IBM System/360 (S/360) is a family of mainframe computer systems
that was announced by
IBM on April 7, 1964, and delivered between 1965
and 1978. It was the first family of computers designed to cover
the complete range of applications, from small to large, both
commercial and scientific. The design made a clear distinction between
architecture and implementation, allowing
IBM to release a suite of
compatible designs at different prices. All but the incompatible model
44 and the most expensive systems used microcode to implement the
instruction set, which featured
8-bit byte addressing and binary,
decimal and (hexadecimal) floating-point calculations.
The launch of the System/360 family introduced IBM's Solid Logic
Technology (SLT), a new technology that was the start of more powerful
but smaller computers.
The slowest System/360 model announced in 1964, the Model 30, could
perform up to 34,500 instructions per second, with memory from 8 to
64 KB. High performance models came later. The 1967 IBM
System/360 Model 91 could do up to 16.6 million instructions per
second. The larger 360 models could have up to 8 MB of main
memory, though main memory that big was unusual—a large
installation might have as little as 256 KB of main storage, but
512 KB, 768 KB or 1024 KB was more common. Up to 8
megabytes of slower (8 microsecond) Large Capacity Storage (LCS) was
IBM 360 was extremely successful in the market, allowing customers
to purchase a smaller system with the knowledge they would always be
able to migrate upward if their needs grew, without reprogramming of
application software or replacing peripheral devices. Many consider
the design one of the most successful computers in history,
influencing computer design for years to come.
The chief architect of System/360 was Gene Amdahl, and the project was
managed by Fred Brooks, responsible to Chairman Thomas J. Watson
Jr. The commercial release was piloted by another of Watson's
lieutenants, John R. Opel, who managed the launch of IBM’s System
360 mainframe family in 1964.
Application level compatibility (with some restrictions) for
System/360 software is maintained to the present day with the System z
1 System/360 history
1.1 A family of computers
1.3 Backward compatibility
1.4 Successors and variants
1.5 Table of System/360 models
2 Technical description
2.1 Influential features
2.2 Architectural overview
2.3.1 Byte-multiplexor and selector channels
2.3.2 Block multiplexer channel
2.4 Basic hardware components
2.5 Operating system software
3 Component names
4.1 Direct access storage devices (DASD)
4.2 Tape drives
4.3 Unit record devices
5 Remaining machines
6 Summary of models announced but never shipped
6.1 Summary of models shipped
8 In popular culture
9 See also
12 External links
12.1 From the
IBM Journal of Research and Development
IBM Systems Journal
IBM System/360 Model 20 CPU with front panels removed, with IBM
2560 MFCM (Multi-Function Card Machine)
IBM System/360 Model 30 CPU (red, middle of picture), tape drives to
its left, and disk drives to its right, at the Computer History Museum
IBM System/360 Model 50 CPU, computer operator's console, and
peripherals at Volkswagen
System/360 Model 65 operator's console, with register value lamps and
toggle switches (middle of picture) and "emergency pull" switch (upper
A family of computers
Contrasting with at-the-time normal industry practice,
IBM created an
entire new series of computers, from small to large, low to high
performance, all using the same instruction set (with two exceptions
for specific markets). This feat allowed customers to use a cheaper
model and then upgrade to larger systems as their needs increased
without the time and expense of rewriting software. Before the
introduction of System/360, business and scientific applications used
different computers with different instruction sets and operating
systems. Different-sized computers also had their own instruction
IBM was the first manufacturer to exploit microcode technology
to implement a compatible range of computers of widely differing
performance, although the largest, fastest, models had hard-wired
This flexibility greatly lowered barriers to entry. With most other
vendors customers had to choose between machines they could outgrow
and machines that were potentially too powerful and thus too costly.
This meant that many companies simply did not buy computers.
IBM initially announced a series of six computers and forty common
IBM eventually delivered fourteen models, including rare
one-off models for NASA. The least expensive model was the Model 20
with as little as 4096 bytes of core memory, eight 16-bit registers
instead of the sixteen
32-bit registers of other System/360 models,
and an instruction set that was a subset of that used by the rest of
The initial announcement in 1964 included Models 30, 40, 50, 60, 62,
and 70. The first three were low- to middle-range systems aimed at the
IBM 1400 series market. All three first shipped in mid-1965. The last
three, intended to replace the 7000 series machines, never shipped and
were replaced by the 65 and 75, which were first delivered in November
1965, and January 1966, respectively.
Later additions to the low-end included models 20 (1966, mentioned
above), 22 (1971), and 25 (1968). The Model 20 had several sub-models;
sub-model 5 was at the higher end of the model. The Model 22 was a
recycled Model 30 with minor limitations: a smaller maximum memory
configuration, and slower I/O channels, which limited it to slower and
lower-capacity disk and tape devices than on the 30.
The Model 44 (1966) was a specialized model, designed for scientific
computing and for real-time computing and process control, featuring
some additional instructions, and with all storage-to-storage
instructions and five other complex instructions eliminated.
This image of the
IBM System 360 Model 91 operator's console, was
NASA sometime in the late 1960s.
