IBM System/360

IBM System/360

Company type	Computer system family
Industry	Computer hardware
Founded	April 7, 1964 (1964-04-07)
Founder	Gene Amdahl (chief architect), Fred Brooks (software development director), Thomas J. Watson Jr. (Chairman)
Headquarters	Armonk , United States
Products	Mainframe computers, peripherals
Parent	IBM

The IBM System/360 (S/360) represents a series of computer systems unveiled by IBM on April 7, 1964, with production spanning from 1965 through 1978. This line was groundbreaking in delivering a unified platform catering to both commercial and scientific needs, encompassing a broad spectrum from entry-level units to expansive mainframe computers. Its architecture distinctly separated the instruction set from hardware implementation, allowing IBM to produce compatible machines at multiple price levels. Apart from the somewhat compatible Model 44 and the top-tier models, most systems employed microcode to execute their instruction sets, characterized by 8-bit byte addressing and supporting fixed-point binary, decimal, as well as hexadecimal floating-point operations. The System/360 family introduced IBM’s Solid Logic Technology (SLT), which increased transistor density on circuit cards to enhance performance in more compact devices, although it avoided integrated circuits due to concerns over their maturity at that time.

The principal architect behind System/360 was Gene Amdahl, with project oversight by Fred Brooks, both reporting directly to Chairman Thomas J. Watson Jr.. The commercial rollout was managed by John R. Opel. The entry-level Model 30 delivered up to 34,500 instructions per second and supported memory capacities between 8 and 64 KB. Subsequent higher-end models followed; most notably, the 1967 IBM System/360 Model 91 could perform up to 16.6 million instructions per second. Larger configurations accommodated as much as 8 MB of main memory—though installations typically ranged from 256 KB to 1 MB—and up to 8 MB of slower Large Capacity Storage (LCS).

The success of the IBM System/360 stemmed from its ability to enable customers to begin with smaller systems and expand without the need for rewriting software or replacing peripherals. Its architectural principles have left a lasting impact on computer design for many years, with application-level compatibility continuing in today’s System z mainframe platforms.

Background

In the early 1960s, IBM confronted difficulties maintaining five incompatible computer product lines aimed at various market segments. Customers transitioning from machines such as the IBM 1401 (used for accounting) to the IBM 7040 (geared toward engineering tasks) faced significant costs due to new hardware and software requirements, which undermined customer retention.

To address this issue, IBM established the SPREAD task force (Systems Programming, Research, Engineering and Development) in 1961 to develop a cohesive strategic approach. Convening in Greenwich, Connecticut, SPREAD envisioned a next-generation family of computers utilizing emerging technologies like small-scale integrated circuits alongside an architecture based on an 8-bit byte.

Contrary to previous fragmented designs, SPREAD proposed a single instruction set architecture (ISA) that would support binary arithmetic, floating-point operations, decimal calculations, string manipulation, character set translation, and extensive file management capabilities. This unified ISA aimed to consolidate functionality previously spread across multiple incompatible systems into one platform.

Although there were initial reservations about launching another platform that could potentially push customers toward competitors, IBM committed to the SPREAD initiative within six months. A dedicated team led by Bob Evans secured endorsement from CEO Thomas J. Watson Jr. Hardware design was headed by Gene Amdahl as chief architect; Fred Brooks directed software development; and Erich Bloch managed IBM’s efforts in hybrid integrated circuits leveraging Solid Logic Technology.

"Family" concept

Creating a unified system design that combined comprehensive features at prices affordable for entry-level clients while delivering the performance demanded by high-end users proved nearly unfeasible. The SPREAD concept addressed this by decoupling the specified feature set from the internal architecture, enabling a family of systems with diverse performance levels and internal configurations.

Depending on the specific model, some functionalities were realized through microcode—small programs stored in read only memory (ROM)—whereas others were implemented purely in hardware. For instance, certain models executed instructions using microcode within the central processing unit (CPU), while others depended entirely on hardware logic. Similarly, some systems utilized cycle-stealing microcode in the CPU for managing I/O channels, whereas others incorporated dedicated hardware controllers. This adaptable design, now recognized as microcode, empowered IBM to customize machines across various price points and performance tiers within a single product family.

