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HKEY_CLASSES_ROOT

This root key defines the standard class objects used by Windows. A class object is a named group of functions that define what you can do with the object it represents. Pretty much everything that has to do with files on the system is defined by a class object. For example, the Registry uses two class objects to define the popular MP3 sound file. If you search the Registry for the .MP3 file extension, you will find the first class object, which associates the .MP3 file extension with the name “Winamp.File” on this computer (Figure 4-47).

Figure 4-47 Association of .MP3 with Winamp

Ah, but what are the properties of Winamp.File? That’s what the HKEY_CLASSES_ ROOT root key is designed to handle. Search this section again for “Winamp.File” (or whatever it said is the value for your MP3 file) and look for a subkey called “open.” This variable determines the file association (Figure 4-48), which is the Windows term for what program to use to open a particular type of file.

This subkey tells the system everything it needs to know about a particular software item, from which program to use to open a file, to the type of icon used to show the file, to what to show when you right-click on that file type. Although it is possible to change most of these settings in the Registry Editor, the normal way is to choose more user-friendly methods. In Windows XP, for example, you can right-click on a file and select Properties, and then click the Change button on the General tab to open the Open With dialog box (Figure 4-49). From there you can browse to select the program you want to use.

Figure 4-48 Winamp file settings

Figure 4-49 Changing the file association the easy way

HKEY_CURRENT_USER and HKEY_USERS

Windows is designed to support more than one user on the same PC, storing personalized information such as desktop colors, screensavers, and the contents of the desktop for every user that has an account on the system. HKEY_CURRENT_USER stores the current user settings, and HKEY_USERS stores all of the personalized information for all users on a PC. While you certainly can change items such as the screensaver here, the better way is to right-click on the desktop and select Properties.

HKEY_LOCAL_MACHINE

This root key contains all the data for a system’s non-user-specific configurations. This encompasses every device and every program in your PC. For example, Figure 4-50 shows the description of a DVD disc drive.

Figure 4-50 Registry information for a DVD drive

HKEY_CURRENT_CONFIG

If the values in HKEY_LOCAL_MACHINE have more than one option, such as two different monitors, this root key defines which one is currently being used. Because most people have only one type of monitor and similar equipment, this area is almost never touched.

Page File

Windows uses a portion of the hard drive as an extension of system RAM, through what’s called a RAM cache. A RAM cache is a block of cylinders on a hard drive set aside as what’s called a page file, swap file, or virtual memory. When the PC starts running out of real RAM because you’ve loaded too many programs, the system swaps programs from RAM to the page file, opening more space for programs currently active. All versions of Windows use a page file, so here’s how one works.

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EXAM TIP The default and recommended page file size is 1.5 times the amount of installed RAM on your computer.

Let’s assume you have a PC with 4 GB of RAM. Figure 4-51 shows the system RAM as a thermometer with gradients from 0 to 4 GB. As programs load, they take up RAM, and as more and more programs are loaded (labeled A, B, and C in the figure), more RAM is used.

Figure 4-51 A RAM thermometer showing that more programs take more RAM

At a certain point, you won’t have enough RAM to run any more programs (Figure 4-52). Sure, you could close one or more programs to make room for yet another one, but you can’t keep all of the programs running simultaneously.