In the early S/360™ days a system had a single processor, which was also known as the central processing unit (CPU). The terms system, processor, and CPU were used interchangeably. However, these terms became confusing when systems became available with more than one processor.

This overlap is shown in Figure 1.

Figure 1. Terminology overlap

Processor and CPU can refer to either the complete system box, or to one of the processors (CPUs) within the system box. Although the meaning may be clear from the context of a discussion, even mainframe professionals must clarify which processor or CPU meaning they are using in a discussion. IBM® uses the term central processor complex (CPC) to refer to the physical collection of hardware that includes main storage, one or more central processors, timers, and channels. (Some system programmers use the term central electronic complex (CEC) to refer to the mainframe "box," but the preferred term is CPC.)

Briefly, all the S/390® or z/Architecture® processors within a CPC are processing units (PUs). When IBM delivers the CPC, the PUs are characterized as CPs (for normal work), Integrated Facility for Linux® (IFL), Integrated Coupling Facility (ICF) for Parallel Sysplex® configurations, and so forth.

Mainframe professionals typically use system to indicate the hardware box, a complete hardware environment (with I/O devices), or an operating environment (with software), depending on the context. They typically use processor to mean a single processor (CP) within the CPC.