The original S/360™ architecture— based on CPUs, memory, channels, control units, and devices, and they way these are addressed— is fundamental to understanding mainframe hardware, because the concepts and terminology of the original design still permeate today's mainframe descriptions and designs.

The ability to partition a large system into multiple smaller systems, called logical partitions or LPARs, is now a core requirement in practically all mainframe installations. The flexibility of the hardware design, allowing any processor to access and accept interrupts for any channel, control unit, and device connected to a given LPAR, contributes to the flexibility, reliability, and performance of the complete system.

Unique to mainframes is the availability of a pool of processors that can be configured, or characterized, to do specific work. Having this processor capability provides great flexibility in meeting customer requirements, some of which are based on the cost structures of some mainframe software.

In addition to this pool of primary processors, mainframes have a network of controllers (special microprocessors) that control the system as a whole. These controllers are not visible to the operating system or application programs.