ACM Turing Lecture 1972.docx

1、ACM Turing Lecture 1972ACM Turing Lecture 1972EWD340The Humble ProgrammerbyEdsger W. DijkstraAs a result of a long sequence of coincidences I entered the programming profession officially on the first spring morning of 1952 and as far as I have been able to trace, I was the first Dutchman to do so i

2、n my country. In retrospect the most amazing thing was the slowness with which, at least in my part of the world, the programming profession emerged, a slowness which is now hard to believe. But I am grateful for two vivid recollections from that period that establish that slowness beyond any doubt.

3、After having programmed for some three years, I had a discussion with A. van Wijngaarden, who was then my boss at the Mathematical Centre in Amsterdam, a discussion for which I shall remain grateful to him as long as I live. The point was that I was supposed to study theoretical physics at the Unive

4、rsity of Leiden simultaneously, and as I found the two activities harder and harder to combine, I had to make up my mind, either to stop programming and become a real, respectable theoretical physicist, or to carry my study of physics to a formal completion only, with a minimum of effort, and to bec

5、ome., yes what? A programmer? But was that a respectable profession? For after all, what was programming? Where was the sound body of knowledge that could support it as an intellectually respectable discipline? I remember quite vividly how I envied my hardware colleagues, who, when asked about their

6、 professional competence, could at least point out that they knew everything about vacuum tubes, amplifiers and the rest, whereas I felt that, when faced with that question, I would stand empty-handed. Full of misgivings I knocked on van Wijngaardens office door, asking him whether I could speak to

7、him for a moment; when I left his office a number of hours later, I was another person. For after having listened to my problems patiently, he agreed that up till that moment there was not much of a programming discipline, but then he went on to explain quietly that automatic computers were here to

8、stay, that we were just at the beginning and could not I be one of the persons called to make programming a respectable discipline in the years to come? This was a turning point in my life and I completed my study of physics formally as quickly as I could. One moral of the above story is, of course,

9、 that we must be very careful when we give advice to younger people; sometimes they follow it!Another two years later, in 1957, I married and Dutch marriage rites require you to state your profession and I stated that I was a programmer. But the municipal authorities of the town of Amsterdam did not

10、 accept it on the grounds that there was no such profession. And, believe it or not, but under the heading profession my marriage act shows the ridiculous entry theoretical physicist!So much for the slowness with which I saw the programming profession emerge in my own country. Since then I have seen

11、 more of the world, and it is my general impression that in other countries, apart from a possible shift of dates, the growth pattern has been very much the same.Let me try to capture the situation in those old days in a little bit more detail, in the hope of getting a better understanding of the si

12、tuation today. While we pursue our analysis, we shall see how many common misunderstandings about the true nature of the programming task can be traced back to that now distant past.The first automatic electronic computers were all unique, single-copy machines and they were all to be found in an env

13、ironment with the exciting flavour of an experimental laboratory. Once the vision of the automatic computer was there, its realisation was a tremendous challenge to the electronic technology then available, and one thing is certain: we cannot deny the courage of the groups that decided to try and bu

14、ild such a fantastic piece of equipment. For fantastic pieces of equipment they were: in retrospect one can only wonder that those first machines worked at all, at least sometimes. The overwhelming problem was to get and keep the machine in working order. The preoccupation with the physical aspects

15、of automatic computing is still reflected in the names of the older scientific societies in the field, such as the Association for Computing Machinery or the British Computer Society, names in which explicit reference is made to the physical equipment.What about the poor programmer? Well, to tell th

16、e honest truth: he was hardly noticed. For one thing, the first machines were so bulky that you could hardly move them and besides that, they required such extensive maintenance that it was quite natural that the place where people tried to use the machine was the same laboratory where the machine h

17、ad been developed. Secondly, his somewhat invisible work was without any glamour: you could show the machine to visitors and that was several orders of magnitude more spectacular than some sheets of coding. But most important of all, the programmer himself had a very modest view of his own work: his

18、 work derived all its significance from the existence of that wonderful machine. Because that was a unique machine, he knew only too well that his programs had only local significance and also, because it was patently obvious that this machine would have a limited lifetime, he knew that very little

19、of his work would have a lasting value. Finally, there is yet another circumstance that had a profound influence on the programmers attitude to his work: on the one hand, besides being unreliable, his machine was usually too slow and its memory was usually too small, i.e. he was faced with a pinchin

20、g shoe, while on the other hand its usually somewhat queer order code would cater for the most unexpected constructions. And in those days many a clever programmer derived an immense intellectual satisfaction from the cunning tricks by means of which he contrived to squeeze the impossible into the c

21、onstraints of his equipment.Two opinions about programming date from those days. I mention them now, I shall return to them later. The one opinion was that a really competent programmer should be puzzle-minded and very fond of clever tricks; the other opinon was that programming was nothing more tha

22、n optimizing the efficiency of the computational process, in one direction or the other.The latter opinion was the result of the frequent circumstance that, indeed, the available equipment was a painfully pinching shoe, and in those days one often encountered the naive expectation that, once more po

23、werful machines were available, programming would no longer be a problem, for then the struggle to push the machine to its limits would no longer be necessary and that was all what programming was about, wasnt it? But in the next decades something completely different happened: more powerful machine

24、s became available, not just an order of magnitude more powerful, even several orders of magnitude more powerful. But instead of finding ourselves in the state of eternal bliss of all progamming problems solved, we found ourselves up to our necks in the software crisis! How come?There is a minor cau

25、se: in one or two respects modern machinery is basically more difficult to handle than the old machinery. Firstly, we have got the I/O interrupts, occurring at unpredictable and irreproducible moments; compared with the old sequential machine that pretended to be a fully deterministic automaton, thi

26、s has been a dramatic change and many a systems programmers grey hair bears witness to the fact that we should not talk lightly about the logical problems created by that feature. Secondly, we have got machines equipped with multi-level stores, presenting us problems of management strategy that, in

27、spite of the extensive literature on the subject, still remain rather elusive. So much for the added complication due to structural changes of the actual machines.But I called this a minor cause; the major cause is. that the machines have become several orders of magnitude more powerful! To put it q

28、uite bluntly: as long as there were no machines, programming was no problem at all; when we had a few weak computers, programming became a mild problem, and now we have gigantic computers, programming had become an equally gigantic problem. In this sense the electronic industry has not solved a sing

29、le problem, it has only created them, it has created the problem of using its products. To put it in another way: as the power of available machines grew by a factor of more than a thousand, societys ambition to apply these machines grew in proportion, and it was the poor programmer who found his jo

30、b in this exploded field of tension between ends and means. The increased power of the hardware, together with the perhaps even more dramatic increase in its reliability, made solutions feasible that the programmer had not dared to dream about a few years before. And now, a few years later, he had t

31、o dream about them and, even worse, he had to transform such dreams into reality! Is it a wonder that we found ourselves in a software crisis? No, certainly not, and as you may guess, it was even predicted well in advance; but the trouble with minor prophets, of course, is that it is only five years

32、 later that you really know that they had been right.Then, in the mid-sixties, something terrible happened: the computers of the so-called third generation made their appearance. The official literature tells us that their price/performance ratio has been one of the major design objectives. But if y

33、ou take as performance the duty cycle of the machines various components, little will prevent you from ending up with a design in which the major part of your performance goal is reached by internal housekeeping activities of doubtful necessity. And if your definition of price is the price to be paid for the hardware, little will prevent yo