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Sec 11.2 - 376 The Silicon Web Physics for the Internet Age...

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376 The Silicon Web: Physics for the Internet Age as binary information. In Chapter 10, Section 10.5, we discussed how transistors are used to perform logic operations much faster and less expensively than could be accom- plished using electromechanical switches and relays. From our discussions in the chapters mentioned above, we can state the following two underlying principles of computing: Physical Principles of Computing (i, ii) (i) Computers require that physical devices be used for representing and storing logic bit values. Each object can be in one of two states, or confi gurations, representing logic values 1 or 0. (ii) Computers require physical switches (usually electronic) for performing logic operations. These imply that the ultimate limits of computing machinery are subject to the laws of physics. Computing does not exist in an abstract, purely mathematical realm. It exists in the physical world. In this chapter, we will see that in order to make electric circuits that can store bit values (i.e., act as memory) we need to consider logic circuits that are somewhat differ- ent from those we studied in Chapter 6. We will also see that the physical devices we use for short-term memory are different from those we use for long-term memory. We will discuss the hierarchy of memory types used in computers and see how this leads to effi cient use of hardware. Finally, we will discuss how the components of a computer are organized, and how information is processed through these components. 11.2 SEQUENTIAL LOGIC FOR COMPUTER MEMORY In this text we will not delve deeply into how logic instructions are carried out in a computer. Rather, we will focus on the physics underlying some of the important components. Especially important is implementation of computer memory. What is memory? In your brain, memory is a physical record of some earlier event. How can we build electronic circuits that have this property? We will see that the underlying physics of each type of memory technology determines whether it is best suited for high-speed, temporary memory or for low-speed, permanent memory, or for something in between. Switches and logic gates are likely candidates for building memory, but there is a catch. The circuits that we studied in Chapter 6, called combinational logic circuits , cannot function as memory devices. This is because their output values are determined completely by the present values of their inputs. They are not infl uenced by the values that the inputs had previously. Recall the rules for combinational logic circuits: Rule 1. It is forbidden to combine two wires into one wire.
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