Almost all computers contain great amounts of information that users often want to protect and keep confidential. This information may include e-mail, business plans, tax returns, and much more. It is up to the operating system to manage the system security so that files, for instance, are only accessible to authorized users.
As a simple example, just to get an idea of how security can work, consider UNIX. Files in UNIX are protected by assigning each one a 9-bit binary protection code. The protection code comprises three 3-bit fields, one for the owner, one for other members of the owner's group (users are divided into groups by the system administrator), and one for everyone else. Each field has a bit for read access, a bit for write access, and a bit for execute access. These 3 bits are known as the rwx bits. For instance, the protection code rwxr-x--x means that the owner can read, write, or execute the file, other group members can read or execute (but not write) the file, and everyone else can execute (but not read or write) the file. For a directory, x indicates search permission. A dash means that the corresponding permission is absent.
As well as file protection, there are many other security issues. Protecting the system from unwanted intruders, both human and nonhuman (e.g., viruses) is one of them. We will look at various security issues in "SECURITY".