The Microsoft Computer Dictionary, Fifth Edition, defines the registry as:
A central hierarchical database used in Microsoft Windows 98, Windows CE, Windows NT, and Windows 2000 used to store information that is necessary to configure the system for one or more users, applications and hardware devices.
The Registry contains information that Windows continually references during operation, such as profiles for each user, the applications installed on the computer and the types of documents that each can create, property sheet settings for folders and application icons, what hardware exists on the system, and the ports that are being used.
The Registry replaces most of the text-based .ini files that are used in Windows 3.x and MS-DOS configuration files, such as the Autoexec.bat and Config.sys. Although the Registry is common to several Windows operating systems, there are some differences among them.
A registry hive is a group of keys, subkeys, and values in the registry that has a set of supporting files that contain backups of its data. The supporting files for all hives except HKEY_CURRENT_USER are in the %SystemRoot%\System32\Config folder on Windows NT 4.0, Windows 2000, Windows XP, Windows Server 2003, and Windows Vista. The supporting files for HKEY_CURRENT_USER are in the %SystemRoot%\Profiles\Username folder. The file name extensions of the files in these folders indicate the type of data that they contain. Also, the lack of an extension may sometimes indicate the type of data that they contain.
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Registry hive Supporting files
HKEY_LOCAL_MACHINE\SAM Sam, Sam.log, Sam.sav
HKEY_LOCAL_MACHINE\Security Security, Security.log, Security.sav
HKEY_LOCAL_MACHINE\Software Software, Software.log, Software.sav
HKEY_LOCAL_MACHINE\System System, System.alt, System.log, System.sav
HKEY_CURRENT_CONFIG System, System.alt, System.log, System.sav, Ntuser.dat, Ntuser.dat.log
HKEY_USERS\DEFAULT Default, Default.log, Default.sav
In Windows 98, the registry files are named User.dat and System.dat. In Windows Millennium Edition, the registry files are named Classes.dat, User.dat, and System.dat.
Note Security features in Windows NT, Windows 2000, Windows XP, Windows Server 2003, and Windows Vista let an administrator control access to registry keys.
The following table lists the predefined keys that are used by the system. The maximum size of a key name is 255 characters.
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Folder/predefined key Description
HKEY_CURRENT_USER Contains the root of the configuration information for the user who is currently logged on. The user's folders, screen colors, and Control Panel settings are stored here. This information is associated with the user's profile. This key is sometimes abbreviated as "HKCU."
HKEY_USERS Contains all the actively loaded user profiles on the computer. HKEY_CURRENT_USER is a subkey of HKEY_USERS. HKEY_USERS is sometimes abbreviated as "HKU."
HKEY_LOCAL_MACHINE Contains configuration information particular to the computer (for any user). This key is sometimes abbreviated as "HKLM."
HKEY_CLASSES_ROOT Is a subkey of HKEY_LOCAL_MACHINE\Software. The information that is stored here makes sure that the correct program opens when you open a file by using Windows Explorer. This key is sometimes abbreviated as "HKCR." Starting with Windows 2000, this information is stored under both the HKEY_LOCAL_MACHINE and HKEY_CURRENT_USER keys. The HKEY_LOCAL_MACHINE\Software\Classes key contains default settings that can apply to all users on the local computer. The HKEY_CURRENT_USER\Software\Classes key contains settings that override the default settings and apply only to the interactive user. The HKEY_CLASSES_ROOT key provides a view of the registry that merges the information from these two sources. HKEY_CLASSES_ROOT also provides this merged view for programs that are designed for earlier versions of Windows. To change the settings for the interactive user, changes must be made under HKEY_CURRENT_USER\Software\Classes instead of under HKEY_CLASSES_ROOT. To change the default settings, changes must be made under HKEY_LOCAL_MACHINE\Software\Classes. If you write keys to a key under HKEY_CLASSES_ROOT, the system stores the information under HKEY_LOCAL_MACHINE\Software\Classes. If you write values to a key under HKEY_CLASSES_ROOT, and the key already exists under HKEY_CURRENT_USER\Software\Classes, the system will store the information there instead of under HKEY_LOCAL_MACHINE\Software\Classes.
HKEY_CURRENT_CONFIG Contains information about the hardware profile that is used by the local computer at system startup.
Note The registry in 64-bit versions of Windows XP, Windows Server 2003, and Windows Vista is divided into 32-bit and 64-bit keys. Many of the 32-bit keys have the same names as their 64-bit counterparts, and vice versa. The default 64-bit version of Registry Editor that is included with 64-bit versions of Windows XP, Windows Server 2003, and Windows Vista displays the 32-bit keys under the following node:
HKEY_LOCAL_MACHINE\Software\WOW6432Node
For more information about how to view the registry on 64-Bit versions of Windows, click the following article number to view the article in the Microsoft Knowledge Base:
305097 (http://support.microsoft.com/kb/305097/ ) How to view the system registry by using 64-bit versions of Windows
The following table lists the data types that are currently defined and that are used by Windows. The maximum size of a value name is as follows:
* Windows Server 2003, Windows XP, and Windows Vista: 16,383 characters
* Windows 2000: 260 ANSI characters or 16,383 Unicode characters
* Windows Millennium Edition/Windows 98/Windows 95: 255 characters
Long values (more than 2,048 bytes) must be stored as files with the file names stored in the registry. This helps the registry perform efficiently. The maximum size of a value is as follows:
* Windows NT 4.0/Windows 2000/Windows XP/Windows Server 2003/Windows Vista: Available memory
* Windows Millennium Edition/Windows 98/Windows 95: 16,300 bytes
Note There is a 64K limit for the total size of all values of a key.
