"Mainframe" is define as a large computer, in particular one to which other computers can be connected so that they can share facilities the mainframe provides (for example, a System/370 computing system to which personal computers are attached so that they can upload and download programs and data). The term usually refers to hardware only, namely, main storage, execution circuitry and peripheral units.

The central component of an IBM mainframe computer system is the processor, just as it is on a PC. IBM introduced its first system processors even earlier, in the 1950’s. However, these processors were based on vacuum tube technology and were typically designed to address a specific set of applications rather than a more general, multipurpose environment.

IBM introduced its first system processors even earlier, in the 1950’s. However, these processors were based on vacuum tube technology and were typically designed to address a specific set of applications rather than a more general, multipurpose environment.

The System/360, or S/360, processors released in 1964 were IBM’s first attempt to standardize mainframe systems. Their design was so forward-looking that many of the architectural principles introduced in these early systems are still used in today’s processors.

The success of the S/360 system gave way to the next big step in mainframe processor development. In 1970, IBM introduced the first of what was to be a long family of System/370, or S/370, processors. Originally, S/370 processors were based on 24-bit technology. That meant that the processor had the capacity to address the highest number of memory locations possible in a 24-bit binary numbering system. Over time the need for greater memory capacity led to the introduction of 31-bit S/370 processors.

In 1990, IBM introduced the System/390 (S/390) family of processors. One of the major benefits S/390 systems had over earlier S/370 systems was the introduction of Enterprise Systems Connection, or ESCON, architecture. ESCON uses fiber optic cables to connect I/O devices to the processor over long distances. Before ESCON, I/O devices were connected to channels using heavy copper cables that could be no longer than 400 feet. ESCON extended that limit to anywhere from 3km to 60km away from the processor unit. In addition, ESCON allows for information to flow at a higher speed between the devices and the channel subsystem on the processor.

Incidentally, IBM has recently started referring to their mainframe computers as mainframe servers, a term that helps associate their mainframe processors with e-business strategy. This book will use the terms server and mainframe computer interchangeably.

Although today’s systems are much more complex and powerful than the systems available only a few years ago, IBM has maintained a high degree of compatibility between all of its processors over the years.