Concepts Glossary 160 Getting Started Guide function key, F1 to F6. To switch back to the graphical mode, running on console 7, press Alt+F7. See also command line Virtual Memory The Linux kernel has the ability to use a section of your hard disk as if it were system memory (RAM). This features is called virtual memory because it acts like system memory for your applications, though it really doesn’t exist as mem- ory on your system. Although the hard disk is always much slower to access than system memory, the ability to use extra hard disk space allows Linux to work with more programs at the same time, without requiring as much system memory. Applications that are not in use (that are paused or waiting for input, for exam- ple), are moved (“swapped”) to the hard disk area called the Swap Partition. This is done by the Linux kernel and is transparent to the application itself. As soon as the application becomes active again, the kernel moves it back to system memory (moving something else to the swap partition if necessary). If you don’t have enough system memory for the programs you’re trying to run, you will notice that the Linux system slows down as it repeatedly swaps pro- grams to and from the virtual memory area. This is called thrashing, and indi- cates that you need to run fewer programs or add system memory to your computer To track the usage of the virtual memory resources on your system, use the vmstat command (see man vmstat for information on the output of vmstat). See also swap partition, kernel. X Window System The X Window System is the underlying graphical system that is used by virtu- ally every Linux and UNIX operating system. The X Window System, often called simply X, includes several layers: • The X server, which provides the most basic interface with your video hardware • The window manager, which managing windows and objects on the graphical screen