Editing IBM mainframe (section)

==IBM System/360==
{{main|IBM System/360}}

[[File:IBM system 360-50 console - MfK Bern.jpg|thumb|right|IBM System/360 Model 50]]

IBM announced the System/360 (S/360) line of mainframes in April 1964.<ref>{{cite web| url = http://www.ibm.com/ibm/history/exhibits/mainframe/mainframe_PR360.html| title = IBM Archives: System/360 Announcement| website = [[IBM]]| date = January 23, 2003| access-date = December 22, 2020| archive-date = January 2, 2021| archive-url = https://web.archive.org/web/20210102231457/https://www.ibm.com/ibm/history/exhibits/mainframe/mainframe_PR360.html| url-status = live}}</ref> The System/360 was a single series of compatible models for both commercial and scientific use. The number "[[360 (number)|360]]" suggested a "360 [[degree (angle)|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 engineering and scientific line (such as [[floating-point arithmetic]]). Some of the arithmetic units and addressing features were optional on some models of the System/360. However, models were upward compatible and most were also downward compatible. The System/360 was also the first computer in wide use to include dedicated hardware provisions for the use of [[operating system]]s. Among these were supervisor and application mode programs and instructions, as well as built-in memory protection facilities. Hardware memory protection was provided to protect the operating system from the user programs (tasks) and user tasks from each other. The new machine also had a larger [[address space]] than the older mainframes, 24 bits addressing 8-bit bytes vs. a typical 18 bits addressing 36-bit words. 

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. 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 included [[OS/360]] (with PCP, MFT, and [[Multiprogramming with a Variable number of Tasks|MVT]]), [[BOS/360]], [[TOS/360]], and [[DOS/360]].

The System/360 later evolved into the [[System/370]], the [[System/390]], and the [[64-bit]] [[IBM System z|zSeries, System z, and zEnterprise]] machines. System/370 introduced [[virtual memory]] capabilities in all models other than the first System/370 models; the [[OS/VS1]] variant of OS/360 MFT, the [[OS/VS2 (SVS)]] variant of OS/360 MVT, and the DOS/VS variant of DOS/360 were introduced to use the virtual memory capabilities, followed by [[MVS]], which, unlike the earlier virtual-memory operating systems, ran separate programs in separate address spaces, rather than running all programs in a single virtual address space. The virtual memory capabilities also allowed the system to support [[virtual machine]]s; the [[VM (operating system)|VM/370]] [[hypervisor]] would run one or more virtual machines running either standard System/360 or System/370 operating systems or the single-user [[Conversational Monitor System]] (CMS). A [[time-sharing]] VM system could run multiple virtual machines, one per user, with each virtual machine running an instance of CMS.