`
`/COMPUTING
`SYSTEM
`FUNDAMENTALS,
`An Approach
`Based on Microcomputers
`
`KENNETH J. DANHOF
`CAROL L. SMITH
`
`Southern Illinois University at Carbondale
`
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`v ADDISON-WESLEY PUBLISHING COMPANY
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`London * Amsterdam « Don Mills, Ontario » Sydney
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`This bookis in the
`ADDISON-WESLEY SERIES IN COMPUTER SCIENCE
`
`Consulting Editor: Michael A. Harrison
`
`Library of Congress Cataloging in Publication Data
`
`Danhof, Kenneth J
`Computing system fundamentals.
`
`Includes bibliographical references and index.
`1. Electronic digital computers.
`2. Microcomputers.
`I. Smith, Carol L., joint author.
`II. Title.
`QA76.5.D252
`001.6'4
`79-14933
`ISBN 0-201-01298-7
`
`Copyright © 1981 by Addison-Wesley Publishing Company, Inc. Philippines copyright 1981 by
`Addison-Wesley Publishing Company,Inc.
`All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or
`transmitted, in any form or by any means,electronic, mechanical, photocopying, recording, or
`otherwise, without the prior written permission of the publisher. Printed in the United States of
`America. Published simultaneously in Canada. Library of Congress Catalog Card No. 79-14933,
`ISBN 0-201-01298-7
`ABCDEFGH-MA-89876543210
`
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`102
`
`Introduction to Microcomputer Architecture
`
`5.2
`
`5.2 MICROPROCESSORS AND ASSOCIATED COMPONENTS
`
`We now consider the structure of the microcomputer in somewhat greater
`detail and examine somespecific products.
`Asindicated earlier, the heart of any microcomputersystem is the micro-
`processor. The microprocessoris a central processing unit, which, through the
`use of the latest techniques in electronic circuit miniaturization, is typically
`contained on a 40-pin package knownas a DIP (dualinline pins). The ‘“‘chip”’
`or portion of the DIP that contains the circuitry may be less than one square
`centimeter in size and contain more than 20,000 transistors (the building
`blocks from which gates are formed).
`Weshall consider both the M6800 and [8085 microprocessors noting com-
`mon elements as well as some of the differences.
`
`5.2.1 The M6800 System
`The Motorola M6800 microprocessoris described in the block diagram in Fig.
`5.5. The various buffers indicated in the diagram can be viewedasregisters for
`holding data until it can be read by the appropriate source. Lines Ap-Aj5 are
`address lines by which the microprocessor addresses memoryand otherexter-
`nal devices. Lines Dp-D;are bidirectional data lines. The remaining lines are
`for processor and bus control. In particular, the read/write (R/W)line indi-
`cates whether the microprocessor is in a Read or Write state and the Valid
`Memory Address line (VMA) signals to memory (and peripherals) that the
`
`address lines contain a valid address. A bar over the label onaline(e.g.,
`HALTin Fig. 5.5) indicates that the line is low (or 0) active as opposed to the
`normal high (or 1) active.
`The microprocessor operates in a synchronous manner; its operations are
`synchronized by the cycles of a clock (actually two phases of the clock are
`required). The minimal clock-cycle time for the M6800 is one microsecond (1
`us). As we shall see, relative to the execution of a given instruction, the proces-
`sor must perform a specific series of subinstructions or microinstructions,
`which consist of the most basic acts the processor can perform. (Thisis exactly
`the passage from Level2 to Level 1 in Fig. 1.1.) One or more microinstructions
`can be performedperclock cycle, and thus eachinstruction involves a number
`of cycles.
`Before considering the details of instruction execution, we must regard
`the microprocessor as a componentin a larger system. A typical small micro-
`computer system is displayed in Fig. 5.6.
`In Fig. 5.6, the RAM is a Random Access Memory—a memoryunit that
`can be both written into and read from. RAMsare constructed from basic
`memoryelements (flip-flops), as indicated in Sec. 5.1, and are vo/atile, so that
`when the power to a RAMis turnedoff, its contents are lost. A RAM might
`typically be organized as 128 (K/8) eight-bit words and be placed ona single
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`5.2
`
`Microprocessors and Associated Components
`
`103
`
`Ajs
`
`A, Ao
`
`Clock,
`Clock,
`NMI
`Reset
`
`
`OUTPUT BUFFERS
`
`
`tastruction a Pp Prosram
`Program
`Program
`
`Stack
`
`Index
`
`Stack
`
`Index
`
`R/W Accemulator
`
`Halt
`IRQ
`
`decode
`and
`
`Three-state control tack|[=={|| pointer Lcontrol
`
`
`Data bus enable
`(CU)
`VMAFgindex, ne
`Bus available
`
`B
`
`instruction [eed Accumulator— A
`
`
`
`Condition
`code
`register
`a
`
`DATA
`BUFFERS
`
`D
`
`7
`
`D
`_ 0
`
`Fig. 5.5 Block diagram for M6800.
`
`24-pin DIP (or it might occur as one of several components on a DIP). The
`individual storage cells of a RAM maybeeitherstatic or dynamic.If static, the
`device need only be addressed when data is read or written. However,
`in
`dynamic RAMsthe data must constantly be refreshed (read out and written
`back in) to preserve the quality of the data.
`In the M6800-based system, lines A,4, A,5 are used as RAM-selectlines.
`Lines Ap, ..., Ag select a particular word within the RAM and lines
`Aj, ..., Ajy3 select one from among several RAMs. The R/W controlline
`selects the Read or Write mode within the RAM.
`The ROM of Fig. 5.6 is a Read Only Memory. A ROM is essentially a
`combinationalcircuit that is written into once and maythen be repeatedly read
`from. ROMs are nonvolatile and are consequently used to hold programs that
`are needed repeatedly in a computing system. For example, monitor programs
`
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`104
`
`Introduction to Microcomputer Architecture
`
`5.2
`
`MICROPROCESSOR
`
`
`
`
`Addressbus
`Databus
`
`
`
`
`
`
`INPUT/OUTPUT
`INTERFACE
`DEVICE
`
`Fig. 5.6 Typical small system.
`
`To peripheral devices
`
`for microcomputersare generally stored in ROMs. Although ROMsare often
`‘‘mask-programmed’’ at
`the factory, Programmable ROMs (PROMs) are
`somewhateasier and less expensive to program on a low-volumebasis. Some
`PROMs are erasable: EPROMs canbeerased by exposureto ultravioletlight.
`In the M6800-based system, lines Aj4, Ajs are again used as ROM-select
`lines. Ajo,
`.
`.
`-
`» Aj3 Select one of several ROMs and Ao,
`.
`.
`.
`, Ag select a par-
`ticular word within a ROM.
`In general, the instructions of a program arestored in consecutive loca-
`tions ina RAM or ROM. Priorto execution of the program, the addressof the
`first instruction of the program is loaded into the microcomputer’s program
`counter (PC).
`
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