Access Methods to Data | A Level Notes Computer Studies 9608 скачать в хорошем качестве

Access Methods to Data | A Level Notes Computer Studies 9608

Computers can store large volumes of data. The difficulty is to be able to get it back. In order to be able to retrieve data it must be stored in some sort of order. Imagine the phone book. It contains large volumes of data which can be used, fairly easily, to look up a particular telephone number because they are stored in alphabetic order of the subscriber's name. Imagine how difficult it would be to find a number if they had just been placed in the book at random. The value of the book is not just that it contains all the data that may be needed, but that it has a structure that makes it accessible. Similarly, the structure of the data in a computer file is just as important as the data that it contains. There are a number of ways of arranging the data that will aid access under different circumstances. Serial access Data is stored in the computer in the order in which it arrives. This is the simplest form of storage, but the data is effectively unstructured, so finding it again can be very difficult. This sort of data storage is only used when it is unlikely that the data will be needed again, or when the order of the data should be determined by when it is input. A good example of a serial file is what you are reading now. The characters were all typed in, in order, and that is how they should be read. Reading this book would be impossible if all the words were in alphabetic order. Another example of the use of a serial file will be seen in section 1.4.n. Serial access Data is stored in the computer in the order in which it arrives. This is the simplest form of storage, but the data is effectively unstructured, so finding it again can be very difficult. This sort of data storage is only used when it is unlikely that the data will be needed again, or when the order of the data should be determined by when it is input. A good example of a serial file is what you are reading now. The characters were all typed in, in order, and that is how they should be read. Reading this book would be impossible if all the words were in alphabetic order. Another example of the use of a serial file will be seen in section 1.4.n. Indexed sequential Imagine a large amount of data, like the names and numbers in a phone book. To look up a particular name will still take a long time even though it is being held in sequence. Perhaps it would be more sensible to have a table at the front of the file listing the first letters of peoples‟ names and giving a page reference to where those letters start. So, to look up Jawad, a J is found in the table which gives the page number 232, the search is then started at page 232 (where all the Js will be stored). This method of access involves looking up the first piece of information in an index which narrows the search to a smaller area, having done this, the data is then searched alphabetically in sequence. This type of data storage is called Index Sequential. Random access A file that stores data in no order is very useful because it makes adding new data or taking data away very simple. In any form of sequential file an individual item of data is very dependent on other items of data. Jawad cannot be placed after Mahmood because that is the wrong „order‟. However, it is necessary to have some form of order because otherwise the file cannot be read easily. What would be wonderful is if, by looking at the data that is to be retrieved, the computer can work out where that data is stored. In other words, the user asks for Jawad's record and the computer can go straight to it because the word Jawad tells it where it is being stored. How this can be done is explained in section 1.4.l. #componentsofacomputer #computersystem #modesofuse #computerstudies9608 #alevelnotes #notesreels #Chapter1.4 #systemsoftware #operatingsystem #data #DataItsRepresentation #StructureandManagement #AccessMethods #MethodstoData ‪@Notesreel‬