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Chapter 36 D825-a multiple-computer system for command and control 455

a card reader, a card punch, a supervisory printer, and an electrostatic line printer.

D825 characteristics are summarized in Table 1.

Summary and conclusion

It is the belief of the authors that modular systems (in the sense discussed above) are a natural solution to the problem of obtaining greater computational capacity-more natural than simply to build larger and faster machines. More specifically, the organizational structure of the D825 has been shown to be a suitable basis for the data processing facility for command and control. Although the investigation leading toward this structure proceeded as an attack upon a number of diverse problems, it has become evident that the requirements peculiar to this area of application are, in effect, aspects of a single characteristic, which might be called structural freedom. Furthermore, it is now clear that the most unique characteristic of the structure realized-integrated operation of freely intercommunicating, totally modular elements- provides the means for achieving structural freedom.

For example, one requirement is that some specified minimum of data processing capability be always available, or that, under any conditions of system degradation due to failure or maintenance, the equipment remaining on line be sufficient to perform primary system functions. In the D825, module failure results in a reduction of the on-line equipment configuration but permits normal operation to continue, perhaps at a reduced rate. The individual modules are designed to be highly reliable and maintainable, but system availability is not derived solely from this source, as is necessarily the case with more conventional systems. The modular configuration permits operation, in effect, with active spares, eliminating the need for total redundancy.

A second requirement is that the working configuration of the system at a given moment be instantly reconstructable to new forms more suited to a dynamically and unpredictably changing work load. In the D825, all communication routes are public, all modules are functionally decoupled, all assignments are scheduled dynamically, and assignment patterns are totally fluid. The system of interrupts and priorities controlled by the AOSP and the switching interlock permits instant adaptation to any work load, without destruction of interrupted programs.

The requirement for expansibility calls simply for adaptation on a greater time scale. Since all D825 modules are functionally decoupled, modules of any types may be added to the system simply by plugging into the switching interlock or the I/O exchange. Expansion in all functional areas may be pursued far beyond that possible with conventional systems.

It is clear, however, that the D825 system would have fallen far short of the goals set for it if only the hardware had been considered. The AOSP is as much a part of the D825 system structure as is the actual hardware. The concept of a "floating" AOSP as the force that molds the constituent modules of an equipment complement into a system is an important notion having an effect beyond the implementation of the D825. One interesting by-product of the design effort for the D825 has, in fact, been a change of perspective; it has become abundantly clear that computers do not run programs, but that programs control computers.

References

AndeJ62; KrogM61; LeinA57; PortR60; ThomR63

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