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Once you purchase a microcomputer system, you are really dealing with threedifferent assets: the hardware (the actual machinery), the software(preprogrammed sets of instructions that allow you to perform variousprocessing tasks), and data (facts that you enter and store using thesoftware). What happens if your computer system is stolen, destroyed in afire, or you accidentally erase a key file or, much worse, all your data?If you have properly insured your system, your investment is partiallyprotected in the first two instances. You can purchase like hardware toreplace the previous system. You can also purchase new copies of the softwareprograms. You are not, however, usually covered in the third instance. Yourcomputer's data, while extremely valuable to you, cannot be replaced by simplybuying "new" data. Many times this leads to hours of work to replace thedata. Some data, unfortunately, may be impossible to replace, and its lossmay lead to very serious business problems.The responsibility of protecting the data lies with the individual using acomputer system. Protecting your data can involve any one of three possibleoperations: you can protect a single file via the Copy command; you can protectan entire diskette via the Disk Copy command; and, finally, you can protectlarge files on hard disk via the Back-up command.The simplest way to protect data from an accident or, more probably, anerror on your part is to make a copy of important data files. This is known asthe back-up process and is easily accomplished by using the Copy command foundin any computer's disk operating system.The Copy command lets you take a file, like a month-end status report, thatis contained on a diskette or on a hard disk and transfer it to anotherdiskette. Once the Copy command has been executed, two copies of the fileexist, an original and a duplicate. The diskette with the duplicate copyshould now be placed in a safe location where it cannot be harmed (desk, filingcabinet, etc.). These steps are performed after major changes or additionsare made to the report.Later, without realizing what you are doing, suppose you inadvertentlyerase the month-end status report from your diskette. What do you do? Yourfirst impulse, of course, is to get the diskette with the duplicate copy ofyour report and start using that. This is absolutely the worst thing that youcan do. If the file was indeed erased from the diskette, the erasure wasperformed at your direction. You are now concerned with protecting yourvaluable data from your own worst enemy, yourself.Rather than using the duplicate copy of the data file, copy the file toanother diskette. This leaves you with a usable copy and a backup. Thus, ifyou are having one of those days where absolutely everything goes wrong, youcan at least partially protect yourself from your own actions. The process ofreconstructing your data files is known as recovery. The back-up process savescopies of your files, and the recovery process returns these files to activeuse.The Copy command is most appropriate when you have just one or two filesthat you wish to back-up. The Copy command, however, is inappropriate whenyou wish to make a backup copy of an entire diskette. Most disk operatingsystems provide a Disk Copy type of command for this process. The Disk Copycommand cuts the time involved in copying an entire disk to about a minute anda half.The only thing that you have to keep track of using the Disk Copy commandis the difference between a source diskette and a target diskette. The sourcediskette is the original diskette that you wish to back-up. The targetdiskette holds the copy, or back up, of the original. Woe to the person thatconfuses these two. If you place a blank, formatted diskette in the sourcedrive and your original in the target drive, you end up with two blank,formatted diskettes and a serious case of gastric distress.In the above examples, the process of backup and recovery was simple andstraightforward. For applications like word processing or spreadsheets thatrequire files that fit easily on a diskette, the back-up and recovery process iseasy. Applications like database management that require master/transactionfile relationships, however, complicate the process of back up and recovery.Before we look at the backup and recovery process, let's first describe themaster and/or transaction file relationship through an inventory application.An inventory master file is a semi permanent file that holds data about allinventory items carried. The information about each inventory item is storedin a record. An inventory record contains such data as item number, itemname, supplier name/reference, and units on hand; reorder point, as well asother pieces of data. Most of these data remains unchanged; hence the termsemi permanent.One of the pieces of datum (fields) that changes is the units-on-hand field.This particular piece of datum is affected by a number of possible actions:sales of existing inventory, returns of inventory, and purchase/arrival ofmore inventory. Each one of the possible above actions is recorded in atransaction file. A inventory transaction record then simply records oneaction that affects one item in the inventory master file.This two-file relationship is typically further complicated by the size ofthe master file. Many master files are contained on hard disk and are of a sizethat will not easily fit on one diskette. In such a situation, the regularCopy command is of little use because it will result in an insufficient storagespace error if it is used. How do you copy such a


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USA CIS 150 - LECTURE NOTES

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