448 Part 5 The PMS level

must have the ability to be dynamically and automatically restructured to a working configuration that is responsive to the problem-mix environment.

Expansibility criteria. The requirement of expansibility is not unique to command and control, but is a desirable feature in any application of data processing equipment. However, the need for expansibility is more acute in command and control because of the dependence of much of the efficacy of the system upon an ability to meet the changing requirements brought on by the very rapidly changing technology of warfare. Further, it must be possible to incorporate new functions in such a way that little or no transitional downtime results in any hardware area.

Expansion should be possible without incurring the costs of providing more capability than is needed at the time. This ability of the system to grow to meet demands should apply not only to the conventionally expansible areas of memory and I/O but to computational devices, as well.

Programming criteria. Expansion of the data-processing facility should require no reprogramming of old functions, and programs for new functions should be easily incorporated into the overall system. To achieve this capability, programs must be written in a manner which is independent of system configuration or problem mix, and should even be interchangeable between sites performing like tasks in different geographic locales. Finally, because of the large volume of routines that must be written for a command and control system, it should be possible for many different people, in different locations and of different areas of responsibility, to write portions of programs, and for the programs to be subsequently linked together by a suitable operating system.

Concomitant with the latter requirement and with that of configuration-independent programs is the desirability of orienting system design and operation toward the use of a high-level procedure-oriented language. The language should have the features of the usual algorithmic languages for scientific computations, but should also include provisions for maintaining large files of data sets which may, in fact, be ill-structured. It is also desirable that the language reflect the special nature of the application; this is especially true when the language is used to direct the storage and retrieval of data.

The three requirements of availability, adaptability, and expansibility were the motivating considerations in developing the D825 design. In arriving at the final systems design, several existing and proposed schemes for the organization of data processing systems were evaluated in light of the requirements listed above. Many of the same conclusions regarding these and other schemes in the use of computers in command and control were reached independently in a more recent study conducted for the Department of Defense by the Institute for Defense Analysis [Kroger et al., 1961].

The single-computer system. The most obvious system scheme, and the least acceptable for command and control, is the single-computer system. This scheme fails to meet the availability requirement simply because the failure of any part-computer, memory, or I/O control-disables the entire system. Such a system was not given serious consideration.

Replicated single-computer systems. A system organization that had been well known at the time these considerations were active involves the duplication (or triplication, etc.) of single-computer systems to obtain availability and greater processing rates. This approach appears initially attractive, inasmuch as programs for the application may be split among two or more independent single-computer systems, using as many such systems as needed to perform all of the required computation. Even the availability requirement seems satisfied, since a redundant system may be kept in idle reserve as backup for the main function.

On closer examination, however, it was perceived that such a system had many disadvantages for command and control applications. Besides requiring considerable human effort to coordinate the operation of the systems, and considerable waste of available machine time, the replicated single computers were found to be ineffective because of the highly interrelated way in which data and programs are frequently used in command and control applications. Further, the steps necessary to have the redundant or backup system take over the main function, should the need arise, would prove too cumbersome, particularly in a time-critical application where constant monitoring of events is required.

Partially shared memory schemes. It was seen that if the replicated computer scheme were to be modified by the use of partially shared memory, some important new capabilities would arise. A partially shared memory can take several forms, but provides principally for some shared storage and some storage privately allotted to individual computers. The shared storage may be of any kind-tapes, discs, or core-but frequently is core. Such a system, by providing a direct path of communication between computers, goes a long way toward satisfying the requirements listed above.