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562 Part 6 Computer families Section 3 The IBM System/360-a series of planned machines which span a wide performance range

The architecture* of the newly announced IBM System /360 features four innovations:

1 An approach to storage which permits and exploits very large capacities, hierarchies of speeds, read-only storage for microprogram control, flexible storage protection, and simple program relocation.

2 An input/output system offering new degrees of concurrent operation, compatible channel operation, data rates approaching 5,000,000 characters/second, integrated design of hardware and software, a new low-cost, multiple-channel package sharing mainframe hardware, new provisions for device status information, and a standard channel interface between central processing unit and input/output devices.

3 A truly general-purpose machine organization offering new supervisory facilities, powerful logical processing operations, and a wide variety of data formats.

4 Strict upward and downward machine-language compatibility over a line of six models having a performance range factor of 50.

The above four featured innovations are all stated as IBM Corporation design results. It seems better to analyze them in terms of design constraints and implementation results. It appears that the design constraints, from marketing and management directions, were compatibility (item 4 above) and the use of common peripheral equipment (item 2 above). Thus we can measure the 360 design in terms of how well it meets these constraints. With some minor exceptions, all the peripheral components existed at the time of the design and had been used with other IBM computers; thus a goal was already realized. A measure of the design can also be based on a comparison with alternative designs. In the following sections we suggest that several forms of multiprocessing would yield higher performance at lower cost. A difficult and important constraint, though not mentioned above, is the necessity of program compatibility with almost all earlier IBM computers.

It should be noted that, at the outset of the IBM System /360 announcement, another company, RCA, adopted the 360 ISP as a design constraint for its own future computer development. Although some price-performance characteristics appear to be better in the RCA series, the implementation scheme is similar. The lower RCA prices do not reflect entirely implementation and technology but include RCA marketing and profit strategy. In addition, of course, there should have been lower development costs.

An interesting aspect of the design is the method used to implement the individual computer models (of the range) and their associated costs. From the standpoint of innovation, the 360 was the first computer series to cover a wide range. The more basic P's (Models 20 ~ 65) were implemented via a microprogrammed processor. This is based on a computer program within an M(read only), i.e., a Read Only Storage/ROS, to interpret the common ISP. A payoff from this implementation strategy is a solution to the "compatibility design constraint," which is the ability to provide compatibility with the customer's previous (IBM) machine, which of course was not a member of the 360 series. This is undoubtedly the most difficult constraint to meet in the P designs, and probably the most significant real innovation. From the marketing viewpoint, it provided the user with a crutch to go from a former IBM computer to the System/360. This is accomplished through "emulation," which (as defined by IBM) means the ability of one C to interpret another's programs at a reasonable performance level. These emulations are realized by various microprogrammed P's being designed to interpret both the 360 ISP and one or more of IBM 704, 709, 1401, 1410, 1440, 1460, 1620, 7010, 7040, 7044, 7070, 7074, 7090, 7094.

Most of the above ISP's have a different structure from the 360 ISP. For example, the 1401 (Chap. 18) series instructions and data are variable-length character strings; the 1620 has variable-length data strings; the 704 series process fixed- and floating-point data with single-address instructions; and the 7070 is a fixed-word decimal computer. Thus the 360 C's represent the first machines to be two logical processors in the same physical implementation.

The emulated speeds are often better than that of the original hardwired computer. This is not surprising, considering the change in technology; it is a very attractive feature. The 360 Mp performance is often a factor of 5 to 10 times the "emulated" computers; and the M(ROS) data rates are a factor of 25 times the Mp's. For example, the Model 65 emulating a 7090 runs faster than a hardwired 7090 (Table 1). The use of an M(ROS) for defining an ISP is questionable if we ignore the emulation constraint. Note, by way of evidence, that the hard-wired models 91 and 44 have the lowest cost-to-performance ratios in the series.

There are minor deviations in the particular models, but all

*The term architecture is used here to describe the attributes of a system as seen by the programmer, i.e., the conceptual structure and functional behavior, as distinct from the organization of the data flow and controls, the logical design, and the physical implementation.

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