A succession of high-end machines included the Model 67 (1966,
mentioned below, briefly anticipated as the 64 and 66), 85 (1969),
91 (1967, anticipated as the 92), 95 (1968), and 195 (1971). The 85
design was intermediate between the System/360 line and the follow-on
System/370 and was the basis for the 370/165. There was a System/370
version of the 195, but it did not include Dynamic Address
The implementations differed substantially, using different native
data path widths, presence or absence of microcode, yet were extremely
compatible. Except where specifically documented, the models were
architecturally compatible. The 91, for example, was designed for
scientific computing and provided out-of-order instruction execution
(and could yield "imprecise interrupts" if a program trap occurred
while several instructions were being read), but lacked the decimal
instruction set used in commercial applications. New features could be
added without violating architectural definitions: the 65 had a
dual-processor version (M65MP) with extensions for inter-CPU
signalling; the 85 introduced cache memory. Models 44, 75, 91, 95, and
195 were implemented with hardwired logic, rather than microcoded as
all other models.
The Model 67, announced in August 1965, was the first production IBM
system to offer dynamic address translation hardware to support
time-sharing. "DAT" is now more commonly referred to as an MMU. An
experimental one-off unit was built based on a model 40. Before the
IBM had announced models 64 and 66, DAT versions of the 60 and 62,
but they were almost immediately replaced by the 67 at the same time
that the 60 and 62 were replaced by the 65. DAT hardware would
reappear in the
S/370 series in 1972, though it was initially absent
from the series. Like its close relative, the 65, the 67 also offered
IBM stopped marketing all System/360 models by the end of 1977.
IBM's existing customers had a large investment in software that
executed on second-generation machines. Four models offered the option
of emulation of the customer's previous computer using a combination
of special hardware, special microcode and an emulation program
that used the emulation instructions to simulate the target system, so
that old programs could run on the new machine.
1401, 1440 and 1460
1401, 1440 and 1460
1410 and 7010
1401, 1440 and 1460
1410 and 7010
7070, 7072 and 7074
7070, 7072 and 7074
709, 7090, 7094 7094 II, 7040 and 7044
However, customers initially had to halt the computer and load the
IBM later added features and modified emulator programs to allow
emulation of the 1401, 1440, 1460, 1410 and 7010 under the control of
an operating system. The Model 85 and later
System/370 maintained the
precedent, retaining emulation options and allowing emulator programs
to execute under operating system control alongside native
Successors and variants
System/360 (excepting the Model 20) was replaced by the compatible
System/370 range in 1970 and Model 20 users were targeted to move to
IBM System/3. (The idea of a major breakthrough with FS technology
was dropped in the mid-1970s for cost-effectiveness and continuity
reasons.) Later compatible
IBM systems include the 3090, the ES/9000
family, 9672 (
System/390 family), the zSeries, System z9, System z10
IBM zEnterprise System.
Computers that were mostly identical or compatible in terms of the
machine code or architecture of the System/360 included Amdahl's 470
family (and its successors), Hitachi mainframes, the
series, Fujitsu as the Facom, the
Spectra 70 series,[NB 1] and
English Electric System 4.[NB 2] The System 4 machines were built
under license to RCA.
RCA sold the Spectra series to what was then
UNIVAC, where they became the
UNIVAC Series 70.
UNIVAC also developed
UNIVAC Series 90 as successors to the 9000 series and Series
Soviet Union produced a System/360 clone named the ES
IBM 5100 portable computer, introduced in 1975, offered an option
to execute the System/360's APL.SV programming language through a
IBM used this approach to avoid the costs and delay
of creating a 5100-specific version of APL.
Special radiation-hardened and otherwise somewhat modified
System/360s, in the form of the
System/4 Pi avionics computer, are
used in several fighter and bomber jet aircraft. In the complete
32-bit AP-101 version, 4 Pi machines were used as the replicated
computing nodes of the fault-tolerant Space Shuttle computer system
(in five nodes). The U.S.
Federal Aviation Administration
Federal Aviation Administration operated the
IBM 9020, a special cluster of modified System/360s for air traffic
control, from 1970 until the 1990s. (Some 9020 software is apparently
still used via emulation on newer hardware.)
Table of System/360 models
(in (binary) KB)
IBM 2361 Large Capacity Storage (LCS)
60 - 62
Replaced by Model 65
Replaced by Model 75
Replaced by Model 92
IBM System/360 Model 91
16-bit, low end, limited partially incompatible instruction set
Available on special bid beginning Nov 1964:388
64 - 66
Replaced by Model 67
Dynamic address translation for time sharing
Specialized for scientific computing
Performance estimated as 2× Model 91:p.394
16-32 KB cache memory, extended-precision floating point
32 KB IC cache memory. Performance estimated as 3× Model 85:p.422
A re-manufactured Model 30
IBM System 360-40
The System/360 introduced a number of industry standards to the
marketplace, such as:
8-bit byte (against financial pressure during development to
reduce the byte to 4 or 6 bits), rather than adopting the 7030 concept
of accessing bytes of variable size at arbitrary bit addresses.
Byte-addressable memory (as opposed to bit-addressable or
The bus and tag I/O channel standardized in FIPS-60
Commercial use of microcoded CPUs
IBM Floating Point Architecture
EBCDIC character set[NB 5]
Nine track magnetic tape
IBM System/360 architecture
The System/360 series had a computer system architecture
specification. This specification makes no assumptions on
the implementation itself, but rather describes the interfaces and
expected behavior of an implementation. The architecture describes
mandatory interfaces that must be available on all implementations,
and optional interfaces. Some aspects of this architecture are:
Big endian byte ordering
A processor with
32-bit general purpose registers (R0-R15)
64-bit program status word (PSW), which describes (among other
A condition code
A 24-bit instruction address
An interruption mechanism, maskable and unmaskable interruption
classes and subclasses
An instruction set. Each instruction is wholly described and also
defines the conditions under which an exception is recognized in the
form of program interruption.
