IBM System/360

The IBM System/360 was a family of computers announced by IBM on April 7, 1964. These computers were special because they could do both business and scientific work. They came in many sizes, from small to very large ones called mainframes. This made it easy to start with a small computer and upgrade later if needed.

One key idea of the System/360 was to separate the computer’s design from how it was built. This let IBM make many models that all worked together, even though they cost different amounts. Most used a smart way to control actions called microcode. They could handle many types of numbers, like whole numbers, decimals, and special science numbers called hexadecimal and floating-point.

The System/360 was led by two important people: Gene Amdahl, who designed it, and Fred Brooks, who managed the project. The slowest model, the Model 30, could do about 34,500 actions each second. A faster model from 1967, the IBM System/360 Model 91, could do up to 16.6 million instructions per second. These computers could store a lot of information, up to 8 MB, though many held less.

The IBM System/360 became very popular because people could start with a small computer and know they could add more power later without changing their programs or equipment. It had a big effect on how computers were made for many years and is often called one of the most successful computers ever. Even today, ideas from the System/360 are still used in modern IBM Z mainframes.

System/360 history

Background

In the early 1960s, IBM had many different kinds of computers. It was hard to keep up with them all. People who bought an IBM computer for accounting couldn’t easily use it for engineering work. This meant they had to spend more money on new machines and programs for small improvements.

To solve this, IBM created a new plan called SPREAD. They wanted to build a family of computers that could all work together. These new machines would use the same basic instructions, no matter what job they were doing — like accounting or engineering. This way, people could switch between different machines without starting over with new programs.

"Family" concept

Instead of making one perfect computer for every job, IBM made a family of machines that shared the same basic design but had different speeds and features. Some parts of the machine could be changed with small programs stored inside, making it easier to adjust how the machine worked.

This flexibility let companies start with a smaller, cheaper machine and upgrade to a bigger one when they needed more power, without changing their programs. For example, a company using a computer for accounting could later add engineering work without buying a whole new system. The name “360” came from the idea of a full circle, showing how these machines could connect and work together in many ways.

Models

IBM first announced six computers and many parts that could work with them. Over time, they made 14 different models, including special ones for NASA. The smallest model, the Model 20, had just a little memory and fewer features, while bigger models like the Model 95 were for more complex jobs.

Some models were made just for specific tasks, like scientific work or controlling machines in real time. Even though these computers were very different inside, they all worked with each other in important ways. For example, the Model 67 was the first to have special hardware that let it handle many users at once.

Backward compatibility

IBM knew that many of their customers had already spent a lot of money on programs for older computers. So, some of the new System/360 models could mimic the old machines. This meant customers could keep using their old programs on the new computers, just by loading a special program first. Later, IBM made it even easier, so these old programs could run alongside the new ones.

Successors and variants

The System/360 was later replaced by the System/370, which was very similar. Some smaller System/360 models led to the IBM System/3. Many other companies also made computers that worked like the System/360, and even today, some special versions of these machines are still used in places like airplanes and air traffic control.

Table of System/360 models

Model summary

• Six of the twenty IBM System/360 models were never sold.
• Fourteen of the twenty IBM System/360 models were sold.

Technical description

Influential features

The System/360 brought several important standards to computers. It used an 8-bit byte, which was bigger than some earlier designs. It had memory that could be accessed by its position, and it used 32-bit words. The system also introduced a special way to connect devices and used a unique set of instructions for handling numbers. It included a special character set for text and a way to store information on magnetic tape.

Architectural overview

The System/360 was designed so that all its machines could work together even if they were different sizes. It had a basic set of rules for how the computer should act, including how it could handle many tasks at once and how it could protect its information. The system could also handle simple tasks by itself and had ways to fix problems if something went wrong.

Some extra features were available on certain models, like special ways to handle numbers and extra security for memory.

All System/360 machines, except two smaller models, followed these design rules.

The System/360 could handle basic math and logic tasks. Some models could also handle more complex number tasks if the right extra features were added. The system used bytes, words, and other sized pieces of data, and its instructions could have different lengths.

The System/360 separated the part of the computer that managed everything from the part that did the actual work. This helped keep the system safe and made it easier to fix problems.

