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 organization 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.
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