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This chapter is from the book

This chapter is from the book

Getting to Know the Registry

The Registry may be a dangerous tool, but you can mitigate that danger somewhat by becoming familiar with the layout of the Registry and what it various bits and parts are used for. This will help you avoid sensitive areas and stick to those Registry neighborhoods where it's safe to poke around. The next few sections introduce you to the major parts of the Registry.

Navigating the Keys Pane

The Registry Editor is reminiscent of Windows Explorer, and it works in sort of the same way. The left side of the Registry Editor window is similar to Explorer's Folders pane, except that rather than folders, you see keys. For lack of a better phrase, I'll call the left pane the Keys pane.

The Keys pane, like Explorer's Folders pane, is organized in a tree-like hierarchy. The five keys that are visible when you first open the Registry Editor are special keys called handles (which is why their names all begin with HKEY). These keys are collectively referred to as the Registry's root keys. I'll tell you what to expect from each of these keys later (see the section called "Getting to Know the Registry's Root Keys" later in this chapter).

These keys all contain subkeys, which you can display by clicking the arrow to the left of each key, or by highlighting a key and pressing the plus-sign key on your keyboard's numeric keypad. To close a key, click the minus sign or highlight the key and press the minus-sign key on the numeric keypad. Again, this is just like navigating folders in Explorer.

You often have to drill down several levels to get to the key you want. For example, Figure 12.2 shows the Registry Editor after I've opened the HKEY_CURRENT_USER key, and then the Control Panel subkey, and then clicked the Mouse subkey. Notice how the status bar tells you the exact path to the current key, and that this path is structured just like a folder path.

Figure 12.2

Figure 12.2 Open the Registry's keys and subkeys to find the settings you want to work with.

Understanding Registry Settings

If the left side of the Registry Editor window is analogous to Explorer's Folders pane, the right side is analogous to Explorer's Contents pane. In this case, the right side of the Registry Editor window displays the settings contained in each key (so I'll call it the Settings pane). The Settings pane is divided into three columns:

  • Name—This column tells you the name of each setting in the currently selected key (analogous to a filename in Explorer).
  • Type—This column tells you the data type of the setting. There are six possible data types:

    REG_SZ—This is a string value.

    REG_MULTI_SZ—This is a series of strings.

    REG_EXPAND_SZ—This is a string value that contains an environment variable name that gets "expanded" into the value of that variable. For example, the %SystemRoot% environment variable holds the folder in which Windows 7 was installed. So, if you see a Registry setting with the value %SystemRoot%\System32\, and Windows 7 is installed in C:\Windows, the setting's expanded value is C:\Windows\System32\.

    REG_DWORD—This is a double word value: a 32-bit hexadecimal value arranged as eight digits. For example, 11 hex is 17 decimal, so this number would be represented in DWORD form as 0x00000011 (17). (Why "double word"? A 32-bit value represents four bytes of data, and because a word in programming circles is defined as two bytes, a four-byte value is a double word.)

    REG_QWORD—This is a quadruple word value: a 64-bit hexadecimal value arranged as 16 digits. Note that leading zeros are suppressed for the high 8 digits. Therefore, 11 hex appears as 0x00000011 (17), and 100000000 hex appears as 0x1000000000 (4294967296).

    REG_BINARY—This value is a series of hexadecimal digits.

  • Data—This column displays the value of each setting.

Getting to Know the Registry's Root Keys

The root keys are your Registry starting points, so you need to become familiar with what kinds of data each key holds. The next few sections summarize the contents of each key.

HKEY_CLASSES_ROOT

HKEY_CLASSES_ROOT—usually abbreviated as HKCR—contains data related to file extensions and their associated programs, the objects that exist in the Windows 7 system, as well as applications and their automation information. There are also keys related to shortcuts and other interface features.

The top part of this key contains subkeys for various file extensions. You see .bmp for bitmap (Paint) files, .txt for text (Notepad) files, and so on. In each of these subkeys, the Default setting tells you the name of the registered file type associated with the extension. (I discussed file types in more detail in Chapter 3, "Customizing the File System.") For example, the .txt extension is associated with the txtfile file type.

  • See "Understanding File Types," p. 46.

These registered file types appear as subkeys later in the HKEY_CLASSES_ROOT branch, and the Registry keeps track of various settings for each registered file type. In particular, the shell subkey tells you the actions associated with this file type. For example, in the shell\open\command subkey, the Default setting shows the path for the executable file that opens. Figure 12.3 shows this subkey for the txtfile file type.

Figure 12.3

Figure 12.3 The registered file type subkeys specify various settings associated with each file type, including its defined actions.

HKEY_CLASSES_ROOT is actually a copy (or an alias, as these copied keys are called) of the following HKEY_LOCAL_MACHINE key:

HKEY_LOCAL_MACHINE\Software\Classes

The Registry creates an alias for HKEY_CLASSES_ROOT to make these keys easier for applications to access and to improve compatibility with legacy programs.