Channels

See also: Channel I/O

Devices like printers and disks connected to the System/360 using special pathways called channels. These channels were like helpers that moved information between the main part of the computer and the outside devices. There were different types of channels for faster or slower devices.

Byte-multiplexor and selector channels

There were two main types of channels. Byte-multiplexor channels worked with slower devices like card readers and printers. Selector channels worked with faster devices like disks and tapes. Most System/360 machines had both types of channels, but smaller models might have only one type that could do both jobs.

The byte-multiplexor channel could talk to many devices at once, sharing the connection. Devices could send information in small or large groups, depending on how fast they were.

Selector channels were for very fast devices and could work with many devices attached to special units called control units.

Block multiplexer channel

Later models of the System/360 introduced a new type of channel called the block multiplexer channel. This channel could pause its work to let other devices use it, making it more efficient. It was first used with special disks and later became a standard feature.

Basic hardware components

The System/360 used special building blocks made from tiny parts placed on small squares. These blocks were then put together to make bigger parts of the computer. The way these parts were arranged let many small pieces work together to make the computer's brain.

Operating system software

Main article: System/360 operating systems

Smaller System/360 machines used simple operating systems like BOS/360 or DOS/360. Bigger machines used OS/360, which came in different versions to handle more tasks. One version, MVT, took longer to finish but became very popular.

For the Model 67, IBM also created TSS/360, a system that let many people use the computer at the same time. However, it had problems and was eventually replaced by other systems like CP-67 and VM/370.

The smallest machines could use a tape-based system called TPS or DPS to help with jobs and data, using just a little memory.

Component names

IBM made a new way to name the parts for the System/360. Some old names, like IBM 1403 and IBM 1052, were still used. In the new system, parts got four-digit numbers that started with 2. The second digit told what kind of part it was.

Peripherals

IBM made new devices to work with the System/360. These devices could work well together, letting different machines share the same equipment.

System/360 computers could also use some older devices, like the IBM 1403 chain printer, which was reliable and known for its strong performance.

The System/360 offered many ways for operators to control the computer, including special keyboards and screens. Some larger machines could even have very expensive graphical displays for more advanced tasks.

Direct access storage devices (DASD)

Further information: Direct-access storage device

See also: IBM S/360 and other IBM mainframe HDDs

The first disk drives for System/360 were the IBM 2302s^: 60–65  and IBM 2311s.^: 54–58  The first drum for System/360 was the IBM 7320.^: 41 

The 2311, with a removable disk pack, was based on the IBM 1311.

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. The disk packs for the 2311 and 2314 were large by today's standards.

Some of the most powerful early System/360s used high-speed drum storage devices. The 3,500 RPM 2301, which replaced the 7320, was part of the original System/360 announcement. The 303.8 kbit/s IBM 2303^: 74–76  was announced on January 31, 1966.

The 6,000 RPM 2305 appeared in 1970, with capacities of 5 MB (2305–1) or 11 MB (2305–2) per module.

Magnetic Tape drives

With System/360, IBM switched from IBM 7-track to 9-track tape format. Some 2400-series drives could be purchased that read and wrote 7-track tapes for compatibility with the older IBM 729 tape drives. In 1968, the IBM 2420 tape system was released, offering much higher data rates. It remained in the product line until 1979.

Unit record devices

• Punched card devices included the 2501 card reader and the 2540 card reader punch. Virtually every System/360 had a 2540.
• Line printers were the IBM 1403 and the slower IBM 1443.
• A paper tape reader, the IBM 2671, was introduced in 1964.
• Optical character recognition (OCR) devices 1287 and later the 1288 were available on the 360's.
• Magnetic ink character recognition (MICR) was provided by the IBM 1412 and 1419 cheque sorters. 1412/1419 and 1445 were mainly used by banking institutions.

Remaining machines

Many IBM System/360 computers were sold or leased in the past, but only a few still exist today. Most of these old computers are kept in museums or by collectors and are not working.

You can see these historic computers at the Computer History Museum in Mountain View, California. They are also at the Museum of Transport and Technology in Auckland, New Zealand, and the Vienna University of Technology in Austria. Other examples are stored at the University of Western Australia Computer Club and shown at the KCG Computer Museum of Kyoto Computer Gakuin in Japan. Two more are on loan to the System Source Computer Museum in the UK for display and restoration.