Computer Systems Week 10: File Organisation Alma Whitfield

2 Previously covered Components of a Computer System How a program in main memory is executed making use of input and output devices connecting with other computers

3 This Week See how programs and data are permanently stored on a computer system Organise files using the Windows Explorer Graphical User Interface

4 Recap The CPU processes instructions Before this can happen the Instructions need to be fetched from main memory (RAM) Compared with the hundreds/thousands of applications on your computer RAM is small e.g. 2 GB of RAM 400 GB of hard disk (1 GB = 1 gigabyte = 1,000 million bytes)

5 Recap When you switch off the computer the contents of RAM are lost Where does RAM get its data from?

6 Files The instructions and data are organised into files stored on secondary storage such as a hard disk A file can be described as a named collection of bytes representing instructions or data

7 Secondary Storage Secondary Storage is effectively a huge data structure for storing files Data structures include Tables, Pointers and Linked Lists (more on this in SPS after Christmas) How the data structure organises and stores the data is dependent on the type of Table, Pointers and Linked List being used

8 File names Early PCs File names restricted to 8 characters maximum followed by up to 3 characters after a full stop e.g. FileOrga.doc Modern PCs File names can be any length up to 256 characters Can include spaces e.g. FileOrganisation.doc

9 Files and Secondary Storage Devices such as hard disc and CDs, DVDs and pen drives can store files permanently i.e. even when the device is switched off these devices are referred to as secondary storage

10 File Types Data files Program files Source Code Files

11 Data files Bytes represent text, pictures, sounds, movies The files can be opened or played or edited by appropriate programs e.g. Notepad program for text files e.g. Word program for Word document files e.g. Windows Media Player program for video and audio files e.g. Data processing programs for customer record files

12 Program files Bytes represent instructions Two main varieties of program file exe files – bytes are machine code instructions (as in 3-Bit) batch files – bytes are text containing operating system commands

13 Program files 1. Application Programs: Microsoft word Internet Explorer etc. 2. System Programs: operating systems like Windows or Linux language translators such as Java, Pascal, C, C++ that translate high level language programs into machine code

14 Source code files Bytes represent text (they can be regarded also as text files) The text is human-readable program instructions e.g. written in Java before they get translated into machine code by compilers

15 Organising files Thousands of files can be held in secondary storage Organising the files is a job for the operating system

16 Operating System and Files Q. When you double click on a file, how does the Operating System know what to do with that file? A. Filename extensions (also known as file extensions for short)

17 File Extensions Files have names They also have something else called extensions (the letters that follow the last dot (. ) in a file’s name. The extension reveals the type of each file e.g. cars.jpg, (this is an image extension) sales1.doc (this is a Microsoft Word document)

18 For example The following extensions indicate that the data contained within the file is executable.exe.com.bat.cmd

19 Saving data from Applications When you save data from an application the application will usually automatically tag on the extension.doc.ppt.mdb.txt You could also tell the application to save as a different file type such as.htm

20 File Attributes Files have attributes associated with them. An attribute is another word for a charactistic For example you could make a file READ- ONLY, HIDDEN, ARCHIVE Right Mouse click and select properties on the file to set these attributes

21 Operations on Files Create a file / Delete a file Open a file / Close a file Read data from a file Write data to a file Reposition the current file pointer in a file Append data to the end of a file Truncate a file (delete part of its contents) Rename a file Copy a file

22 Using the PC Filing System PCs running popular operating systems such as Windows and Linux organise computer files into directories (sometimes called folders) on your secondary storage (e.g. hard drive). These directories are based on a hierarchical structure, where the root directory (denoted by \ ) is at the top of that structure.

23 Directory Trees A directory of files can be contained within another directory The directory containing another is usually called the parent directory, and the one inside is called a subdirectory Directory tree A logical view of a file system; a structure showing the nested directory organization of a file system Root directory The directory at the highest level

24 The file directory A directory is a table listing files and their location on the disc Directories are called folders in Windows 24

25 The file directory 25 In practice, the black blobs are numbers giving the addresses of the locations of the files on the disc surface

26 Directory entries can be other directories leading to a hierarchical directory structure i.e. folders within folders i.e. a tree data structure 26

27 Path: a file’s full name Write down the full path of POOLS \LETTERS\BEGGING\POOLS 27

28 MS Windows hierarchical directory structure 28

29 MS Windows hierarchical directory structure 29

30 File Handling in Microsoft Windows Microsoft windows is based on a graphical user interface (GUI). This makes handling files and directories simple. Using Windows Explorer you can:  Find files and folders  Copy files and folders  Move files and folders  Delete files and folders  Create folders Demo... more in the seminar

31 Next week File handling in MS-DOS