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the addition of the assignment arrow. Thus in the figure we write that i <_ i+1, meaning that the value of the variable i takes on a value equal to its current value, plus 1. If we were to use 'the equality sign, i = i +1, this might be confused with the statement that the variable i is equal it itself plus 1 (which is universally false). Some programming languages make use of the equality sign in this way but it is preferable to use a separate notation. Thus, whenever the equality sign occurs it implies an assertion of the equality of two quantities (e.g., in the diamond in the figure where i = N+1 asks if i and N+1 have the same value).

The language of flowcharts is adequate for most of what we do in this book. However, it is not adequate for all tasks of specification. For instance, it is not, adequate to specify i desired computer, prior to constructing an RT system that would realize the computer. For this one wants a language for describing computer instruction sets. Flowcharts have in fact been used for this on occasion but they are usually a bit awkward. Special languages have been proposed and in Chapter 6, which discusses the design of small computers, we will use one called ISP (for Instruction Set Processor), giving there as much of the notation as we need. The language was introduced in Bell and Newell (1971) where complete details can be found. Our reason for mentioning it here is to emphasize that there are many specification languages, adapted to the demands of different task environments.

We have now provided a framework within which to proceed with the design of RT systems. We introduced digital systems, both what they are good for and what they are constructed of. We established that we would operate at the register-transfer level, distinguishing this from the many other levels at which digital systems could be considered. We posed the design task in general terms, just so you would be clear on what you were being asked to do. Then, we introduced the languages for the two main boundaries of the design task: the components out of which RT systems are to be built and the specifications for desired behavior.

There is nothing left to do now except start designing. Chapter 2 gives the full specifications of the PDP-16 modules, and from there you can begin.

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