46 Part 1 The structure of computers

The most striking fact about function is the existence of only a single dimension, and with only a few values. Perhaps we have taken a simplistic view of the functions that computers perform, but we think our computer space represents reality: To wit, there is remarkably little shaping of computer structure to fit the function to be performed.

At the root of this lies the general-purpose nature of computers, in which all the functional specialization occurs at the time of programming and not at the time of design. However, it might seem that specialized environments would not require all the generality, so that functional adaptation would still be possible. But this appears not to be so for two reasons. First, the level of operations of the Pc (as defined in the ISP) is too basic to reflect the kind of specialization offered by the environment (think of information-transfer or conditional-transfer operations). Second, all environments ultimately require a variety of tasks in addition to the main specialized task. These include at least language compilation or assembly, readable formatted output, debugging aids, and other utility routines. By the time these have been added, a substantial requirement for generality has been generated.

However, this is not the whole story. A second part is the difference between the computer type and the specific configuration assembled for a task. The latter is often carefully specialized to the function to be performed. But this is mostly the amount of Mp, the amount of types of Ms, and the number and types of T's. Within limits, these are all items that can be attached to any type of computer (i.e., to any Pc) and are handled in an environment- independent way. Thus there is little specialization of computer types, but great specialization of particular configurations. That this should be the case indicates something about the nature of the functional specialization-that it can be expressed adequately in gross PMS terms, as more bits of storage and more data rate.

There is still more to the story. Some functional specialization exists, as indicated in the dimension. This depends primarily on two kinds of things beyond the reach of the configurational adaptation described above. The first consists of demands for reliability, ruggedness, small size, etc. These have strong effects on design, but below the ISP and PMS levels. The second consists of demands for large amounts of processing power. One response to this again affects design at the lower levels of logic, devices, and circuitry and has little impact on design at the ISP and PMS level. But response is also possible in terms of the data-types that are built into the ISP. Large machines have data-types that are appropriate to their tasks (with operations to match), and these affect the