IBM System/370

The IBM System/370 is a family of powerful computers that IBM introduced on June 30, 1970, as the successors to the System/360. These machines were important because they could run older programs easily, thanks to their backward compatibility, and they offered better speed and performance. Like the System/360 before them, the System/370 became very popular and successful for IBM.

One big change in the System/370 was inside the computer chips. They moved from older technology to newer, smaller circuits that could fit more features into the same space. This allowed the computers to include things like floating-point support as standard, making them better for many tasks.

When the System/370 was first introduced, many people wanted a feature called virtual memory, which the early models did not have. This changed in 1972 when IBM added this feature with new hardware called dynamic address translation (DAT). After that, many new operating systems could use this feature, making the computers even more powerful and flexible.

The System/370 was IBM's main large computer for many years, from the 1970s through the 1980s. Other companies tried to compete with IBM by making similar machines. In 1990, IBM introduced a new line of computers called the System/390, which replaced the System/370 with even more advanced technology.

Evolution

The original IBM System/370 line started on June 30, 1970. The first machines, Models 155 and 165, began shipping in early 1971. Over its 20 years, the System/370 saw many improvements.

These machines kept most of the features from the earlier System/360 but added some new ones. They also used newer technology inside, which made them more reliable and powerful.

Some new features included better ways to handle data, new instructions for calculations, and support for more memory. These changes helped make the System/370 very successful and popular for big business uses.

Series and models

Models sorted by date introduced (table)

The following table shows the main IBM System/370 series and models. Many models could work in different ways, so some could use more than one type of technology. A special name, "System/370-compatible," was sometimes used by IBM for certain machines. Other companies made systems that could run the same software, and IBM seemed to ignore them even though they were strong competitors.

Models grouped by Model number (detailed)

IBM called the System/370 family many different machines with three-digit numbers:

System/370 Model 115

The IBM System/370 Model 115 started on March 13, 1973. It was good for people who already used older IBM machines. It came with at least two disk drives and could have up to four.

System/370 Model 125

The IBM System/370 Model 125 began on October 4, 1972. It had up to four big disk storage units and different amounts of memory.

System/370 Model 135

The IBM System/370 Model 135 started on March 8, 1971. It had choices for memory size and could copy programs from older IBM machines.

System/370 Model 138

The IBM System/370 Model 138 began on June 30, 1976. It had more memory than the Model 135 and could be updated later.

System/370 Model 145

The IBM System/370 Model 145 started on September 23, 1970. It was the first to use a new kind of memory and could copy programs from older machines.

System/370 Model 148

The IBM System/370 Model 148 had the same start and end dates as the Model 138. Older Model 135 and 145 machines could be updated to work like the 148.

System/370 Model 155

The IBM System/370 Model 155 and the Model 165 started on June 30, 1970. They could copy programs from many older IBM machines. In 1972, the Model 155 got an update to add more features.

System/370 Model 158

The IBM System/370 Model 158 and the 370/168 began on August 2, 1972. They could handle new kinds of memory and work with other machines.

System/370 Model 165

The IBM System/370 Model 165 was stronger than the 155. It started in April 1971 and could copy programs from older IBM machines. In 1972, it got an update to add more features.

System/370 Model 168

The IBM System/370 Model 168 had up to eight megabytes of memory, which was twice as much as the 158. It could handle new kinds of memory and was IBM's top machine at the time.

System/370 Model 195

The IBM System/370 Model 195 began on June 30, 1970 and was IBM's most powerful machine then.

System/370-compatible

Starting in 1977, IBM began calling new machines "A compatible member of the System/370 family."

IBM 303X

Main article: IBM 303X

The first of these, the IBM 3033, started on March 25, 1977. It was very fast and later got a version with two processors.

IBM 308X

Main article: IBM 308X

Next came the IBM 308X series:

• The 3081 (started November 12, 1980) had two processors
• The 3083 (started March 31, 1982) had one processor
• The 3084 (started September 3, 1982) had four processors

IBM 3090

Main article: IBM 3090

The IBM 3090 series started on February 12, 1985. They had two or four processors and later got more models with different powers.

IBM 4300

Main article: IBM 4300

The IBM 4300 series started on January 30, 1979. They were smaller machines but still worked like the System/370.

IBM 9370

Main article: IBM 9370

The IBM 9370 began on October 7, 1986. It was small enough for offices and could grow as needs changed.

Clones

Other companies made machines that could run IBM software. These included:

• Amdahl Corporation 470 series
• ES EVM
• Fujitsu
• Hitachi
• Magnuson Computer Systems
• Mitsubishi
• Siemens
• Two Pi Corporation
• Univac

Architecture details

The IBM System/370 has a special way of organizing data, called big endian, where the most important piece of data comes first. It includes many parts that help it run programs, like registers for storing numbers, a program status word that keeps track of important settings, and a way to handle interruptions.

The system also has memory with 8 bits in each byte and special areas for communication. It includes ways for operators to start up the system, interrupt it, reset it, and debug issues. There are also options to add features like virtual memory and extra instructions for handling floating-point numbers.

IBM made sure that most programs written for the older System/360 would still work on the System/370. They carefully planned changes so that regular programs would keep running, even when new features were added. For example, a small part of the system’s settings tells whether a program needs the older 24-bit addressing or the newer 31-bit addressing, allowing both to work together. However, some parts, especially those needed for the operating system, had to be updated to work with the new features.

S/370 replacement

IBM updated the System/370 line to the System/390 in the 1990s, making a small change that kept all older programs working. Later, in 2000, the System/390 became the zSeries, now known as IBM Z. This brought a big improvement with 64-bit z/Architecture, while still supporting older programs from the original S/360.

GCC and Linux on the S/370

The GNU Compiler Collection (GCC) used to support the S/370 computers, but this support became outdated. It was replaced with a version that works on newer S/390 machines. Today, GCC works best on machines with the full abilities of System/390 Generation 5 (G5), which was used when Linux/390 first came out. However, there is a special version of GCC called GCCMVS that still works on older S/370 systems.

Although the S/370 and S/390 use very similar instructions, the older support for S/370 was stopped.

I/O evolutions

I/O evolution from original S/360 to S/370

Main article: IBM System/360 § Channels

The block multiplexer channel, which was only on certain earlier machines, became a standard part of the system. It could also work as a selector channel. These channels came in two speeds: one for single bytes and one for double bytes.

I/O evolution since original S/370

When new updates were announced, IBM improved the channels with a feature called Indirect Data Address Lists.

Data streaming channels started at a speed of 3.0 MB/s and later increased to 4.5 MB/s.

If one processor in a system stopped working or needed maintenance, another processor could take over its tasks.

System/370-XA introduced a new channel subsystem that handled tasks the operating system used to manage.

The System/390 brought in the ESCON channel, which used optical fiber and could reach up to 43 kilometers. It started at 10 Mbyte/s and later increased to 17 Mbyte/s.

Later, FICON became the standard channel for IBM mainframes. Fibre Channel Protocol allowed connecting certain devices using the same system as FICON.