This strategy drastically lowered entry obstacles. Unlike competitors who compelled customers to pick between underperforming or prohibitively expensive machines, IBM’s range allowed organizations to choose systems aligned with their requirements and upgrade without sacrificing software compatibility. Businesses could begin with a system geared for accounting tasks and subsequently expand to engineering workloads on the same platform. Transitioning to higher-performance models featuring capabilities such as floating-point hardware was straightforward, maintaining support for existing peripherals like high-speed line printers.

The idea that a single architecture could serve multiple application domains inspired the "360" designation, symbolizing 360 degrees in a circle—a theme IBM emphasized in its promotional efforts.

Models

IBM initially introduced six computers alongside forty peripheral devices but ultimately released fourteen models, including unique units developed for NASA. The most economical was the Model 20, equipped with as little as 4096 bytes of core memory, eight 16-bit registers (compared to sixteen 32-bit registers found in other variants), and a simplified instruction set.

The 1964 launch featured Models 30, 40, 50, 60, 62, and 70. The first trio targeted the market segment served by the IBM 1400 series and became available mid-1965. The latter three were intended as successors to the 7000 series but never reached production; instead, they were replaced by the 65 and 75, which shipped in late 1965 and early 1966 respectively.

Lower-end models added later included Models 20 (1966), 22 (1971), and 25 (1968). The Model 22 was a scaled-down version of the Model 30 featuring reduced memory capacity and slower I/O channels, which limited peripheral compatibility.

The specialized Model 44 (1966) was designed for scientific and real-time applications, offering additional instructions while omitting some complex ones.

A succession of high-performance systems followed: the Model 67 (1966), 85 (1969), 91 (1967), 95 (1968), and 195 (1971). The Model 85 acted as a transition between System/360 and its successor, the System/370, serving as the foundation for the 370/165. The System/370 edition of the 195 lacked Dynamic Address Translation capabilities.

Despite variations in hardware implementations—such as differences in data path widths and microcode usage—all models maintained robust architectural compatibility. For example, the high-end 91 supported out-of-order instruction execution but did not include decimal instructions commonly used in commercial environments. New capabilities were introduced without compromising compatibility: the dual-processor M65MP extended the Model 65; cache memory was incorporated into the Model 85. Models 44, 75, 91, 95, and 195 utilized hardwired logic rather than microcode.

The Model 67, announced in August 1965, was IBM's first production machine equipped with dynamic address translation (virtual memory) hardware supporting time-sharing. Earlier DAT prototypes (Models 64 and 66) were superseded by the Model 67 alongside the replacement of Models 60 and 62 with the Model 65. DAT technology reemerged in the S/370 series launched in 1972. Like the Model 65, the Model 67 offered dual CPU configurations.

By the close of 1977, IBM ceased marketing all System/360 models.

Backward compatibility

IBM’s clientele had already invested heavily in software designed for second-generation systems. To accommodate this, multiple System/360 variants incorporated the capability to emulate earlier machines through dedicated hardware and microcode solutions, allowing legacy applications to function seamlessly on the new architecture.

System/360 model	Emulated systems
	1401	1440, 1460	1410, 7010	7070, 7072, 7074	7080	709, 7090, 7094, 7094 II, 7040, 7044
Model 20	Yes
Model 30	Yes	Yes
Model 40	Yes	Yes	Yes
Model 50	Yes	Yes	Yes	Yes
Model 65				Yes	Yes	Yes
Model 85						Template:Partial

Initially, operating the emulation software required customers to stop their systems. Subsequently, IBM advanced these emulators to function under the control of the operating system for models such as the 1401, 1440, and 1460. This methodology was preserved in the Model 85 and later in the System/370 series, enabling emulators to run concurrently with native applications within the OS environment.

