IBM mainframes, though perceived as synonymous with mainframe computers in general due to their marketshare, are now technically and specifically IBM's line of business computers that can all trace their design evolution to the IBM System/360.
IBM initially sold its computers without any software, expecting customers to write their own; programs were manually initiated, one at a time. Later, IBM provided compilers for the newly developed higher-level programming languages Fortran and COBOL. The first operating systems for IBM computers were written by IBM customers who did not wish to have their very expensive machines ($2M USD in the mid-1950s) sitting idle while operators set up jobs manually, and so they wanted a mechanism for maintaining a queue of jobs. It is generally thought that the first operating system used for real work was GM-NAA I/O, produced by General Motors' Research division in 1956. IBM enhanced one of GM-NAA I/O's successors and provided it to customers under the name IBSYS. 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.
The second generation (transistor-based) products were a mainstay of IBM's business and IBM continued to make them for several years after the introduction of the System/360. (Some IBM 7094s remained in service into the 1980s.)
IBM had difficulty getting customers to upgrade from the smaller machines to the mainframes because so much software had to be rewritten. The 7010 was introduced in 1962 as a mainframe-sized 1410. The later Systems 360 and 370 could emulate the 1400 machines. A desk size machine with a different instruction set, the IBM 1130, was released concurrent with the System/360 to address the 1620s niche. It used the same EBCDIC character encoding as the 360 and was mostly programmed in Fortran, which was relatively easy to adapt to larger machines when necessary.
Midrange computer is a designation used by IBM for a class of computer systems which fall in between mainframes and microcomputers.
All that changed with the announcement of the System/360 (S/360) in April, 1964. The System/360 was a single series of compatible models for both commercial and scientific use. The number "360" suggested a "360 degree," or "all-around" computer system. System/360 incorporated features which had previously been present on only either the commercial line (such as decimal arithmetic and byte addressing) or the technical line (such as floating point arithmetic). The System/360 was also the first computer in wide use to include dedicated hardware provisions for the use of operating systems. Among these were supervisor and application mode programs and instructions, as well as built-in memory protection facilities. The new machine also had a larger address space than the older mainframes, 24 bits vs. a typical 18 bits.
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 second generation to the new line, IBM used the 360's microprogramming capability to emulate the more popular older models. Thus 360/30s with this added cost feature could run 1401 programs and the larger 360/65s could run 7094 programs. To run old programs, the 360 had to be halted and restarted in emulation mode. 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.
Operating systems for the System/360 family and its successors included OS/360 (with PCP, MFT, and MVT), BOS, TOS, DOS, and SVS. The original OS/360 and early MVS and VM/CMS versions did not include a copyright literal in the object code and therefore not protected by U.S. Copyright Laws and are available for free use.
There are other supporting processors typically installed inside mainframes such as cryptographic accelerators (CryptoExpress), the OSA-Express networking processor, and FICON Express disk I/O processors.