IBM System

The IBM System/360 (S/360) was a mainframe computer system family first announced by IBM on April 7, 1964, and sold between 1964 and 1978. It was the first family of computers designed to cover the complete range of applications, from small to large, both commercial and scientific. The design made a clear distinction between architecture and implementation, allowing IBM to release a suite of compatible designs at different prices. All but the most expensive systems used microcode to implement the instruction set, which featured 8-bit byte addressing and binary, decimal and floating-pointcalculations.

The slowest System/360 models announced in 1964 ranged in speed from 0.0018 to 0.034 MIPS the fastest System/360 models were approximately 50 times as fast with 8 kB and up to 8 MB of internal main memory, though the latter was unusual, and up to 8 megabytes of slower Large Core Storage (LCS). A large system might have 256 kB of main storage.

The 360s were extremely successful in the market, allowing customers to purchase a smaller system with the knowledge they would always be able to migrate upward if their needs grew, without reprogramming of application software. The design is considered by many to be one of the most successful computers in history, influencing computer design for years to come.

The chief architect of the S/360 was Gene Amdahl, and the project was managed by Fred Brooks, responsible to Chairman Thomas J. Watson Jr.

Application level compatibility (with some restrictions) for System/360 software is maintained until present day with the IBM zSeries computers.

System/360 history

An IBM System/360 in use at Volkswagen.

A family of computers

Contrasting with at-the-time normal industry practice, IBM created an entire series of computers (or CPUs) from small to large, low to high performance, all using the same instruction set (with two exceptions for specific markets). This feat allowed customers to use a cheaper model and then upgrade to larger systems as their needs increased without the time and expense of rewriting software. IBM was the first manufacturer to exploit microcode technology to implement a compatible range of computers of widely differing performance, although the largest, fastest, models had hard-wired logic instead.

This flexibility greatly lowered barriers to entry. With other vendors (with the notable exception of ICT), customers had to choose between machines they could outgrow and machines that were potentially overpowered (and thus too expensive). This meant that many companies simply did not buy computers.