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NATO SOFTWARE ENGINEERING CONFERENCE 1968
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8. Invited Addresses
‘inventory’ need not be nearly 300 times the cost of keeping one routine. Automated techniques exist for generating approximations of different degrees of precision. Various editing and binding techniques are possible for inserting or deleting code pertinent to each degree of robustness. Perhaps only the floating-vs-fixed dichotomy would actually necessitate fundamentally different routines. Thus it seems that the basic inventory would not be hard to create.
The example of the sine routine re-emphasizes an interesting fact about this business. It is safe to assert that almost all sines are computed in floating point these days, yet that would not justify discarding the fixed point option, for that could well throw away a large part of the business in distinct tailor-made routines for myriads of small process-control and other real-time  applications on all sorts of different hardware. ‘Mass production’ of software means multiplicity of what manufacturing industry would call ‘models,’ or ‘sizes’ rather than multiplicity of replicates of each.
Parameterized Families of Components
One phrase contains much of the secret of making families of software components: ‘binding time.’ This is an ‘in’ phrase this year, but it is more popular in theory than in the field. Just about the only applications of multiple binding times I can think of are sort generators and the so-called ‘Sysgen’ types of application: filling in parameters at the time routines are compiled to control table sizes, and to some extent to control choice among several bodies of code. The best known of these, IBM’s OS/360 Sysgen is indeed elaborate — software houses have set themselves up as experts on this job. Sysgen differs, though, in a couple of ways from what I have in mind as the way a software components industry might operate.
First, Sysgen creates systems not by construction, but rather by excision, from an intentionally fat model. The types of adjustment in Sysgen are fairly limited. For example it can allocate differing amounts of space to a compiler, but it can’t adjust the width of list link fields in proportion to the size of the list space. A components industry on the other hand, not producing components for application to one specific system, would have to be flexible in more dimensions, and would have to provide routines whose niches in a system were less clearly delineated.
Second, Sysgen is not intended to reduce object code or running time. Typically Sysgen provides for the presetting of defaults, such as whether object code listings are or are not standard output from a compiler. The entire run-time apparatus for interrogating and executing options is still there, even though a customer might guarantee he’d never use it were it indeed profitable to refrain. Going back to the sine routine, this is somewhat like building a low precision routine by computing in high precision and then carefully throwing away the less significant bits.
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Having shown that sysgen isn’t the exact pattern for a components industry, I hasten to add that in spirit it is almost the only way a successful components industry could operate. To purvey a rational spectrum of high quality components a fabricator would have to systemize his production. One could not stock 300 sine routines unless they were all in some sense instances of just a few models, highly parameterized, in which all but a few parameters were intended to be permanently bound before run time. One might call these early-bound parameters ‘sale time’ parameters.
Many of the parameters of a basic software component will be qualitatively different from the parameters of routines we know today. There will be at least
Choice of Precision. Taken in a generalized sense precision includes things like width of characters, and size of address or pointer fields.
Choice of Robustness. The exact tradeoff between reliability and compactness in space and time can strongly affect the performance of a system. This aspect of parameterization and the next will probably rank first in importance to customers.
Choice of Generality. The degree to which parameters are left adjustable at run time.
Choice of Time-space behavior.
Choice of Algorithm. In numerical routines, as exemplified by those in the CACM, this choice is quite well catered for already. For nonnumerical routines, however, this choice must usually be decided on the basis of folklore. As some nonnumerical algorithms are often spectacularly unsuitable for particular hardware, a wide choice is perhaps even more imperative for them.