A memory (called storage) subsystem with
8 bits per byte
A special processor communication area starting at address 0
Manual control operations that allow
A bootstrap process (a process called
Initial Program Load
Initial Program Load or IPL)
Resetting the system
Basic debugging facilities
Manual display and modifications of the system's state (memory and
An Input/Output mechanism - which does not describe the devices
Some of the optional features are:
Binary-coded decimal instructions
Floating point instructions
Timing facilities (interval timer)
Key-controlled memory protection
All models of System/360, except for the Model 20 and Model 44,
implemented that specification.
Binary arithmetic and logical operations are performed as
register-to-register and as memory-to-register/register-to-memory as a
standard feature. If the Commercial Instruction Set option was
installed, packed decimal arithmetic could be performed as
memory-to-memory with some memory-to-register operations. The
Scientific Instruction Set feature, if installed, provided access to
four floating point registers that could be programmed for either
64-bit floating point operations. The Models 85 and 195
could also operate on 12
8-bit extended-precision floating point
numbers stored in pairs of floating point registers, and software
provided emulation in other models. The System/360 used an
64-bit double-word, and 4-bit nibble. Machine
instructions had operators with operands, which could contain register
numbers or memory addresses. This complex combination of instruction
options resulted in a variety of instruction lengths and formats.
Memory addressing was accomplished using a base-plus-displacement
scheme, with registers 1 through F (15). A displacement was encoded in
12 bits, thus allowing a 4096-byte displacement (0-4095), as the
offset from the address put in a base register.
Register 0 could not be used as a base register nor as an index
register (nor as a branch address register), as "0" was reserved to
indicate an address in the first 4 KB of memory, that is, if
register 0 was specified as described, the value 0x00000000 was
implicitly input to the effective address calculation in place of
whatever value might be contained within register 0 (or if specified
as a branch address register, then no branch was taken, and the
content of register 0 was ignored, but any side effect of the
instruction was performed).
This specific behavior permitted initial execution of an interrupt
routines, since base registers would not necessarily be set to 0
during the first few instruction cycles of an interrupt routine. It
isn't needed for IPL ("Initial Program Load" or boot), as one can
always clear a register without the need to save it.
With the exception of the Model 67, all addresses were real memory
Virtual memory was not available in most
System/370 series. The Model 67 introduced a virtual memory
architecture, which MTS, CP-67, and
TSS/360 used—but not IBM's
mainline System/360 operating systems.
The System/360 machine-code instructions are 2 bytes long (no memory
operands), 4 bytes long (one operand), or 6 bytes long (two operands).
Instructions are always situated on 2-byte boundaries.
Operations like MVC (Move-Characters) (Hex: D2) can only move at most
256 bytes of information. Moving more than 256 bytes of data required
multiple MVC operations. (The
System/370 series introduced a family of
more powerful instructions such as the MVCL "Move-Characters-Long"
instruction, which supports moving up to 16 MB as a single
An operand is two bytes long, typically representing an address as a
4-bit nibble denoting a base register and a 12-bit displacement
relative to the contents of that register, in the range 000–FFF
(shown here as hexadecimal numbers). The address corresponding to that
operand is the contents of the specified general-purpose register plus
the displacement. For example, an MVC instruction that moves 256 bytes
(with length code 255 in hexadecimal as FF) from base register 7, plus
displacement 000, to base register 8, plus displacement 001, would be
coded as the 6-byte instruction "D2FF 8001 7000"
The System/360 was designed to separate the system state from the
problem state. This provided a basic level of security and
recoverability from programming errors. Problem (user) programs could
not modify data or program storage associated with the system state.
Addressing, data, or operation exception errors made the machine enter
the system state through a controlled routine so the operating system
could try to correct or terminate the program in error. Similarly, it
could recover certain processor hardware errors through the machine
See also: Channel I/O
Peripherals interfaced to the system via channels. A channel was a
specialized processor with the instruction set optimized for
transferring data between a peripheral and main memory. In modern
terms, this could be compared to direct memory access (DMA).
Byte-multiplexor and selector channels
There were initially two types of channels; byte-multiplexer channels
(known at the time simply as "multiplexor channels"), for connecting
"slow speed" devices such as card readers and punches, line printers,
and communications controllers, and selector channels for connecting
high speed devices, such as disk drives, tape drives, data cells and
drums. Every System/360 (except for the Model 20, which was not a
standard 360) had a byte-multiplexer channel and 1 or more selector
channels. The smaller models (up to the model 50) had integrated
channels, while for the larger models (model 65 and above) the
channels were large separate units in separate cabinets, such as the
IBM 2860 and 2870. (The 60, 62, and 70 had allowed only for 2860
selector channels, on the assumption that they would all have smaller
360s attached, which would do the slow-speed work.)