Successors and variants

With the exception of Models 20, 44, and 67, the System/360 lineup was replaced by the compatible System/370 series starting in 1970. Users of Model 20 were transitioned toward the IBM System/3. IBM discontinued its efforts to develop a significant breakthrough using FS technology during the mid-1970s to focus on affordability and system continuity. Subsequent compatible platforms include the IBM 4300 family, 308x family, 3090, ES/9000 family, 9672 (System/390), and the modern IBM Z series.

Several third-party manufacturers created systems largely compatible with the System/360 architecture: these include Amdahl Corporation’s 470 series, Hitachi mainframes, the UNIVAC 9000 series, Fujitsu’s Facom machines, RCA’s Spectra 70 line, and the English Electric System 4. The latter was licensed from RCA and engineered specifically for real-time operations featuring multiple processor states. After RCA was acquired by UNIVAC, their Spectra series evolved into the UNIVAC Series 70; UNIVAC also developed Series 90 as successors. Additionally, the Soviet Union produced a clone of the System/360 known as the ES EVM.

The IBM 5100 portable computer introduced in 1975 offered an option to run System/360’s APL.SV language through hardware emulation, which helped lower development expenses.

Specialized versions of the System/360 hardened against radiation were developed as part of the System/4 Pi avionics computer series used in fighter jets and bombers. The complete AP-101 variant served as fault-tolerant processing units within the U.S. Space Shuttle’s onboard computing system. From approximately 1970 through the early 1990s, the U.S. Federal Aviation Administration utilized the IBM 9020—a cluster of adapted System/360 machines—for air traffic control; some of this software reportedly continues to function today via emulation on modern hardware.

Table of System/360 Models

data-sort-type="number" Template:Verth	Announcement Date	Shipping Date	Scientific Throughput (kIPS)	Commercial Throughput (kIPS)	data-sort-type="number" Template:Verth	data-sort-type="number" Template:Verth	Memory Capacity (in KiB)	Mass (lbs)	Remarks
30	Apr 1964	Jun 1965	10.2	29	1.3	0.7	8–64	1,700 lb (771 kg)
40	Apr 1964	Apr 1965	40	75	3.2	0.8	16–256	1,700–2,310 lb (771–1,048 kg) weight varies with memory
50	Apr 1964	Aug 1965	133	169	8.0	2.0	64–512	4,700–7,135 lb (2,132–3,236 kg) weight varies with memory
Models 60 and 62	Apr 1964	N/A							Superseded by Model 65
Model 70	Apr 1964	N/A							Supplanted by Model 75
Model 90	Apr 1964	N/A							Replaced by Model 92
Model 92	Aug 1964	N/A						Renamed as IBM System/360 Model 91
20	Nov 1964	Mar 1966	2.0	2.6		4–32 1,200–1,400 lb (544–635 kg) Entry-level model with a partly incompatible instruction set.
91	Jan 1966	Oct 1967	1,900	1,800	133	164	1,024–4,096	Offered via special bid starting November 1964.
Models 64 and 66	Apr 1965	N/A					Discontinued in favor of Model67.
65	Apr1965	Nov1965	563	567	40	21	128–1,024	4,290–8,830 pounds (1,950–4,010 kg) weight depends on memory and processor configuration. Compatible with IBM2361 Large Capacity Storage (LCS).
75	Apr1965	Jan1966	940	670	41	43	256–1,024	5,125–5,325 pounds (2,325–2,415 kg) varies according to memory. Supported LCS.
67	Aug1965	May1966		40	512–2,048	3,674 pounds (1,666 kg) processor only. Features dynamic address translation for time-sharing.
44	Aug1965	Sep1966	118	185	16	4.0	32–256	2,900–4,200 pounds (1,300–1,900 kg) weight varies with memory size. Designed specifically for scientific applications.
	95		special order		Feb1968		3,800 est.		3,600 est.		133		711		5,220		Performance estimated at double that of Model91.
	IBM System/360 Model25		Jan1968		Oct1968		9.7		25		1.1		2.2		16–48		2,050 pounds (930 kg)
	IBM System/360 Model85		Jan1968		Dec1969		3,245		3,418		100		67		512–4,096		14,428 pounds (6,544 kg) processor only. Includes a cache of 16–32 KB and extended precision floating-point capability.
	IBM System/360 Model195		Aug1969		Mar1971		10,000 est.		10,000 est.		148		169		1,024–4,096		13,450–28,350 pounds (6,100–12,860 kg) weight depends on memory configuration. Equipped with a 32 KB IC cache; performance approximated as triple that of the Model85.
	IBM System/360 Model22		Apr1971		Jun1971										24–32		1,500 \lb A refurbished version of the Model30.

