Reading: Chapter 14 and 15, Singhal and Shivratari.
An operating system needs to provide protection mechanisms to prevent undesirable operations such as:
A process being able to manipulate hardware translation tables, access arbitrary main memory locations, or perform arbitrary operations on files on disk.
An external agent being able to read or modify the information being communicated between two machines on a network.
An arbitrary user being able to log on to a computer.
These mechanisms can then be used to make a system secure.
Note that the presence of protection mechanisms are not enough to make a
system secure, since these mechanisms may be improperly used or may not be sufficient.