HKEY_CURRENT_USER

HKEY_CURRENT_USER—usually abbreviated as HKCU—contains data that applies to the user that's currently logged on. It contains user-specific settings for Control Panel options, network connections, applications, and more. Note that if a user has group policies set on his account, his settings are stored in the HKEY_USERS\sid subkey (where sid is the user's security ID). When that user logs on, these settings are copied to HKEY_CURRENT_USER. For all other users, HKEY_CURRENT_USER is built from the user's profile file, ntuser.dat (located in %UserProfile%).

Here's a summary of the most important HKEY_CURRENT_USER subkeys:

AppEvents

Contains sound files that play when particular system events occur (such as maximizing of a window)

Control Panel

Contains settings related to certain Control Panel icons

Keyboard Layout

Contains the keyboard layout as selected via Control Panel's Keyboard icon

Network

Contains settings related to mapped network drives

Software

Contains user-specific settings related to installed applications and Windows

HKEY_LOCAL_MACHINE

HKEY_LOCAL_MACHINE (HKLM) contains non-user-specific configuration data for your system's hardware and applications. You'll use the following three subkeys most often:

Hardware

Contains subkeys related to serial ports and modems, as well as the floating-point processor.

Software

Contains computer-specific settings related to installed applications. The Classes subkey is aliased by HKEY_CLASSES_ROOT. The Microsoft subkey contains settings related to Windows (as well as any other Microsoft products you have installed on your computer).

System

Contains subkeys and settings related to Windows startup.

HKEY_USERS

HKEY_USERS (HKU) contains settings that are similar to those in HKEY_CURRENT_USER. HKEY_USERS is used to store the settings for users with group policies defined, as well as the default settings (in the .DEFAULT subkey) which get mapped to a new user's profile.

HKEY_CURRENT_CONFIG

HKEY_CURRENT_CONFIG (HKCC) contains settings for the current hardware profile. If your machine uses only one hardware profile, HKEY_CURRENT_CONFIG is an alias for HKEY_LOCAL_MACHINE\SYSTEM\ControlSet001. If your machine uses multiple hardware profiles, HKEY_CURRENT_CONFIG is an alias for HKEY_LOCAL_MACHINE\SYSTEM\ControlSetnnn, where nnn is the numeric identifier of the current hardware profile. This identifier is given by the CurrentConfig setting in the following key:

HKLM\SYSTEM\CurrentControlSet\Control\IDConfigDB

Understanding Hives and Registry Files

The Registry database actually consists of a number of files that contain a subset of the Registry called a hive. A hive consists of one or more Registry keys, subkeys, and settings. Each hive is supported by several files that use the extensions listed in Table 12.1.

Table 12.1. Extensions Used by Hive Supporting Files

Extension

Descriptions

None

A complete copy of the hive data.

.log1

A log of the changes made to the hive data.

.log, .log2

These files are created during the Windows 7 setup, but remain unchanged as you work with the system.

Table 12.2 shows the supporting files for each hive. (Note that not all of these files might appear on your system.)

Table 12.2. Supporting Files Used by Each Hive

Hive

Files

HKLM\BCD00000000

%SystemRoot%\System32\config\BCD-Template

%SystemRoot%\System32\config\BCD-Template.LOG

HKLM\COMPONENTS

%SystemRoot%\System32\config\COMPONENTS

%SystemRoot%\System32\config\COMPONENTS.LOG

%SystemRoot%\System32\config\COMPONENTS.LOG1

%SystemRoot%\System32\config\COMPONENTS.LOG2

HKLM\SAM

%SystemRoot%\System32\config\SAM

%SystemRoot%\System32\config\SAM.LOG

%SystemRoot%\System32\config\SAM.LOG1

%SystemRoot%\System32\config\SAM.LOG2

HKLM\SECURITY

%SystemRoot%\System32\config\SECURITY

%SystemRoot%\System32\config\SECURITY.LOG

%SystemRoot%\System32\config\SECURITY.LOG1

%SystemRoot%\System32\config\SECURITY.LOG2

HKLM\SOFTWARE

%SystemRoot%\System32\config\SOFTWARE

%SystemRoot%\System32\config\SOFTWARE.LOG

%SystemRoot%\System32\config\SOFTWARE.LOG1

%SystemRoot%\System32\config\SOFTWARE.LOG2

HKLM\SYSTEM

%SystemRoot%\System32\config\SYSTEM

%SystemRoot%\System32\config\SYSTEM.LOG

%SystemRoot%\System32\config\SYSTEM.LOG1

%SystemRoot%\System32\config\SYSTEM.LOG2

HKU\.DEFAULT

%SystemRoot%\System32\config\DEFAULT

%SystemRoot%\System32\config\DEFAULT.LOG

%SystemRoot%\System32\config\DEFAULT.LOG1

%SystemRoot%\System32\config\DEFAULT.LOG2

Also, each user has his or her own hive, which maps to HKEY_CURRENT_USER during logon. The supporting files for each user hive are stored in \Users\user, where user is the username. In each case, the ntuser.dat file contains the hive data, and the ntuser.dat.log1 file tracks the hive changes. (If a user has group policies set on her account, the user data is stored in an HKEY_USERS subkey.)

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