Summary of Models

• Out of the twenty announced models in the IBM System/360 series, six were never released or shipped.
• Fourteen models were commercially available on the market.

Key innovations

The System/360 established several foundational industry standards, among them:

• Adoption of the 8-bit byte, maintained despite economic pressures to downsize
• Memory addressable by individual bytes
• Use of 32-bit integer word size
• Standardized Bus and Tag I/O channel interface (FIPS-60)
• Introduction of microcoded CPUs for commercial applications
• IBM’s hexadecimal floating-point format
• Implementation of the EBCDIC character encoding
• Utilization of nine-track magnetic tape storage

Architecture summary

The System/360 architecture specifies both required and optional interfaces but leaves implementation specifics open. Its principal characteristics include:

• Big-endian byte order convention
• A processor containing sixteen 32-bit general-purpose registers labeled R0 through R15
• A 64-bit program status word (PSW) that manages interrupt masks, privilege levels, condition codes, and a 24-bit program counter
• Support for both maskable and non-maskable interrupts
• A detailed instruction set with clearly defined exception conditions
• Memory organized into 8-bit bytes with 24-bit addressing, including a dedicated processor communication area at address zero
• Manual controls for bootstrap (Initial Program Load), operator interrupt handling, system reset, debugging functions, and state adjustments
• An Input/Output system specifying interface protocols without dictating specific devices

Optional architectural elements encompass binary-coded decimal instructions, floating-point operations, interval timers, and memory protection governed by keys.

All models except the Model 20 and Model 44 fully adhered to this architectural specification.

Arithmetic and logical operations in binary occur either between registers or from memory to registers. The Commercial Instruction Set supports packed decimal arithmetic operating directly in memory. The Scientific Instruction Set provides four floating-point registers capable of 32- or 64-bit precision; Models 85 and 195 extend this to 128-bit extended precision. The System/360 employs an 8-bit byte and a 32-bit word, with instruction formats varying in length and complexity.

Memory addressing uses a base-plus-displacement scheme involving registers 1 through 15 combined with a 12-bit displacement field (enabling up to 4096-byte offsets). Register 0 is reserved as a zero value in addressing calculations, facilitating interrupt routines without requiring initial register setup.

Virtual memory was not part of the original architecture except on the Model 67, which introduced virtual memory concepts later adopted by systems such as MTS and CP-67. All other models operated with real memory addresses; virtual addressing became standard only with the System/370 series.

Machine instructions vary from two to six bytes in length and are aligned on two-byte boundaries. For instance, the MVC (Move-Characters) instruction can transfer up to 256 bytes per execution; moving larger data blocks necessitates multiple instructions. Subsequent architectures like System/370 introduced enhanced move instructions capable of handling up to 16 MB.

Operands typically combine a nibble identifying the base register with a 12-bit displacement value. For example, an MVC instruction transferring 256 bytes from the address formed by base register 7 plus zero displacement to base register 8 plus displacement one would be encoded as "D2FF 8001 7000."

The System/360 enforces a separation between supervisor state and problem state to enhance security and facilitate error recovery. User-level programs are prohibited from altering supervisor data or code. Exceptions cause controlled transitions into supervisor state for error management or process termination. Hardware errors are managed through machine check routines operating within this framework.

Channels

Peripherals interface with the system through specialized processors called channels, which are designed to efficiently manage data transfers between peripherals and main memory, similar in function to direct memory access (DMA). In the System/360 architecture, these channels connect to control units via bus and tag cables. In subsequent IBM systems, these were superseded by Enterprise Systems Connection (ESCON) and Fibre Connection (FICON) channels, introduced well after the System/360 generation.

