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80
NATO SOFTWARE ENGINEERING CONFERENCE 1968
8. Invited Addresses
The pinnacle of software is systems — systems to the exclusion of almost all other considerations. Components, dignified as a hardware field, is unknown as a legitimate branch of software. When we undertake to write a compiler, we begin by saying ‘What table mechanism shall we build?’ Not, ‘What mechanism shall we use? but ‘What mechanism shall we build? I claim we have done enough of this to start taking such things off the shelf.
Software Components
My thesis is that the software industry is weakly founded, and that one aspect of this weakness is the absence of a software components subindustry. We have enough experience to perceive the outline of such a subindustry. I intend to elaborate this outline a little, but I suspect that the very name ‘software components’ has probably already conjured up for you an idea of how the industry could operate. I shall also argue that a components industry could  be immensely useful, and suggest why it hasn’t materialized. Finally I shall raise the question of starting up a ‘pilot plant’ for software components.
The most important characteristic of a software components industry is that it will offer families of routines for any given job. No user of a particular member of a family should pay a penalty, in unwanted generality, for the fact that he is employing a standard model routine. In other words, the purchaser of a component from a family will choose one tailored to his exact needs. He will consult a catalogue offering routines in varying degrees of precision, robustness, time-space performance, and generality. He will be confident that each routine in the family is of high quality — reliable and efficient. He will expect the routine to be intelligible, doubtless expressed in a higher level language appropriate to the purpose of the component, though not necessarily instantly compilable in any processor he has for his machine. He will expect families of routines to be constructed on rational principles so that families fit together as building blocks. In short, he should be able safely to regard components as black boxes.
Thus the builder of an assembler will be able to say ‘I will use a String Associates A4 symbol table, in size 500x8,’ and therewith consider it done. As a bonus he may later experiment with alternatives to this choice, without incurring extreme costs.
A Familiar Example
Consider the lowly sine routine. How many should a standard catalogue offer? Offhand one thinks of several dimensions along which we wish to have variability:
Precision, for which perhaps ten different approximating functions might suffice
Floating-vs-fixed computation
Argument ranges 0-π/2, 0-2 π, also -π/2 to π/2, -π to π, -big to +big
Robustness — ranging from no argument validation through signaling of complete loss of significance, to signaling of specified range violations

We have here 10 precisions, 2 scalings, 5 ranges and 3 robustnesses. The last range option and the last robustness option are actually arbitrary parameters specifiable by the user. This gives us a basic inventory of 300 sine routines. In addition one might expect a complete catalogue to include a measurement-standard sine routine, which would deliver (at a price) a result of any accuracy specified at run time. Another dimension of variability, which is perhaps difficult to implement, as it caters for very detailed needs is
Time-space tradeoff by table lookup, adjustable in several ‘subdimensions’:
(a) Table size
(b) Quantization of inputs (e.g., the inputs are known to be integral numbers of degrees)
Another possibility is
(c) Taking advantage of known properties of expected input sequences, for example profiting from the occurrence of successive calls for sine and cosine of the same argument.
A company setting out to write 300 sine routines one at a time and hoping to recoup on volume sales would certainly go broke. I can’t imagine some of their catalogue items ever being ordered. Fortunately the cost of offering such an