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Sequential Logic

Sequential Logic - 03-CLements-Chap03.qxd 11:10 PM Page 101...

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3 Sequential Logic CHAPTER MAP 2 Logic elements and Boolean algebra We begin our introduction to the computer with the basic building block from which we construct all computers, the gate. A combinational digital circuit such as an adder is composed of gates and its output is a Boolean (logical) function of its inputs only. 3 Sequential logic The output of a sequential circuit is a function of both its current inputs and its past inputs; that is, a sequential circuit has memory. The building blocks used to construct devices that store data are called flip-flops. In this chapter we look at basic sequential elements and the counters, registers, and shifters that are constructed from flip-flops. 4 Computer arithmetic Computer arithmetic concerns the representation of numbers in a computer and the arithmetic used by digital computers.We look at how decimal numbers are converted into binary form and the properties of binary numbers and we demonstrate how operations like addition and subtraction are carried out.We also look at how computers deal with negative numbers and fractional numbers. 5 The instruction set architecture In this chapter we introduce the computer’s instruction set architecture (ISA), which describes the low-level programmer’s view of the computer.The ISA describe the type of operations a computer carries out.We are interested in three aspects of the ISA: the nature of the instructions, the resources used by the instructions (registers and memory), and the ways in which the instructions access data (addressing modes).The 68K microprocessor is used to illustrate the operation of a real device. INTRODUCTION We now introduce a new type of circuit that is constructed from devices that remember their previous inputs.The logic circuits in Chapter 2 were all built with combinational elements whose outputs are functions of their inputs only . Given a knowledge of a combinational circuit’s inputs and its Boolean function, we can always calculate the state of its outputs.The output of a sequential circuit depends not only on its current inputs, but also on its previous inputs. Even if we know a sequential circuit’s Boolean equations, we can’t determine its output state without knowing its past history (i.e. its previous internal states).The basic building blocks of sequential circuits are the flip-flop, bistable , and latch just as the basic building block of the combinational circuit is the gate. It’s not our intention to deal with sequential circuits at anything other than an introductory level, as their full treatment forms an entire branch of digital engineering. Sequential circuits can’t be omitted from introductory texts on computer hardware because they are needed to implement registers, counters, and shifters, all of which are fundamental to the operation of the central processing unit.
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