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The System/370

A Complete History Of Mainframe Computing
The System/370

Although the System/360 was very successful, and in some ways, revolutionary and innovative, it also eschewed leading-edge technologies that left opportunities for other companies to exploit. To its credit, however, it was still selling well even six years after its announcement, and it laid a foundation for generations that followed it, of which the System/370 was first.

The initial launch of the System/370 in 1970 consisted of just two machines, the charismatically named 155 (running at almost 8.70 MHz) and 165 (running at 12.5 MHz). Naturally, both machines were compatible with programs written for the System/360 and could even use the same peripherals. Additionally, the performance was greatly improved, with the System/370 165 offering close to five times the performance of the System/360 65, the fastest machine available from that line when it was released in November 1965.

There were also several new technologies for the System/370, compared to the System/360. IBM finally moved to the integrated circuit, a change many people thought long overdue. Most models in the line had transistor memory rather than core memory. The System/370 also finally supported dynamic address translation (on all but the initial two models), which was an important technology for time sharing and virtual memory. There was also a very high-speed memory cache (80 ns for the 165), which IBM called a buffer. This was used by the processor to mitigate the relatively slow (two microsecond or 2,000 ns) main memory access time. Another important consideration was that the System/370 was built from the beginning with dual processors and multiprogramming in mind.

So, while the System/370 was not a spectacular announcement, it did plug up some glaring holes in the System/360, improved speed considerably, expanded the instruction set, and maintained a high degree of compatibility. It was a solid step forward and maintained IBM's dominance in the mainframe world.

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