IBM mainframe

IBM
Industry	Computer hardware, mainframe computers
Founded	June 16, 1911; 115 years ago in Endicott, New York, United States
Headquarters	Armonk, United States
Products	Mainframe computers (IBM 700/7000 series, System/360, System/370, IBM Z series)
Website	www.ibm.com/mainframe

IBM mainframes refer to the extensive computer systems developed by IBM starting in 1952. Throughout the 1960s and 1970s, IBM held a commanding position in the computing industry with its 7000 series, followed by the introduction of the System/360 and System/370. The modern mainframe computer models in IBM's portfolio are evolutions based on the original System/360 architecture.

First and second generation

Between 1952 and the late 1960s, IBM released several large-scale computers collectively known as the IBM 700/7000 series. The initial generation, the 700 series, utilized vacuum tube technology, whereas the subsequent 7000 series adopted transistor components. These systems cemented IBM's leadership in electronic data processing ("EDP"). IBM categorized these machines into two groups: one designed for engineering and scientific applications (models such as 701, 704, 709, 7030, 7090, 7094, 7040, 7044), and another aimed at commercial data processing tasks (including models like 702, 705, 705-II, 705-III, 7080, 7070, 7072, 7074, and 7010). Although many peripherals were interchangeable between these categories, their instruction set architectures differed significantly and were often incompatible even within each group.

In IBM's early sales approach, computers were offered without accompanying software, assuming that clients would develop their own programs; job execution was manually controlled. Eventually, IBM began supplying compilers for higher-level programming languages such as Fortran, COMTRAN, and COBOL. Initial operating systems emerged from user organizations aiming to minimize idle time during manual job setups. The first broadly adopted system was GM-NAA I/O, created by General Motors in 1956. IBM then advanced this with the SHARE Operating System, which it marketed as IBSYS. As software grew more complex and hardware diversity increased costs, IBM moved toward consolidating its product line with the unified System/360 platform and its associated operating systems.

The transistor-based second generation remained integral to IBM's lineup well beyond the debut of System/360; certain IBM 7094 units continued operation into the early 1980s.

Smaller machines

Prior to System/360's release, IBM manufactured smaller-scale computers that did not classify as mainframes but were still substantial and expensive by modern standards. These included:

IBM 650 (using vacuum tube logic with a decimal architecture and drum memory, serving both business and scientific purposes)
IBM 305 RAMAC (vacuum tube-based system notable for being the first to incorporate disk storage)
IBM 1400 series (focused on business data processing; highly popular with peripherals that were later integrated into System/360)
IBM 1620 (employed decimal architecture for engineering, scientific research, and educational use)

IBM encountered difficulties persuading customers to transition from these smaller systems to mainframes due to the significant effort required for software redevelopment. The model 7010 introduced in 1962 was essentially a mainframe-sized version of the 1410. Subsequent Systems/360 and /370 offered emulation capabilities for the IBM 1400 series. Alongside System/360’s launch, the compact desk-oriented IBM 1130 targeted users of the 1620 by sharing the same EBCDIC character encoding and primarily utilizing Fortran, facilitating smoother upgrades to larger systems.

Following System/360's introduction, IBM expanded its range of smaller systems:

IBM System/7 (featuring semiconductor memory for process control applications; designed as a successor to the IBM 1800)
IBM Series/1
IBM 3790
IBM 8100
IBM System/3 (which introduced support for 96-column punched cards)

The designation Midrange computer applies to IBM products positioned between traditional mainframes and microcomputer categories.

IBM System/360

Introduced in April 1964, IBM's System/360 (S/360) represented a unified mainframe series tailored for both business and scientific uses. The "360" denoted a complete, all-encompassing system. It integrated capabilities that had previously been separated between commercial systems—such as decimal arithmetic and byte addressing—and scientific machines, which featured floating-point arithmetic available as optional components on certain models. The design ensured compatibility both upwards and largely downwards across different models within the series.

