Mainframes

Mainframe computers (colloquially referred to as "big iron") are computers used primarily by large organizations for critical applications; bulk data processing, such as census, industry and consumer statistics, enterprise resource planning; and transaction processing. They are larger and have more processing power than some other classes of computers: minicomputers, servers, workstations, and personal computers.

The term originally referred to the large cabinets called "main frames" that housed the central processing unit and main memory of early computers. Later, the term was used to distinguish high-end commercial machines from less powerful units. Most large-scale computer system architectures were established in the 1960s, but continue to evolve. Mainframe computers are often used as servers.

Modern mainframe design is characterized less by raw single-task computational speed (typically defined as MIPS (millions of instructions per second) rate or FLOPS (floating-point operations per second)), and more by:

Their high stability and reliability enable these machines to run uninterrupted for decades.

Mainframes have high availability, one of the primary reasons for their longevity, since they are typically used in applications where downtime would be costly or catastrophic. The term reliability, availability and serviceability (RAS) is a defining characteristic of mainframe computers. Proper planning and implementation is required to realize these features. In addition, mainframes are more secure than other computer types: the NIST vulnerabilities database, US-CERT, rates traditional mainframes such as IBM zSeries, Unisys Dorado and Unisys Libra as among the most secure with vulnerabilities in the low single digits as compared with thousands for Windows, UNIX, and Linux. Software upgrades usually require setting up the operating system or portions thereof, and are non-disruptive only when using virtualizing facilities such as IBM z/OS and Parallel Sysplex, or Unisys XPCL, which support workload sharing so that one system can take over another's application while it is being refreshed.

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