Machines that count: the IBM System 360

Computer historian John Pratt, curator of the "Machines that Count" exhibition now on at Auckland's Museum of Transport and Technology, provides weekly insights into the history of computing. This week: the IBM System 360 mainframe

Many people rank IBM’s System 360, alongside the Boeing 707 and the Model T Ford, as one of the most influential innovations of the 20th century. But was it?

The machine was five years and US$5 billion in the making, and it served to put even more distance between IBM and its competitors — none of which ever approached being more than 20% of IBM’s size. The System 360 pioneered the 8-bit byte, wide-area networking and high-volume hard disk storage — virtually all of the features we associate with the mainframe.

The System 360 led to a revolution in industry. For example, the SABRE airline reservation system was introduced on the System 360. And the machine also supported banking applications, just as ERMA (Electronic Recording Machine Accounting) proved that the computerisation of banking was worth the price. One of the first banks to go with a real-time online system anywhere was New Zealand’s ASB Bank, which took delivery of a System 360 in 1967.

Inside, the System 360 was quite pragmatic. The physical architecture of the machine followed the model established with the Model 604 electronic calculator. It was constructed around a tubular-steel chassis that carried cages full of relatively simple business-card sized cards — all interconnected by a wiring loom that would not look out of place in a car.

Still, the machine represented a relatively dense construction for the day. Tiny aluminium cans, 12mm square, contained three or four point-contact germanium transistors and some crude resistors. These were a kind of precursor to the more conventional silicon chips that were to follow with the System 370 in 1970.

They might sound a little prosaic today, but they were practically state of the art when the System 360 was being designed — five years before Intel was even founded. Only Texas Instruments or Fairchild might have done better, but they could not have produced the volumes IBM needed. By 1967, IBM was making these devices at the rate of 90 million every year, well and truly overshadowing other semiconductor industry players.

The System 360 Model 40 currently on display at Auckland’s Museum of Transport and Technology (MOTAT) was good for about 75,000 instructions per second and could manage a maximum of 512KB of RAM. This was provided by core memory. The standard configuration was either 64KB or 128KB.

The top-of-the-range Model 75, used by NASA, was good for a million instructions per second, with a maximum of 1MB of RAM.

Big Blue’s System 360 inherited the vacuum column tape-drives, card-readers and various other peripherals from the 7000-series machines, but it was also ready for the new-generation disk drives being developed by IBM.

This is where SABRE came in. One of the critical requirements of SABRE was access time: American Airlines wanted no more than three seconds of latency. It was a requirement that stimulated the development of the hard drive itself.

The 2314 disk system consisted of nine drawers, each containing a platter of 11 disks of 14 inches. It worked by having heads slide straight across the disk surface. These were located by an electro-hydraulic mechanism that could distinguish 120 tracks per inch. The total system was good for 230MB of storage.

As it became clear what a runaway success the System 360 was, it became the de facto standard for the industry. IBM didn’t just create a market for mainframes, it provided an architectural blueprint for competitors that didn’t just copy Big Blue’s mainframes but provided plug-compatible peripherals, too.

It all sounds a bit old-hat today, but in the 20 years it took microprocessor-based machines to approach mainframe levels of performance, billions of dollars worth of mainframes were sold. The last “conventional” mainframe was, in fact, the System 36, introduced in 1984.

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