The mainframe turns 50, or, why the IBM System/360 launch was the dawn of enterprise IT

In many ways, the modern computer era began in the New Englander Motor Hotel in Greenwich, Connecticut.


In many ways, the modern computer era began in the New Englander Motor Hotel in Greenwich, Connecticut.

It was there in 1961 that a task force of top IBM engineers met in secret to figure out how to build the next-generation IBM computer.

A new design was sorely needed. IBM already sold a number of successful though entirely separate computer lines, but they were becoming increasingly difficult to maintain and update.

"IBM in a sense was collapsing under the weight of having to support these multiple incompatible product lines," said Dag Spicer, chief content officer for the Computer History Museum, which maintains a digital archive on the creation and success of the System/360.

Fifty years ago on April 7, IBM announced the computer that the task force had designed, the System/360.

The system eventually became a huge success for the company -- and a good thing too. IBM's president at the time, Tom Watson, Jr., killed off other IBM computer lines and put the company's full force behind the System/360. IBM's revenue swelled to $8.3 billion by 1971, up from $3.6 billion in 1965. Through the 1970s, more than 70 percent of mainframes sold were IBM's. By 1982, more than half of IBM's revenue came from descendants of the System/360.

But its impact can be measured by more than just the success it brought to IBM.

"IBM was where everyone wanted to work," said Margaret McCoey, an assistant professor of computer science at La Salle University in Philadelphia, who also debugged operating system code for Sperry/Univac System/360 clones in the late 1970s.

The System/360 ushered in a whole new way of thinking about designing and building computer systems, a perspective that seems so fundamental to us today that we may not realise it was rather radical 50 years ago.

Before the System/360 introduction, manufacturers built each new computer model from scratch. Sometimes machines were even built individually for each customer. Software designed to run on one machine would not work on other machines, even from the same manufacturer. The operating system for each computer had to be built from scratch as well.

The idea hatched at the Connecticut hotel was to have a unified family of computers, all under a single architecture.

Gene Amdahl was the chief architect for the system and Fred Brooks was the project leader.

Amdahl would later coin Amdahl's Law, which, roughly stated, holds that any performance gains that come from breaking a computer task into parallel operations is offset by the additional overhead incurred by managing multiple threads. And Brooks would go on to write "The Mythical Man Month," which asserted a similar idea that adding more people to a software development project can actually slow development of the software, due to the additional burden of managing the extra people.

The idea they came up with was to have a common architecture shared among the lower-end, less expensive machines and the priciest high-speed models. The top-end models would perform 40 times as fast as the low-end models. Keep in mind that applying the word "architecture" to the design of a computer was all but unheard of in the early 1960s.

But specifying an architecture, rather than a specific implementation, paved the way for compatibility amongst different models.

"In designing a system with both upward and downward compatibility for both scientific and business customers, IBM was attempting to use a single architecture to meet the needs of an unprecedentedly large segment of its customers," according to a 1987 case study of the System/360 published by the Association for Computing Machinery. In fact, the "360" in the moniker was meant to indicate that the machine could serve all kinds of customers, small or large, business or scientific.

"The System/360 actually unified both business and computing threads in the market into one system," Spicer said.

While the idea seems obvious today, the concept of a unified family of computers had profound consequences for both IBM, its customers and the industry as a whole.

IBM was able to use a single OS for all of its computers (though it ended up creating three variants to span the different use cases). A lot of work writing software for separate computers was eliminated, allowing engineers to concentrate on new applications instead.

IBM saved a lot of resources on the hardware as well. No longer would components, such as processors and memory, need to be designed for each machine. Now different models could share general-purpose components, allowing IBM to enjoy more economies of scale.

Customers benefited as well. They could take code written for one System/360 machine and run it on another. Not only could users move their System/360 code to a bigger machine without rewriting it, but they could port it to a smaller model as well.

When an organisation bought a new computer in the early 1960s it "generally had to throw out all of its software, or at least rejigger it to work on the new hardware," Spicer said. "There was no idea of having computers that could run compatible software over the generations."

IBM has steadfastly maintained backward compatibility in the decades since. Programs for the original System/360s can still run, sometimes with only slight modification, on IBM mainframes today (which is not to say IBM hasn't aggressively urged customers to upgrade to the latest models for performance improvements).

Compare that longevity to one of IBM's largest competitors in the software market. This month, Microsoft ends support for its Windows XP OS after a mere decade since its release.

The System/360 and its successor System/370 continued to sell well into the 1970s, as punch cards were slowly replaced by IBM 3270 terminals, known as green screens.

Green screens changed the way the System360/370 could be used. Originally, they did batch processing, where a job was submitted via punch cards. The machine would churn through the data and return the results. The green screens paved the way for more interactive sessions with a machine, noted Greg Beedy, senior principal product manager at CA and a 45-year veteran of working on mainframes.

Beedy noted that the 3270 terminals were always 80 columns wide -- equal to the number of columns on a punch card.

Even after the introduction of terminals, programming was a much more tedious job back in the 1970s; today, programmers have "instant gratification," McCoey said.

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