System/360 was one of the earliest widely adopted computers to incorporate hardware mechanisms supporting operating systems, including supervisor and user application modes along with memory protection features. These protections prevented user programs from interfering with the OS and isolated different tasks from each other. Its addressing scheme extended to 24-bit addresses of 8-bit bytes, an improvement over earlier mainframes that typically used 18-bit addressing for 36-bit words.

Smaller S/360 models, such as the 360/30, were intended as replacements for the IBM 1400 series while facilitating seamless upgrades to larger systems. IBM employed microprogramming techniques to enable backward compatibility: the 360/30 could execute programs written for the 1401, and more powerful models like the 360/65 were capable of emulating the 7094. However, emulation required halting and restarting the machine; subsequent System/370 models introduced the ability to switch between native and emulation modes dynamically under operating system control.

The operating system lineup for System/360 included OS/360 with its variants PCP, MFT, and MVT, alongside BOS/360, TOS/360, and DOS/360.

The System/360 architecture served as a foundation for later generations such as the System/370, System/390, and the 64-bit zSeries (System z) platforms. With System/370, virtual memory became available on most models; OS versions like OS/VS1, OS/VS2 (SVS), and DOS/VS exploited this feature. Later developments in MVS allowed each program to operate within its own address space rather than sharing a common one. Virtual memory also paved the way for virtual machines; the VM/370 hypervisor could run multiple virtual machines simultaneously, each hosting standard S/360 or S/370 operating systems or single-user environments such as Conversational Monitor System (CMS). Time-sharing VM implementations enabled many users to operate individual CMS instances concurrently on separate virtual machines.

Today's systems

The IBM Z series, which began with the z900 in 2000, supports z/Architecture—an extension of the System/390 mainframe architecture that provides 64-bit processing capabilities.

Processor units

Modern IBM mainframes incorporate various processor types:

CP (central processor): general-purpose processing unit
IFL (Integrated Facility for Linux): dedicated processors optimized for running Linux operating systems (often under z/VM)
ICF (Integrated Coupling Facility): facilitates operations within Parallel Sysplex environments
SAP (System Assist Processor): responsible for system management tasks including accounting and handling I/O channel operations
zAAP (System z Application Assist Processor): specialized in executing Java and XML workloads
zIIP (System z Integrated Information Processor): designed to process particular workloads such as those involving IBM Db2, XML, and IPSec

Although these processors share similar functionalities, they are differentiated primarily for software licensing reasons: all except CP have usage restrictions and are excluded from software license fee calculations, which are mainly based on CP counts. Additional specialized processors include cryptographic accelerators like (CryptoExpress), networking processors such as the OSA-Express, and disk I/O processors like those used in FICON Express.

Neon Enterprise Software once offered "zPrime," a solution intended to enable conventional workloads to run on zIIPs and zAAPs; however, this product was withdrawn in 2011 following legal action initiated by IBM.

Operating systems

Prominent operating systems utilized on IBM mainframes include z/OS, which evolved from MVS/ESA and OS/390, along with z/VM derived from the VM family, z/VSE tracing back to the DOS lineage, and z/TPF, specialized for transaction processing. Additionally, various Linux on IBM Z variants are supported, such as Debian, Red Hat Enterprise Linux, and SUSE Linux Enterprise Server. Certain installations employ MUSIC/SP and UTS (Mainframe UNIX). Although OpenSolaris on System z was launched in 2008, it has since been discontinued.

Middleware

IBM mainframes accommodate a broad spectrum of key enterprise transaction processing platforms and database systems, including but not limited to CICS, IMS, WebSphere Application Server, IBM Db2, and Oracle Database. Many of these middleware components function seamlessly across different mainframe operating systems.

Emulators

There are software emulators available for System/370, System/390, and System z architectures, such as FLEX-ES, which operates on UnixWare or Linux environments, and the open-source Hercules emulator compatible with Linux, FreeBSD, Solaris, macOS, and Windows platforms. IBM also provides the zPDT (System z Personal Development Tool), an emulator designed for development and testing purposes that runs on Linux x86-64 systems.

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