The byte-multiplexer channel was able to handle I/O to/from several
devices simultaneously at the device's highest rated speeds, hence the
name, as it multiplexed I/O from those devices onto a single data path
to main memory. Devices connected to a byte-multiplexer channel were
configured to operate in 1-byte, 2-byte, 4-byte, or "burst" mode. The
larger "blocks" of data were used to handle progressively faster
devices. For example, a 2501 card reader operating at 600 cards per
minute would be in 1-byte mode, while a 1403-N1 printer would be in
burst mode. Also, the byte-multiplexer channels on larger models had
an optional selector subchannel section that would accommodate tape
drives. The byte-multiplexor's channel address was typically "0" and
the selector subchannel addresses were from "C0" to "FF." Thus, tape
drives on System/360 were commonly addressed at 0C0-0C7. Other common
byte-multiplexer addresses were: 00A: 2501 Card Reader, 00C/00D: 2540
Reader/Punch, 00E/00F: 1403-N1 Printers, 010-013: 3211 Printers,
020-0BF: 2701/2703 Telecommunications Units. These addresses are still
commonly used in z/VM virtual machines.
System/360 models 40 and 50 had an integrated 1052-7 console that was
usually addressed as 01F, however, this was not connected to the
byte-multiplexer channel, but rather, had a direct internal connection
to the mainframe. The model 30 attached a different model of 1052
through a 1051 control unit. The models 60 through 75 also used the
Cable used as Bus or Tag cable for
Bus and tag terminators
Selector channels enabled I/O to high speed devices. These storage
devices were attached to a control unit and then to the channel. The
control unit let clusters of devices be attached to the channels. On
higher speed models, multiple selector channels, which could operate
simultaneously or in parallel, improved overall performance.
Control units were connected to the channels with "bus and tag" cable
pairs. The bus cables carried the address and data information and the
tag cables identified what data was on the bus. The general
configuration of a channel was to connect the devices in a chain, like
this: Mainframe—Control Unit X—Control Unit Y—Control Unit Z.
Each control unit was assigned a "capture range" of addresses that it
serviced. For example, control unit X might capture addresses 40-4F,
control unit Y: C0-DF, and control unit Z: 80-9F. Capture ranges had
to be a multiple of 8, 16, 32, 64, or 128 devices and be aligned on
appropriate boundaries. Each control unit in turn had one or more
devices attached to it. For example, you could have control unit Y
with 6 disks, that would be addressed as C0-C5.
There were three general types of bus-and-tag cables produced by IBM.
The first was the standard gray bus-and-tag cable, followed by the
blue bus-and-tag cable, and finally the tan bus-and-tag cable.
Generally, newer cable revisions were capable of higher speeds or
longer distances, and some peripherals specified minimum cable
revisions both upstream and downstream.
The cable ordering of the control units on the channel was also
significant. Each control unit was "strapped" as High or Low priority.
When a device selection was sent out on a mainframe's channel, the
selection was sent from X->Y->Z->Y->X. If the control unit
was "high" then the selection was checked in the outbound direction,
if "low" then the inbound direction. Thus, control unit X was either
1st or 5th, Y was either 2nd or 4th, and Z was 3rd in line. It was
also possible to have multiple channels attached to a control unit
from the same or multiple mainframes, thus providing a rich
high-performance, multiple-access, and backup capability.
Typically the total cable length of a channel was limited to 200 feet,
less being preferred. Each control unit accounted for about 10 "feet"
of the 200-foot limit.
Block multiplexer channel
IBM introduced a new type of I/O channel on the Model 85 and Model
195: the 2880 block multiplexer channel. The channel allowed a device
to suspend a channel program, pending the completion of an I/O
operation and thus to free the channel for use by another device.
These channels could support either standard 1.5 MB/second
connections or, with the 2-byte interface feature, 3 MB/second;
the later used one tag cable and two bus cables.
The initial use for this was the 2305 fixed-head disk, which had 8
"exposures" (alias addresses) and rotational position sensing (RPS).
They were standard on the
System/370 and thereafter.
Block multiplexer channels could operate as a selector channel to
allow compatible attachment of legacy subsystems.
Basic hardware components
Many SLT cards plugged into an SLT board.
Being somewhat uncertain of the reliability and availability of the
then new monolithic integrated circuits,
IBM chose instead to design
custom hybrid integrated circuits using discrete flip chip mounted
glass encapsulated transistors and diodes with silk screened resistors
on a ceramic substrate. This substrate was then either encapsulated in
plastic or covered with a metal lid to create a "Solid Logic
Technology" (SLT) module.
A number of these SLT modules were then mounted onto a small
multi-layer printed circuit "SLT card". Each card had one or two
sockets on one edge that plugged onto pins on one of the computer's
"SLT boards". This was the reverse of how most other company's cards
were mounted, where the cards had pins which plugged into sockets on
the computer's boards.
Up to twenty SLT boards could be assembled side-by-side (vertically
and horizontally) to form a "logic gate". Several gates mounted
together constituted a box-shaped "logic frame". The outer gates were
generally hinged along one vertical edge so they could be swung open
to provide access to the fixed inner gates. The larger machines could
have more than one frame bolted together to produce the final unit,
such as a multi-frame Central Processing Unit (CPU).
Operating system software
Main article: System/360 operating systems
The smaller System/360 models used the Basic Operating System/360
(BOS/360), Tape Operating System (TOS/360), or Disk Operating
System/360 (DOS/360, which evolved into DOS/VS, DOS/VSE, VSE/AF,
VSE/SP, VSE/ESA, and then z/VSE).
The larger models used Operating System/360 (OS/360).
several versions of OS/360, with increasingly powerful features:
Primary Control Program (PCP), Multiprogramming with a Fixed number of
Tasks (MFT), and Multiprogramming with a Variable number of Tasks
(MVT). MVT took a long time to develop into a usable system, and the
less ambitious MFT was widely used. PCP was used on intermediate
machines; the final releases of OS/360 included only MFT and MVT. For
System/370 and later machines, MFT evolved into OS/VS1, while MVT
OS/VS2 (SVS) (Single Virtual Storage), then various
MVS (Multiple Virtual Storage) culminating in the current
When it announced the Model 67 in August 1965,
IBM also announced
TSS/360 (Time-Sharing System) for delivery at the same time as the 67.