Byte-multiplexor and selector channels

Originally, two primary channel types were implemented: byte-multiplexer channels intended for "slow speed" peripherals such as card readers, punches, printers, and communication controllers; and selector channels designed for high-speed devices like disk drives, tape drives, data cells, and drums. Every System/360 model except the Model 20 incorporates at least one byte-multiplexer channel alongside one or more selector channels. The Model 25 features a single channel that can be configured as either type. Smaller systems (up to Model 50) have integrated channels, whereas larger models (65 and above) utilize distinct channel units: the IBM 2870 byte-multiplexor with up to four selector sub-channels, and the IBM 2860 supporting up to three selector channels.

Byte-multiplexer channels enable simultaneous I/O operations at device-specific speeds by multiplexing data streams onto a single pathway to main memory. Devices connected to these channels operate in modes of 1-byte, 2-byte, 4-byte, or "burst," depending on their speed characteristics—for instance, a 2501 card reader processes 600 cards per minute in 1-byte mode, while a 1403-N1 printer functions in burst mode. On larger models, byte-multiplexor channels may optionally include selector subchannels dedicated to tape drives, typically addressed within the range "0C0" to "0C7." These standard byte-multiplexer addresses continue to be relevant within z/VM virtual machine environments.

Models 40 and 50 incorporate an integrated 1052-7 console addressed at 01F with a direct internal connection rather than routing through the byte-multiplexer channel. Models from 60 through 75 also utilize the 1052-7 console; the Model 30 connects a different console model via a 1051 control unit.

Selector channels facilitate connection to high-speed devices grouped under control units that aggregate multiple peripherals. High-end models support multiple selector channels operating concurrently, which allows parallel processing and improved throughput.

Control units attach to channels through pairs of bus-and-tag cables: bus cables transmit address and data signals while tag cables specify the nature of the data on the bus. Devices are arranged in a daisy chain configuration: Mainframe—Control Unit X—Control Unit Y—Control Unit Z. Each control unit manages a defined "capture range" of addresses (for example, X:40–4F, Y:C0–DF, Z:80–9F), which are allocated in multiples of 8, 16, 32, 64, or 128 devices. For instance, control unit Y might oversee six disk drives assigned addresses C0 through C5.

IBM manufactured three variants of bus-and-tag cables—standard gray, blue, and tan—with newer iterations supporting increased speeds or extended distances; certain peripherals require specific cable versions both upstream and downstream.

The sequence of control units on a channel is crucial; each unit is assigned either High or Low priority status. Device selection signals propagate along the chain in both directions (X→Y→Z→Y→X). Units with High priority respond on the outbound signal path; those with Low priority respond on the inbound path. Consequently, unit X may be positioned first or fifth in sequence; Y second or fourth; Z third. Multiple channels can connect to a single control unit from one or more mainframes to enhance performance and provide redundancy.

The total length of channel cabling is restricted to roughly 200 feet, with each control unit consuming about 10 feet of this maximum allowance.

Block multiplexer channel

The 2880 block multiplexer channel was introduced by IBM on the Model 85 and Model 195, becoming a standard feature on the System/370. This channel type allowed a device to pause a channel program during I/O activities, thus freeing the channel to serve other devices. It supports conventional 1.5 MB/s connections or, with an added 2-byte interface option, achieves 3 MB/s speeds using one tag cable and two bus cables. Additionally, the S/370 provides a 3.0 MB/s data streaming channel variant that operates with one bus and one tag cable.

Originally deployed for the 2305 fixed-head disk—which included 8 "exposures" (alternative addresses) and rotational position sensing (RPS)—block multiplexer channels can also act as selector channels to connect older subsystems.

Basic hardware components

IBM addressed early concerns about the dependability of monolithic integrated circuits by designing custom hybrid integrated circuits mounted on 11 mm square ceramic substrates. Resistors were applied via silk screening, while encapsulated discrete transistors and diodes were added before sealing with a metal or plastic cover, creating modules known as "Solid Logic Technology" (SLT).

