History of CP/CMS - IBM and The System/360

IBM and The System/360

In the early 60s, IBM was struggling to define its technical direction. The company had identified a problem with its past computer offerings: incompatibility between the many IBM products and product lines. Each new product family, and each new generation of technology, forced customers to wrestle with an entirely new set of technical specifications. IBM products incorporated a wide variety of processor designs, memory architectures, instruction sets, input/output strategies, etc. This was not, of course, unique to IBM. All computer vendors seemed to begin each new system with a "clean sheet" design. IBM saw this as both a problem and an opportunity. The cost of software migration was an increasing barrier to hardware sales. Customers could not afford to upgrade their computers, and IBM wanted to change this.

IBM embarked on a very risky undertaking: the System/360. This product line was intended to replace IBM's diverse earlier offerings, including the IBM 7000 series, the canceled IBM 8000 series, the IBM 1130 series, and various other specialized machines used for scientific and other applications. The System/360 would span an unprecedented range of processing power, memory size, device support, and cost; and more important, it was based on a pledge of backward compatibility, such that any customer could move software to a new system without modification. In today's world of standard interfaces and portable systems, this may not seem such a radical goal; but at the time, it was revolutionary. Before the System/360, each computer model often had its own specific devices and programs that could not be used with other systems. Buying a bigger CPU also meant buying new printers, card readers, tape drives, etc. In addition, customers would have to rewrite their programs to run on the new CPU, something customers often balked at. With the S/360, IBM wanted to offer a huge range of computer systems, all sharing a single processor architecture, instruction set, I/O interface, and operating system. Customers would be able to "mix and match" to meet current needs; and they could confidently upgrade their systems in the future, without the need to rewrite all their software applications. IBM's focus remained on its traditional customer base: large organizations doing administrative and business data processing.

At the start of System/360 project, IBM did not fully appreciate the amount of risk involved. The System/360 ultimately gave IBM total dominance over the computer industry; but at first, it nearly put IBM out of business. IBM took on one of the largest and most ambitious engineering projects in history, and in the process discovered diseconomies of scale and the mythical man-month. Extensive literature on the period, such as that by Fred Brooks, illustrate the pitfalls.

It was during the period of System/360 panic that Project MAC asked IBM to provide computers with extensive time-sharing capabilities. This was not the direction the System/360 project was going. Time-sharing wasn't seen as important to IBM's main customer base; batch processing was key. Moreover, time-sharing was new ground. Many of the concepts involved, such as virtual memory, remained unproven. For example: At the time, nobody could explain why the troubled Manchester/Ferranti Atlas virtual memory "didn't work". This was later explained as due to thrashing, based on CP/CMS and M44/44X research. As a result, IBM's System/360 announcement in April 1964 did not include key elements sought by the time-sharing advocates, particularly virtual memory capabilities. Project MAC researchers were crushed and angered by this decision. The System/360 design team met with Project MAC researchers, and listened to their objections; but IBM chose another path.

In February 1964, at the height of these events, IBM had launched its Cambridge Scientific Center (CSC), headed by Norm Rassmussen. CSC was to serve as the link between MIT researchers and the IBM labs, and was located in the same building with Project MAC. IBM fully expected to win the Project MAC competition, and to retain its perceived lead in scientific computing and time-sharing.

One of CSC's first projects was to submit IBM's Project MAC proposal. IBM had received intelligence that MIT was leaning toward the GE proposal, which was for a modified 600-series computer with virtual memory hardware and other enhancements; this would eventually become the GE 645. IBM proposed a modified S/360 that would include a virtual memory device called the "Blaauw Box" – a component that had been designed for, but not included in, the S/360. The MIT team rejected IBM's proposal. The modified S/360 was seen as too different from the rest of the S/360 line; MIT did not want to use a customized or special-purpose computer for MULTICS, but sought hardware that would be widely available. GE was prepared to make a large commitment to time-sharing, while IBM was seen as obstructive. Bell Laboratories, another important IBM customer, soon made the same decision, and rejected the S/360 for time-sharing.