TSS/360, a response to Multics, was an ambitious project that included
many advanced features. It never worked properly, was delayed,
canceled, reinstated, and finally canceled again in 1971. It was
replaced by CP-67, MTS (Michigan Terminal System), TSO (Time Sharing
Option for OS/360), or one of several other time-sharing systems.
CP-67, the original virtual machine system, was also known as CP/CMS.
CP/67 was developed outside the
IBM mainstream at IBM's Cambridge
Scientific Center, in cooperation with
MIT researchers. CP/CMS
eventually won wide acceptance, and led to the development of VM/370
(Virtual Machine) which had a primary interactive "sub" operating
system known as
VM/CMS (Conversational Monitoring System). This
evolved into today's z/VM.
The Model 20 offered a simplified and rarely used tape-based system
called TPS (Tape Processing System), and DPS (Disk Processing System)
that provided support for the 2311 disk drive. TPS could run on a
machine with 8 KB of memory; DPS required 12 KB, which was
pretty hefty for a Model 20. Many customers ran quite happily with
4 KB and CPS (Card Processing System). With TPS and DPS, the card
reader was used to read the
Job Control Language cards that defined
the stack of jobs to run and to read in transaction data such as
customer payments. The operating system was held on tape or disk, and
results could also be stored on the tapes or hard drives. Stacked job
processing became an exciting possibility for the small but
adventurous computer user.
A little-known and little-used suite of 80-column punched-card utility
programs known as Basic Programming Support (BPS) (jocularly: Barely
Programming Support), a precursor of TOS, was available for smaller
IBM created a new naming system for the new components created for
System/360, although well-known old names, like
IBM 1403 and
were retained. In this new naming system, components were given
four-digit numbers starting with 2. The second digit described the
type of component, as follows:
Arithmetic processors, for example the
IBM 2030, which was the CPU for
IBM System/360 Model 30.
Power supplies and other equipment intimately associated with
processors, for example the
IBM 2167 Configuration Unit.
Visual output devices, for example the
IBM 2250 and
IBM 2260 CRT
displays, and the
IBM 2203 line printer for the System/360 model 20.
Direct-access storage devices, for example the
IBM 2311 and
disk drives, the
IBM 2321 Data Cell;
Main storage such as the
IBM 2361 Large Capacity Storage (Core
Storage, Large Core Storage or LCS) and the
IBM 2365 Processor
Magnetic tape drives, for example the
IBM 2405 and
Punched card handling equipment, for example the
IBM 2501 (card
IBM 2520 (card punch);
IBM 2540 (reader/punch) and
(Multi-Function Card Machine or MFCM).
Paper tape handling equipment, for example the
IBM 2671 paper tape
Communications equipment, for example the
interactive terminal and the
IBM 2780 batch terminal.
Channels and controllers, for example the
IBM 2821 Control Unit, IBM
Miscellaneous devices, for example the
IBM 2914 Data Channel Switch
IBM 2944 Data Channel Repeater.
IBM developed a new family of peripheral equipment for System/360,
carrying over a few from its older 1400 series. Interfaces were
standardized, allowing greater flexibility to mix and match
processors, controllers and peripherals than in the earlier product
In addition, System/360 computers could use certain peripherals that
were originally developed for earlier computers. These earlier
peripherals used a different numbering system, such as the
chain printer. The 1403, an extremely reliable device that had already
earned a reputation as a workhorse, was sold as the 1403-N1 when
adapted for the System/360.
Also available were optical character recognition (OCR) readers IBM
IBM 1288 which could read Alpha Numeric (A/N) and Numeric
Hand Printed (NHP/NHW) Characters from Cashier's rolls of tape to full
legal size pages. At the time this was done with very large
optical/logic readers. Software was too slow and expensive at that
Most small systems were sold with an
IBM 1052-7 as the console
typewriter. This was tightly integrated into the CPU — the keyboard
would physically lock under program control. Certain high-end machines
could optionally be purchased with a 2250 graphical display, costing
upwards of US $100,000. The 360/85 used a 5450 display console that
was not compatible with anything else in the line; the later 3066
console for the 370/165 and 370/168 used the same basic display design
as the 360/85.
Direct access storage devices (DASD)
IBM 2311 disk drive
IBM S/360 and other
IBM mainframe HDDs
The first disk drives for System/360 were
IBM 2302s:60–65 and
The 156 KB/second 2302 was based on the earlier 1302 and was
available as a model 3 with two 112.79 MB modules:60 or as a
model 4 with four such modules.:60
The 2311, with a removable 1316 disk pack, was based on the
and had a theoretical capacity of 7.2 MB, although actual
capacity varied with record design.:31 (When used with a 360/20,
the 1316 pack was formatted into fixed-length 270 byte sectors, giving
a maximum capacity of 5.4MB.)