These SLT modules were flip-chip mounted onto compact multi-layer printed circuit "SLT cards," each containing one or two sockets that plugged into pins on the computer's "SLT boards" (backplanes). Unlike other manufacturers who used cards with pins fitting into board sockets, IBM reversed this arrangement.

A "logic gate" could be constructed from up to twenty SLT boards arranged in a grid up to four high and five wide. Multiple gates were combined into a box-shaped "logic frame," with outer gates hinged to allow access to internal gates. Larger installations linked several frames together to form complex systems such as multi-frame Central Processing Units (CPUs).

Operating system software

Smaller System/360 configurations operated using Basic Operating System/360 (BOS/360), Tape Operating System (TOS/360), or Disk Operating System/360 (DOS/360), which progressed through iterations including DOS/VS, DOS/VSE, VSE/AF, VSE/SP, VSE/ESA, ultimately evolving into z/VSE.

For more powerful models, Operating System/360 (OS/360) was employed, developed in phases with increasing sophistication: Primary Control Program (PCP), Multiprogramming with a Fixed number of Tasks (MFT), and Multiprogramming with a Variable number of Tasks (MVT). Due to MVT’s extensive development demands, MFT initially saw broader use. PCP supported early large systems and intermediate machines; later OS/360 releases included only MFT and MVT. On System/370 and later platforms, MFT transitioned into OS/VS1, while MVT evolved into OS/VS2 (SVS) and successive MVS versions, culminating in today’s z/OS.

With the introduction of the IBM System/360 Model 67 in August 1965, IBM launched Time-Sharing System (TSS/360) to rival Multics. Despite its advanced capabilities, TSS/360 suffered from performance limitations and was discontinued multiple times by 1971. Customers migrated toward alternatives such as CP-67, Michigan Terminal System (MTS), Time Sharing Option (TSO) for OS/360, or other time-sharing solutions.

CP-67—IBM’s original virtual machine system developed at the Cambridge Scientific Center in collaboration with MIT—progressed into CP/CMS and gained widespread adoption. This foundation led to VM/370 featuring its interactive suboperating system VM/CMS (Conversational Monitoring System), which remains active today as z/VM.

The Model 20 utilized simpler tape-oriented systems: Tape Processing System (TPS) and Disk Processing System (DPS), supporting the 2311 disk drive. TPS could operate with as little as 8 KB of memory; DPS required 12 KB—substantial for the Model 20’s scale. Many users ran with only 4 KB using Card Processing System (CPS). These systems employed Job Control Language cards for batch processing, enabling efficient management of job queues on limited hardware.

A less prominent collection named Basic Programming Support (BPS), serving as a forerunner to TOS, provided fundamental punched-card utilities for smaller systems but experienced limited adoption.

Component designations

IBM implemented a revised naming convention for System/360 components while preserving well-known names such as IBM 1403 and IBM 1052. Components were identified by four-digit codes beginning with the digit 2, where the second digit denoted the category:

20xx:	Arithmetic units, for instance, the IBM 2030 CPU utilized in the IBM System/360 Model 30.
21xx:	Power supply units and processor-associated hardware, such as the IBM 2167 Configuration Unit.
22xx:	Visual display equipment, including IBM 2250 and IBM 2260 CRT monitors, and the IBM 2203 line printer designed for Model 20.
23xx:	Direct access storage devices like the IBM 2311, IBM 2314 disk drives, and the IBM 2321 Data Cell; also main memory units such as IBM 2361 Large Capacity Storage and IBM 2365 Processor Storage.
24xx:	Magnetic tape drives including models like IBM 2401, IBM 2405, and IBM 2415.
25xx:	Punched card machinery such as the IBM 2501 card reader, IBM 2520 card punch, IBM 2540 combined reader/punch, and the IBM 2560 Multi-Function Card Machine.
26xx:	Paper tape equipment exemplified by the IBM 2671 reader.
27xx:	Communication devices including the IBM 2701, IBM 2705, IBM 2741 terminal, and the IBM 2780 batch terminal.
28xx:	Controllers and channels such as the IBM 2821 Control Unit, along with the IBM 2841 and IBM 2844 models.
29xx:	Various miscellaneous peripherals like the IBM 2914 Data Channel Switch and the IBM 2944 Data Channel Repeater.

