IBM mainframe

Later, IBM provided compilers for the newly developed higher-level programming languages Fortran, COMTRAN and later COBOL.

IBM enhanced one of GM-NAA I/O's successors, the SHARE Operating System, and provided it to customers under the name IBSYS.

[1][2] As software became more complex and important, the cost of supporting it on so many different designs became burdensome, and this was one of the factors which led IBM to develop System/360 and its operating systems.

Prior to System/360, IBM also sold computers smaller in scale that were not considered mainframes, though they were still bulky and expensive by modern standards.

These included: IBM had difficulty getting customers to upgrade from the smaller machines to the mainframes because so much software had to be rewritten.

A desk-size machine with a different instruction set, the IBM 1130, was released concurrently with the System/360 to address the niche occupied by the 1620.

The smaller models in the System/360 line (e.g. the 360/30) were intended to replace the 1400 series while providing an easier upgrade path to the larger 360s.

To smooth the transition from the second generation to the new line, IBM used the 360's microprogramming capability to emulate the more popular older models.

Many customers kept using their old software and one of the features of the later System/370 was the ability to switch to emulation mode and back under operating system control.

The different processors on current IBM mainframes are: These are essentially identical, but distinguished for software cost control: all but CP are slightly restricted[a] such they cannot be used to run arbitrary operating systems, and thus do not count in software licensing costs (which are typically based on the number of CPs).

IBM offers an emulator called zPDT (System z Personal Development Tool) which runs on Linux on x86-64 machines.