MAINFRAME COMPUTER

Tt refers to a class of ultra-reliable medium and large-scale servers designed for enterprise-class and carrier-class operations.

The first mainframe vendors were Burroughs, Control Data, GE, Honeywell, IBM, NCR, RCA and Univac, otherwise known as "IBM and the Seven Dwarfs." After GE and RCA's computer divisions were absorbed by Honeywell and Univac respectively, the mainframers were known as "IBM and the BUNCH."

The Vendors

For decades, IBM has been the dominant vendor in the mainframe business. Although many tried to compete by offering IBM-compatible mainframes, only Amdahl (Fujitsu) has remained as a competitor in this arena (see IBM-compatible mainframe). Unisys, Sun and others also make mainframe-class machines that typically run under a version of Unix or Linux.

There Is a Difference

One might wonder why mainframes cost a million dollars or more when the raw gigahertz (GHz) rating of their CPUs is not any higher than a PC costing 1,000 times less. Quite often in fact, the ratings are lower. Here are the reasons.

Lots of Processors, Memory and Channels

Mainframes support symmetric multiprocessing (SMP) with several dozen central processors in one system. They are highly scalable. CPUs can be added to a system, and systems can be added in clusters. Built with multiple ports into high-speed caches and main memory, a mainframe can address thousands of gigabytes of RAM. They connect to high-speed disk subsystems that can hold terabytes of data.

Enormous Throughput

A mainframe provides exceptional throughput by offloading its input/output processing to a peripheral channel, which is a computer itself. Mainframes can support hundreds of channels, and additional processors may act as I/O traffic cops that handle exceptions (channel busy, channel failure, etc.).

All these subsystems handle the transaction overhead, freeing the CPU to do real "data processing" such as computing balances in customer records and subtracting amounts from inventories, the purpose of the computer in the first place.

Super Reliable

Mainframe operating systems are generally rock solid because a lot of circuitry is designed to detect and correct errors. Every subsystem may be conPublish Posttinuously monitored for potential failure, in some cases even triggering a list of parts to be replaced at the next scheduled maintenance. As a result, mainframes are incredibly reliable with mean time between failure (MTBF) up to 20 years!