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Unformatted text preview: table we have represented 'true' by '1' and 'false' by '0', as is common practice when dealing with Boolean variables.) The gate abstraction The point about the gate abstraction is that not only does it greatly simplify the process of designing circuits with great numbers of transistors, but it actually Figure 1.3 The logic symbol and truth table for a NAND gate. 6 An Introduction to Processor Design removes the need to know that the gate is built from transistors. A logic circuit should have the same logical behaviour whether the gates are implemented using field-effect transistors (the transistors that are available on a CMOS process), bipolar transistors, electrical relays, fluid logic or any other form of logic. The implementation technology will affect the performance of the circuit, but it should have no effect on its function. It is the duty of the transistor-level circuit designer to support the gate abstraction as near perfectly as is possible in order to isolate the logic circuit designer from the need to understand the transistor equations. Levels of abstraction It may appear that this point is being somewhat laboured, particularly to those readers who have worked with logic gates for many years. However, the principle that is illustrated in the gate level abstraction is repeated many times at different levels in computer science and is absolutely fundamental to the process which we began considering at the start of this section, which is the management of complexity. The process of gathering together a few components at one level to extract their essential joint behaviour and hide all the unnecessary detail at the next level enables us to scale orders of complexity in a few steps. For instance, if each level encompasses four components of the next lower level as our gate model does, we can get from a transistor to a microprocessor comprising a million transistors in just ten steps. In many cases we work with more than four components, so the number of steps is greatly reduced. A typical hierarc...
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This document was uploaded on 10/30/2011 for the course CSE 378 380 at SUNY Buffalo.
- Spring '09