Peripherals

The System/360 introduced a new series of peripherals featuring standardized interfaces, significantly improving modularity by allowing flexible combinations of processors, controllers, and peripheral devices compared to previous product lines. Certain trusted legacy peripherals, notably the acclaimed IBM 1403 chain printer—rebranded as the 1403-N1 for System/360—were maintained.

Optical character recognition (OCR) readers such as IBM1287 and IBM1288 were capable of interpreting both alphanumeric and numeric hand-printed characters across a range of document sizes—an advanced feature considering software constraints of that era.

Up through the System/360 Model65, an IBM1052–7 console typewriter served as the primary operator interface. The high-tier Model85 equipped with feature5450 incorporated a distinctive display console incompatible with other System/360 models; subsequent consoles like the3066 used in System/370 Models165 and168 adopted similar designs. Models91 and195 utilized graphical displays comparable to the IBM2250 as their main consoles.

Additional operator consoles included optional high-end graphical displays such as the IBM2250—which carried a price exceeding US$100,000—while smaller systems employed more cost-effective alternatives like the IBM2260 or later the IBM3270.

Direct access storage devices (DASD)

The first disk drives introduced for the System/360 were the IBM 2302 and IBM 2311 models, alongside the IBM 7320 drum unit. The 2302, derived from the IBM 1302, was available as model 3 with two modules of 112.79 MB each or model 4 with four such modules. The 2311 utilized a removable IBM 1316 disk pack, an evolution of the IBM 1311 design, offering a theoretical capacity of 7.2 MB, though actual storage varied depending on record formatting. For instance, when used with the 360/20 system, the 1316 pack formatted into fixed-length sectors of 270 bytes could hold up to about 5.4 MB.

In 1966, IBM released the IBM 2314, which accommodated up to eight active drives plus one spare. Each drive employed a removable IBM 2316 disk pack with nearly 28 MB of storage capacity. These disk packs were physically large—approximately in diameter—and contained six platters that provided ten recording surfaces through ten read/write heads moving in unison across concentric tracks. Data was arranged into virtual cylinders to reduce head movement and improve efficiency. Unlike contemporary fixed-sector disks, System/360 supported variable-length records, allowing more adaptable data handling.

High-speed drum storage devices with individual heads per track, such as the 3,500 RPM IBM 2301, debuted alongside System/360 and offered a capacity of 4 MB. The subsequent IBM 2303, introduced in 1966, delivered approximately 3.913 MB at a data transfer rate of 303.8 kbit/s. These drum units were eventually replaced by fixed-head disk drives.

The faster IBM 2305, spinning at 6,000 RPM and launched in 1970, came in modules of either 5 MB (2305–1) or 11 MB (2305–2). Despite their relatively limited capacity, their rapid access times and transfer speeds—up to 3 MB/s—made them well-suited for demanding tasks like overlay linkage and virtual memory paging. Although technically classified as head-per-track disks rather than drums, the 2305 featured twelve recording surfaces.

The uncommon IBM 2321 Data Cell was a mechanically intricate device that stored information on numerous magnetic strips accessed via a cylindrical drum mechanism. It offered a substantial storage capacity of around 400 MB—far exceeding the contemporaneous IBM 2311’s 7.2 MB—intended to fill the gap between high-capacity magnetic tapes and costlier disk drives. However, some users considered its electromechanical design less dependable compared to simpler direct-access storage options.

The Model 44 was distinctive in that it included a single-disk drive as standard equipment using the IBM 2315 "ramkit" cartridge, providing roughly 1.17 MB of storage.