In 1966, the first 2314s shipped. This device had up to eight usable
disk drives with an integral control unit; there were nine drives, but
one was reserved as a spare. Each drive used a removable 2316 disk
pack with a capacity of nearly 28 MB. The disk packs for the 2311
and 2314 were physically large by today's standards — e.g., the 1316
disk pack was about 14 in (36 cm) in diameter and had six
platters stacked on a central spindle. The top and bottom outside
platters did not store data. Data were recorded on the inner sides of
the top and bottom platters and both sides of the inner platters,
providing 10 recording surfaces. The 10 read/write heads moved
together across the surfaces of the platters, which were formatted
with 203 concentric tracks. To reduce the amount of head movement
(seeking), data was written in a virtual cylinder from inside top
platter down to inside bottom platter. These disks were not usually
formatted with fixed-sized sectors as are today's hard drives (though
this was done with CP/CMS). Rather, most System/360 I/O software could
customize the length of the data record (variable-length records), as
was the case with magnetic tapes.
IBM 2314 disk drives and
IBM 2540 card reader/punch at the University
Some of the most powerful early System/360s used high-speed
head-per-track drum storage devices. The 3,500 RPM 2301,
which replaced the 7320, was part of the original System/360
announcement, with a capacity of 4 MB. The 303.8 KB/second
IBM 2303:74–76 was announced on January 31, 1966, with a
capacity of 3.913 MB. These were the only drums announced for
System/360 and System/370, and their niche was later filled by
The 6,000 RPM 2305 appeared in 1970, with capacities of 5 MB
(2305-1) or 11 MB (2305-2) per module. Although these
devices did not have large capacity, their speed and transfer rates
made them attractive for high-performance needs. A typical use was
overlay linkage (e.g. for OS and application subroutines) for program
sections written to alternate in the same memory regions. Fixed head
disks and drums were particularly effective as paging devices on the
early virtual memory systems. The 2305, although often called a "drum"
was actually a head-per-track disk device, with 12 recording surfaces
and a data transfer rate up to 3 MB per second.
Rarely seen was the
IBM 2321 Data Cell, a mechanically complex
device that contained multiple magnetic strips to hold data; strips
could be randomly accessed, placed upon a cylinder-shaped drum for
read/write operations; then returned to an internal storage cartridge.
IBM Data Cell [noodle picker] was among several
"speedy" mass online direct-access storage peripherals (reincarnated
in recent years as "virtual tape" and automated tape librarian
peripherals). The 2321 file had a capacity of 400 MB, at the time
when the 2311 disk drive only had 7.2 MB. The
IBM Data Cell was
proposed to fill cost/capacity/speed gap between magnetic
tapes—which had high capacity with relatively low cost per stored
byte—and disks, which had higher expense per byte. Some
installations also found the electromechanical operation less
dependable and opted for less mechanical forms of direct-access
The Model 44 was unique in offering an integrated single-disk drive as
a standard feature. This drive used the 2315 "ramkit" cartridge and
provided 1,171,200 bytes of storage.:11
IBM 2401 tape drives
The 2400 tape drives consisted of a combined drive and control unit,
plus individual 1/2" tape drives attached. With System/360, IBM
IBM 7 track to
9 track tape
9 track tape format. 2400 drives could be
purchased that read and wrote 7 track tapes for compatibility with the
IBM 729 tape drives. In 1967, a slower and cheaper pair of tape
drives with integrated control unit was introduced: the 2415. In 1968,
IBM 2420 tape system was released, offering much higher data
rates, self-threading tape operation and 1600bpi packing density. It
remained in the product line until 1979.
Unit record devices
IBM 1403 line printer
Punched card devices included the 2501 card reader and the 2540 card
reader punch. Virtually every System/360 had a 2540. The 2560 MFCM
("Multi-Function Card Machine") reader/sorter/punch, listed above, was
for the Model 20 only. It was notorious for reliability problems
(earning humorous acronyms often involving "...Card Muncher" or
"Mal-Function Card Machine").
Line printers were the
IBM 1403 and the slower
A paper tape reader, the
IBM 2671, was introduced in 1964. It had a
rated speed of 1,000 cps. There were also a paper tape reader and
paper tape punch from an earlier era, available only as RPQs (Request
Price Quotation). The 1054 (reader) and 1055 (punch), which were
carried forward (like the 1052 console typewriter) from the
Teleprocessing System. All these devices operated at a maximum of 15.5
characters per second. The paper tape punch from the
IBM 1080 System
was also available by RPQ, but at a prohibitively expensive price.
Optical Character Recognition (OCR) devices 1287 and latter the 1288
were available on the 360's. The 1287 could read handwritten numerals,
some OCR fonts, and cash register OCR paper tape reels. The 1288 'page
reader' could handle up to legal size OCR font typewritten pages, as
well as handwritten numerals. Both of these OCR devices employed a
'flying spot' scanning principle, with the raster scan provided by a
large CRT, and the reflected light density changes were picked up by a
high gain photomultiplier tube.
MICR (Magnetic Ink Character Recognition) was provided by the
and 1419 Cheque Sorters, with Magnetic Ink Printing (for cheque books)
on 1445 Printers (a modified 1443 that used an MICR ribbon). 1412/1419
and 1445 were mainly used by Banking Institutions.
Few of these machines remain. Despite being sold or leased in very
large numbers for a mainframe system of its era, only a few System/360
computers still exist, and few of them still run. Most machines were
scrapped when they could no longer profitably be leased, certainly for
the value of the gold and other precious metal content of their
circuits but possibly also to keep these machines from competing with
IBM's newer computers, such as the System/370. As with all classic
mainframe systems, complete System/360 computers were prohibitively
large to put in storage, and too expensive to maintain.
Living Computer Museum
Living Computer Museum has a Model 20 running, with emulated card
reader and punch, on public display.
