, pending interrupts in a system (or in an LPAR) can be accepted by any processor in the system (or working in the LPAR). Any processor can initiate and manage I/O operations to any channel or device available to the system or LPAR. Channels, I/O devices, interrupts, and memory are owned by the system (or by the LPAR) and not by any specific processor.

This multiprocessor integration appears simple on the surface, but its implementation is complex. It is also important for maximum performance; the ability of any processor to accept any interrupt sent to the system (or to the LPAR) is especially important.

Each processor in a system (or in an LPAR) has a small private area of memory (8 KB starting at real address 0 and always mapped to virtual address 0) that is unique to that processor. This area is the Prefix Storage Area (PSA) and is used for interrupt handling and for error handling. A processor can access another processor's PSA through special programming, although this is normally done only for error recovery purposes. A processor can interrupt other processors by using a special instruction (SIGP, for Signal Processor). Again, this is typically used only for error recovery.