Magnetic Tape drives

The System/360 utilized the 2400-series of half-inch magnetic tape units, including models such as the 2401 and various versions of the 2402 (Models 1-6), the controlled versions of the 2403 (Models 1-6), and controlled variants of the 2404 (Models 1-3), along with Tape Control Units like the 2803 and 2804 Models 1 and 2. In 1967, IBM introduced the combined unit known as the 2415 Magnetic Tape Unit and Control, which integrated two to six tape drives with control electronics into one package; this offered a slower yet more economical alternative. The components of the 2415 were not available for separate purchase. With the System/360 family, IBM transitioned from using IBM 7-track tapes to 9-track tape formats; however, some models within the 2400-series retained support for seven-track tapes to maintain backward compatibility with older IBM 729 tape drives. In 1968, IBM launched the high-performance 2420 tape system featuring enhanced data transfer rates, automatic threading capabilities, and a density of 1600 bits per inch; this system remained in production until its discontinuation in 1979.

Unit record equipment

• Among punched card apparatus, the extensively utilized 2501 card reader and the nearly universal 2540 card reader/punch were prominent. The 2560 MFCM ("Multi-Function Card Machine"), exclusive to the Model 20, experienced notable reliability challenges.
• Line printers included the rapid IBM 1403 and the slower IBM 1443 variants.
• Introduced in 1964, the IBM 2671 paper tape reader achieved speeds up to 1,000 characters per second. Earlier models such as the IBM 1054/1055 paper tape readers and punches were obtainable only through special request (RPQ) and operated at considerably reduced speeds.
• Optical character recognition (OCR) devices like the IBM 1287 and its successor, the 1288, utilized sophisticated 'flying spot' scanning technology to interpret handwritten digits and OCR fonts.
• Magnetic ink character recognition (MICR) functionality was provided by IBM's 1412 and 1419 check sorters along with modified IBM 1445 printers capable of MICR printing, primarily serving financial institutions.

Preserved systems

Despite extensive sales and leases during their time, only a limited number of IBM System/360 mainframes survive today, mainly preserved in museums or private collections. Significant examples include:

• A non-operational IBM System/360 Model 30 is exhibited at the Computer History Museum in Mountain View, California, as well as at the Museum of Transport and Technology in Auckland, New Zealand, and Vienna University of Technology in Austria.
• The University of Western Australia Computer Club maintains a complete Model 40 unit in storage.
• Japan’s first computer school, Kyoto Computer Gakuin’s KCG Computer Museum, displays an IBM System/360 Model 40.
• In 2019, two Model 20 processors with peripherals were acquired by enthusiasts from Nürnberg, Germany, and relocated to Creslow Park in Buckinghamshire, UK. As of September 2024, these machines are on extended loan to the System Source Computer Museum in Hunt Valley, Maryland, USA for exhibition and refurbishment.
• The Living Computers: Museum + Labs previously housed a Model 30 until its closure in February 2020.

A comprehensive listing of operational System/360 units worldwide can be found at World Inventory of remaining System/360 CPUs.

Gallery

This section presents operator consoles featuring register indicator lamps, toggle switches, and emergency pull switches across different System/360 models.

See also

• IBM System/360 architecture
• IBM System/370 architecture
• IBM 9370
• IBM System/370-XA
• IBM Enterprise Systems Architecture
• z/Architecture
• IBM 9020
• History of IBM
• List of IBM products
• IBM System/4 Pi
• Gerrit Blaauw
• Bob O. Evans

External links

• IBM System/360 System Summary 11th edition August 1969
• Generations of the IBM 360/370/3090/390 by Lars Poulsen with multiple links and references
• Description of a large IBM System/360 model 75 installation at JPL
• Illustrations from “Introduction to IBM Data Processing Systems”, 1968: includes photographs of IBM System/360 computers and peripherals
• IBM System 360 RPG Debugging Template and Keypunch Card
• Video: two-hour lecture and panel discussion titled The IBM System/360 Revolution, from the Computer History Museum on 2004-04-07
• Original vintage film from 1964 IBM System/360 Computer History Archives Project
• Photos of a dual processor IBM 360/67 at the University of Michigan's Computing Center in the late 1960s or early 1970s, featured in Dave Mills' article on the Michigan Terminal System (MTS)
• Pictures of an IBM System/360 Model 67 at Newcastle (UK) University