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Section 3

The IBM System/360- a series of planned machines which span a wide performance range

In this introduction, besides making some general comments on the IBM System/360, we will attempt an analysis of the performance and costs of the series. Performance is notoriously difficult to measure, as we noted in Chap. 3, and costs are even more so. With respect to the latter, what is publicly available are price data, not manufacturing-cost data.

These prices reflect not only marketing policies but also accounting policies within the organization for the attribution of costs to product lines. For example, we have had to determine Pc and Mp prices on the basis of incremental Mp prices within a C. Nevertheless, the 360 series provides two things which make a comparative analysis worthwhile. First, the common ISP makes simple performance measures more comparable; second, the common manufacturer makes relative prices more a reflection of relative costs than would otherwise be the case. Neither of these aspects is perfect, as we will note at several points in the discussion. Nevertheless, the 360 series provides as good an opportunity to attempt cost/performance analysis as we know. Indeed, this opportunity has already been grasped in a paper by Solomon [1966], which we have found very valuable and use to provide a basis of Pc power.

Analyses of the type we attempt here produce only rather crude pictures and are subject to question if all the input data are not very carefully checked. We have not done the latter, depending instead on published sources. For the purpose of this book, illustration of the style of analysis seems sufficient. In addition, using a performance measure based only on Pc power measurements, as we do here, leaves many questions unanswered because it does not address the soft areas of analysis relating to throughput, task environment, and the operating system software.

Unlike the other introductions in this book, the reader may find it worthwhile to scan this one, read the chapters in the section, and then return to this introduction when the system has become somewhat familiar.

The IBM System/360 is the name given to a third-generation series of computers which constitute the current primary IBM product line. They all have a common ISP but differ in interpreter speeds and PMS structure. Many PMS elements are used in common, particularly K's, Ms's, and T's.

The System/360 series is presented both because IBM's market dominance makes it the most prevalent current computer and because its implementations span the largest performance and price range of any series. The C('360) models should be compared with one another (Table 1) to be aware of their capabilities. Their introduction dates and their relationship are shown in Fig. 1. Chapters 43, 44, and 32 discuss the logical structure of the system, the implementations,1 and the microprogrammed Model 30.

A succinct description of the design goals and innovations is given in the abstract of the paper Architecture of the IBM System 360 [Amdahl et al., 1964a]:

Fig. 1. IBM System/360 models introduction dates.

1Chapters 43 and 44 are from IBM Systems Journal, vol. 3, no. 2,1964, which was devoted exclusively to the System/360. The other articles (listed in the bibliography) are recommended for additional details.


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