Data Storage

I Data Storage Data Storage Edited by Prof. Florin Balasa In-Tech intechweb.org Published by In-Teh In-Teh Olajnica 19/2, 32000 Vukovar, Croatia Abstracting and non-profit use of the material is permitted with credit to the source. Statements and opinions expressed in the chapters are these of the individual contributors and not necessarily those of the editors or publisher. No responsibility is accepted for the accuracy of information contained in the published articles. Publisher assumes no responsibility liability for any damage or injury to persons or property arising out of the use of any materials, instructions, methods or ideas contained inside. After this work has been published by the In-Teh, authors have the right to republish it, in whole or part, in any publication of which they are an author or editor, and the make other personal use of the work. © 2010 In-teh www.intechweb.org Additional copies can be obtained from: publication@intechweb.org First published April 2010 Printed in India Technical Editor: Maja Jakobovic Cover designed by Dino Smrekar Data Storage, Edited by Prof. Florin Balasa p. cm. ISBN 978-953-307-063-6 V Preface Many different forms of storage, based on various natural phenomena, has been invented. So far, no practical universal storage medium exists, and all forms of storage have some drawbacks. Therefore, a computer system usually contains several kinds of storage, each with an individual purpose. Traditionally, the most important part of a digital computer is the central processing unit (CPU or, simply, a processor), because it actually operates on data, performs calculations, and controls all the other components. Without a memory, a computer would merely be able to perform fixed operations and immediately output the result. It would have to be reconfigured to change its behavior. This is acceptable for devices such as desk calculators or simple digital signal processors. Von Neumann machines differ in that they have a memory in which they store their operating instructions and data. Such computers are more versatile in that they do not need to have their hardware reconfigured for each new program, but can simply be reprogrammed with new in-memory instructions. Most modern computers are von Neumann machines. In practice, almost all computers use a variety of memory types, organized in a storage hierarchy around the CPU, as a trade-off between performance and cost. Generally, the lower a storage is in the hierarchy, the lesser its bandwidth (the amount of transferred data per time unit) and the greater its latency (the time to access a particular storage location) is from the CPU. This traditional division of storage to primary, secondary, tertiary and off-line storage is also guided by cost per bit. Primary storage (or main memory, or internal memory), often referred to simply as memory, is the only one directly accessible to the CPU. The CPU continuously reads instructions stored there and executes them as required. Any data actively operated on is also stored there in uniform manner. As the random-access memory (RAM) types used for primary storage are volatile (i.e., they lose the information when not powered), a computer containing only such storage would not have a source to read instructions from, in order to start the computer. Hence, non-volatile primary storage containing a small startup program is used to bootstrap the computer, that is, to read a larger program from non-volatile secondary storage to RAM and start to execute it. Secondary storage (or external memory) differs from primary storage in that it is not directly accessible by the CPU. The computer usually uses its input/output channels to access secondary storage and transfers the desired data using intermediate area in the primary storage. The secondary storage does not lose the data when the device is powered down – it is ... - tailieumienphi.vn