Computer History Museum
Computer History Museum in
Mountain View, CA
Mountain View, CA has a non-working
Model 30 on display, as do the Museum of Transport and Technology
(Motat) in Auckland, New Zealand and the Vienna University of
Technology in Austria. The
IBM Endicott History and Heritage Center in
Endicott, NY has a non-working Model 30 and an associated 2401
magnetic tape drive on display.
University of Western Australia
University of Western Australia Computer Club has a complete Model
40 in storage.
Smithsonian Institution owns a Model 65, though it is no longer on
IBM museum in
Sindelfingen has two System/360s – a Model 20 and
a Model 91.
The control panel of a complex of System/360 model types built for the
FAA as the
IBM 9020 is on display in the Computer Science department
Stanford University as
IBM 360 display and Stanford Big Iron. In
its maximum configuration it could comprise up to 12 System/360 Model
65s and Model 50s. It was manufactured in 1971 and decommissioned in
Summary of models announced but never shipped
Six of the twenty
IBM System/360 models announced never shipped / were
Models 60, 62, 70: announced along with the Models 30, 40 and 50. The
60 and 62 were replaced by the Model 65, the 70 by the 75.
Models 64 and 66: These were replaced by the Model 67.
What was planned as the Model 70 was replaced by the Model 75.
What was planned as the Model 92 was replaced by the Model 91.
Summary of models shipped
Fourteen of the twenty
IBM System/360 models announced did ship:
20, 22, 25, 30, 40, 44, 50, 65, 67, 75, 85, 91, 95, 195
This gallery shows the operator's console, with register value lamps,
toggle switches (middle of pictures), and "emergency pull" switch
(upper right of pictures) of the various models.
In popular culture
In the US television series
Mad Men (2007–2015), the "
IBM 360" was
featured as a plot device in which a company leased the system to the
advertising agency and was a prominent background in the seventh
History of IBM
Bob O. Evans
Spectra 70 had radically different architecture for
interrupts and I/O. There were compatibility packages to allow
operating systems for System/360 to run on a Spectra/70 and vice
^ Intended for real-time processing, the
English Electric System 4
employed four processor states, each with its own set of general
purpose registers. Instructions available in the user state were
identical to the System 360. The other states were entered according
to the class or severity of interrupt. The fourth (the highest) state
was entered when power failure was imminent, and enabled the processor
to shut itself down in an orderly fashion.
^ Performance calculated (not measured) based on a mix of instructions
typical of scientific applications ("Gibson Mix") with the results in
kilo Instructions Per Second (kIPS) per Longbottom, Roy. "Computer
Speeds From Instruction Mixes - pre-1960 to 1971". Retrieved October
12, 2014. except for M95 and M195. The latter based upon
estimates of performance relative to M65 from Pugh.
^ Using commercial instruction mix ("ADP Mix")
^ In System/360 architecture bit 12 of the program status word (PSW)
controlled selection between the
EBCDIC or a then proposed ASCII-8
mode signed decimal data. The proposed ASCII-8
ANSI standard was in
the approval process when System/360 was announced but it was
subsequently rejected and no
ASCII peripheral devices were made
available. This capability was not included in System/370; bit 12 of
the PSW was redefined to switch between System/360 (BC mode) and
System/370 (EC mode) PSW format.
IBM System/360 Dates and Characteristics". IBM.
^ "Why won't you DIE? IBM's S/360 and its legacy at 50".
^ "System 360/30 announcement". IBM.
^ "System 360 Model 91". IBM.
^ a b "System/360 Announcement" (press release),
IBM Data Processing
Division, April 7, 1964, webpage: IBM-PR360: states cycle time from
"...millionth-of-a-second to only 200 billionths-of-a-second," and
"...memory capacity ranges from 8,000 characters of information to
more than 8,000,000."
^ An Appreciation - John R. Opel, posted on www.ibm.com
^ DIGITAL COMPUTER NEWSLETTER, Office of Naval Research, Mathematical
Sciences Division, July 1965--pages 5-6:
IBM System/360 time-sharing
^ Elliott, Jim (2010). "
IBM Mainframes – 45+ Years of Evolution"
IBM Canada Ltd. p. 17. shows the announcement, ship
and withdrawal dates for all S/360 models other than the transient
models 64 and 66
System/370 Model 165 Theory of Operation (Volume 8)
709/7090/7094/7094-II Compatibility Feature. Second Edition. IBM.
February 1971. SY77-6835-0.
^ System/360, Model 30 1401 Compatibility Feature (PDF). IBM. April
1964. A24-3255-1. Mode status (System/360, Model 30, mode or 1401
compatibility mode) is set during the read-in of the compatibility
^ Emulating the
IBM 7094 on the
IBM Models 85 and 165 using OS/360
Program Number for M/85: 360C-EU-734 Program Number for M/165:
360C-EU-740 OS Release 20. Third Edition. IBM. November 1971.
^ a b Gray, George T.; Smith, Ronald Q. (2001). "Sperry Rand's
Third-Generation Computers 1964-1980". IEEE Annals of the History of
Computing. IEEE Computer Society. 23 (1): 3–16.
^ Account of Soviet cloning of the IBM-360, from Pioneers of Soviet
Computing by Boris Malinovsky
^ a b c d e f g Pugh, Emerson W.; Johnson, Lyle R.; Palmer, John H.
(1991). IBM's 360 and Early 370 Systems. MIT.
