G53OPS - Operating Systems

This course is run at the The University of Nottingham within the School of Computer Science & IT. The course is run by Graham Kendall (EMAIL : gxk@cs.nott.ac.uk)

Introduction to File Systems

Before we look at how a file system can be implemented we are going to take a brief look at filing systems from a users point of view. Most of this will probably be familiar to you but we cover it so that we have a sound basis for the discussions that follow.

We use files almost without thinking of why we need them. We can summarise why we need to use files as follows.

· It allows us to store data between processes. If we simply held data in main memory that data would be lost when the process terminated and certainly when the computer is switched off.
· It allows us to store large volumes of data. Certainly, larger volumes of data than can be stored in memory.
· Using files allows more than one process to access the data at the same time. Although, this is possible using a shared address space it is often more convenient to store data in a file and allow processes to access it as and when they require.

File Naming

Different operating systems have different file naming conventions.

For example, MS-DOS only allows an eight character filename (and a three character extension). This limitation also applies to Windows 3.1 (which, as we said in an earlier handout is not really an operating system - it is just a graphical user interface loaded on top of an operating system).

Windows 95 and Windows NT allow file names up to 255 characters (although the full path name is only allowed to be a maximum of 260 characters).

In addition to the limits of the length of a filename, there are also limits as the characters that can be used.

MS-DOS, for example, does not allow the space character as part of a filename. Windows 95/NT does allow this character.

Other characters which are (typically) illegal in filenames include '?', '/', '\' and '*'. The reasons are that these characters have special significance to the operating system. For example, the asterix and the question mark can be used a wildcard characters when searching for files and the slash characters are used as directory separators.

Some operating systems also distinguish between upper and lower case characters. For example, to MS-DOS, the filename ABC, abc, and AbC all represent the same file. To UNIX these would all represent different files.

 Last Updated : 23/01/2002