by Gerhard Adam

Created on: July 14, 2008

Before we define a page fault, let's explain some terminology that involves the operating system and its use of system memory.

First of all, what is system memory or RAM? In a computer there are two elements which are relatively easy to understand; the hard drive or disk and the processor. The hard drive is a rotating disk on which information is stored and the processor is the chip which is responsible for "understanding" and performing the instructions given to it by programs.

In between these two elements there is another piece of hardware called Random Access Memory (RAM) which is used to hold the data from the hard drive which is being worked on. The reason for this is that the processor is almost a million times faster than the hard drive so it is important that we don't make the processor wait because we have to retrieve information. This is avoided by moving the current program and data into the memory which is significantly faster than the hard drive.

Most systems also use even smaller and faster memory chips to speed up the processing once something is loaded into RAM. These are called the L1 (Level 1) and L2 (Level 2) caches, but that is part of the hardware systems design and architecture and isn't included in this discussion.

The operating system is the set of programs that manages the computer system and provides the underlying "intelligence" to deal with the hardware and applications software you may want to use. This operating system could be Windows, Unix, Linux, or something else, but all of these operating systems must perform comparable functions in order to manage the hardware on which they run.

Early systems used the systems memory or RAM based solely on what was installed in the hardware, so if there was 2 Megabytes (Mega = millions) of RAM, then that was all that was available for the system and applications to use. This usage, unsurprisingly is referred to as "real memory".

It was quickly realized that not all of the real memory was referenced by programs all the time, and there were actually sections that never got referenced at all after they were loaded. The solution to this was "virtual memory" and was controlled by the operating system.

Virtual memory required that all of real memory was divided up into little chunks (4k) which are called pages. By dividing it up in this way, the operating system could keep track of which page was referenced and when more real memory was needed, the unreferenced pages could be moved to the hard drive,

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