ISBN 0-262-16123-0. References are to Appendix A unless
page otherwise noted.
^ a b Padegs, A. (September 1981). "System/360 and Beyond". IBM
Journal of Research and Development. IBM. 25 (5): 377–390.
IBM System/360 Model 30 Functional Characteristics" (PDF). IBM.
IBM System/360 Model 40 Functional Characteristics" (PDF).
IBM System/360 Model 50 Functional Characteristics" (PDF). IBM.
IBM System/360 Model 20 Disk Programming System Control and Service
Programs" (PDF). IBM. March 1969.
IBM System/360 Model 91 Functional Characteristics" (PDF). IBM.
IBM System/360 Model 65 Functional Characteristics" (PDF). IBM.
IBM System/360 Model 75 Functional Characteristics" (PDF).
IBM System/360 Model 67 Functional Characteristics" (PDF). IBM.
IBM System/360 Model 44 Functional Characteristics" (PDF).
IBM System/360 Model 95". IBM.
IBM System/360 Model 25 Functional Characteristics" (PDF). IBM.
IBM System/360 Model 85 Functional Characteristics" (PDF). IBM.
IBM System/360 Model 195 Functional Characteristics" (PDF). IBM.
IBM System/360 Model 22". IBM.
^ NTIS (1979), I/O Channel Interface, National Technical Information
IBM System/360 Principles of Operation (PDF). First Edition. IBM.
IBM System/360 Principles of Operation. Ninth Edition (last
edition). Poughkeepsie, NY: IBM. November 1970. A22-6821-8.
IBM System/360 I/O Interface Channel to Control Unit Original
Equipment Manufacturers' Information (PDF). Fifth Edition. IBM.
IBM System/360 Model 67 Functional Characteristics (PDF). Third
Edition. IBM. February 1972. GA27-2719-2.
System/370 Principles of Operation (PDF). IBM. September 1975.
p. 189. GA22-7000-4. Retrieved December 30, 2015.
^ a b c d e
IBM System/360 Component Descriptions - 2841 and
Associated DASD (PDF). Eighth Edition. IBM. December 1969.
IBM System/360 Component Descriptions--2841 Storage Control Unit,
2302 Disk Storage, Models 3 and 4, 2311 Disk Storage Drive, 2321 Data
Cell Drive, Model 1, 7320 Drum Storage,
IBM Systems Reference Library,
IBM 2301 Drum Storage, Columbia University Computing History
IBM 2305 product announcement
^ Reference Manual for
IBM 2835 Storage Control and
IBM 2305 Fixed
Head Storage Module. Fifth Edition. IBM. November 1980.
IBM 2321 Data Cell Drive, Columbia University Computing History
IBM System/360 Model 44 Functional Characteristics
^ "University Computer Club".
Scanned manuals of
IBM System/360 — at bitsavers.org
IBM's announcement of the System/360
Dates of announcement, first ship and withdrawal of all models of the
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
"The Beginning of I.T. Civilization - IBM's System/360 Mainframe" by
Illustrations from “Introduction to
IBM Data Processing Systems”,
1968: contains photographs of
IBM System/360 computers and peripherals
IBM System 360 RPG Debugging
Template and Keypunch Card
Video of a two-hour lecture and panel discussion entitled The IBM
System/360 Revolution, from the
Computer History Museum
Computer History Museum on 2004-04-07
Original vintage film from 1964
IBM System/360 Computer History
Several photos of a dual processor
IBM 360/67 at the University of
Michigan's academic Computing Center in the late 1960s or early 1970s
are included in Dave Mills' article describing the Michigan Terminal
Pictures of an
IBM System/360 Model 67 at Newcastle (UK) University
Pugh, Emerson W. (1984). Memories That Shaped an Industry: Decisions
IBM System/360. MIT. p. 323.
IBM Journal of Research and Development
Amdahl, G. M.; Blaauw, G. A.; Brooks, F. P. (1964). "Architecture of
IBM Journal of Research and Development. 8 (2):
Davis, E. M.; Harding, W. E.; Schwartz, R. S.; Corning, J. J. (1964).
"Solid Logic Technology: Versatile, High-Performance
IBM Journal of Research and Development. 8 (2):
IBM Systems Journal
Blaauw, G. A.; Brooks, F. P. (1964). "The structure of SYSTEM/360:
Part I—Outline of the logical structure".
IBM Systems Journal. 3
(2): 119–135. doi:10.1147/sj.32.0118.
Stevens, W. Y. (1964). "The structure of SYSTEM/360, Part II: System
IBM Systems Journal. 3 (2): 136–143.
Amdahl, G. M. (1964). "The structure of SYSTEM/360, Part III:
Processing unit design considerations".
IBM Systems Journal. 3 (2):
Padegs, A. (1964). "The structure of SYSTEM/360, Part IV: Channel
IBM Systems Journal. 3 (2): 165–179.
Blaauw, G. A. (1964). "The structure of SYSTEM/360, Part V:
IBM Systems Journal. 3 (2): 181–195.
Tucker, S. G. (1967). "Microprogram control for SYSTEM/360". IBM
Systems Journal. 6 (4): 222–241. doi:10.1147/sj.64.0222.
Wikimedia Commons has media related to
IBM System/360 mainframe line.
This article is based on material taken from the Free On-line
Dictionary of Computing prior to 1 November 2008 and incorporated
under the "relicensing" terms of the GFDL, version 1.3 or later.
IBM System/360 Computers