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Name Data type Description
Binary Value REG_BINARY Raw binary data. Most hardware component information is stored as binary data and is displayed in Registry Editor in hexadecimal format.
DWORD Value REG_DWORD Data represented by a number that is 4 bytes long (a 32-bit integer). Many parameters for device drivers and services are this type and are displayed in Registry Editor in binary, hexadecimal, or decimal format. Related values are DWORD_LITTLE_ENDIAN (least significant byte is at the lowest address) and REG_DWORD_BIG_ENDIAN (least significant byte is at the highest address).
Expandable String Value REG_EXPAND_SZ A variable-length data string. This data type includes variables that are resolved when a program or service uses the data.
Multi-String Value REG_MULTI_SZ A multiple string. Values that contain lists or multiple values in a form that people can read are generally this type. Entries are separated by spaces, commas, or other marks.
String Value REG_SZ A fixed-length text string.
Binary Value REG_RESOURCE_LIST A series of nested arrays that is designed to store a resource list that is used by a hardware device driver or one of the physical devices it controls. This data is detected and written in the \ResourceMap tree by the system and is displayed in Registry Editor in hexadecimal format as a Binary Value.
Binary Value REG_RESOURCE_REQUIREMENTS_LIST A series of nested arrays that is designed to store a device driver's list of possible hardware resources the driver or one of the physical devices it controls can use. The system writes a subset of this list in the \ResourceMap tree. This data is detected by the system and is displayed in Registry Editor in hexadecimal format as a Binary Value.
Binary Value REG_FULL_RESOURCE_DESCRIPTOR A series of nested arrays that is designed to store a resource list that is used by a physical hardware device. This data is detected and written in the \HardwareDescription tree by the system and is displayed in Registry Editor in hexadecimal format as a Binary Value.
None REG_NONE Data without any particular type. This data is written to the registry by the system or applications and is displayed in Registry Editor in hexadecimal format as a Binary Value
Link REG_LINK A Unicode string naming a symbolic link.
QWORD Value REG_QWORD Data represented by a number that is a 64-bit integer. This data is displayed in Registry Editor as a Binary Value and was introduced in Windows 2000.
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Back up the registry
Before you edit the registry, export the keys in the registry that you plan to edit, or back up the whole registry. If a problem occurs, you can then follow the steps in the "Restore the registry" section to restore the registry to its previous state. To back up the whole registry, use the Backup utility to back up the system state. The system state includes the registry, the COM+ Class Registration Database, and your boot files. For more information about how to use the Backup utility to back up the system state, click the following article numbers to view the articles in the Microsoft Knowledge Base:
308422 (http://support.microsoft.com/kb/308422/ ) How to use the Backup utility that is included in Windows XP to back up files and folders
320820 (http://support.microsoft.com/kb/320820/ ) How to use the Backup utility to back up files and folders in Windows XP Home Edition
326216 (http://support.microsoft.com/kb/326216/ ) How to use the backup feature to back up and restore data in Windows Server 2003
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Edit the registry
To modify registry data, a program must use the registry functions that are defined in the following MSDN Web site:
http://msdn2.microsoft.com/en-us/library/ms724875.aspx (http://msdn2.microsoft.com/en-us/library/ms724875.aspx)
Administrators can modify the registry by using Registry Editor (Regedit.exe or Regedt32.exe), Group Policy, System Policy, Registry (.reg) files, or by running scripts such as VisualBasic script files.
Use the Windows user interface
We recommend that you use the Windows user interface to change your system settings instead of manually editing the registry. However, editing the registry may sometimes be the best method to resolve a product issue. If the issue is documented in the Microsoft Knowledge Base, an article with step-by-step instructions to edit the registry for that issue will be available. We recommend that you follow those instructions exactly.
Use Registry Editor
Warning Serious problems might occur if you modify the registry incorrectly by using Registry Editor or by using another method. These problems might require that you reinstall the operating system. Microsoft cannot guarantee that these problems can be solved. Modify the registry at your own risk.
You can use Registry Editor to do the following:
* Locate a subtree, key, subkey, or value
* Add a subkey or a value
* Change a value
* Delete a subkey or a value
* Rename a subkey or a value
The navigation area of Registry Editor displays folders. Each folder represents a predefined key on the local computer. When you access the registry of a remote computer, only two predefined keys appear: HKEY_USERS and HKEY_LOCAL_MACHINE.
Use Group Policy
Microsoft Management Console (MMC) hosts administrative tools that you can use to administer networks, computers, services, and other system components. The Group Policy MMC snap-in lets administrators define policy settings that are applied to computers or users. You can implement Group Policy on local computers by using the local Group Policy MMC snap-in, Gpedit.msc. You can implement Group Policy in Active Directory by using the Active Directory Users and Computers MMC snap-in. For more information about how to use Group Policy, see the Help topics in the appropriate Group Policy MMC snap-in.
Use a Registration Entries (.reg) file
Create a Registration Entries (.reg) file that contains the registry changes, and then run the .reg file on the computer where you want to make the changes. You can run the .reg file manually or by using a logon script. For more information, click the following article number to view the article in the Microsoft Knowledge Base:
310516 (http://support.microsoft.com/kb/310516/ ) How to add, modify, or delete registry subkeys and values by using a Registration Entries (.reg) file
Use Windows Script Host
The Windows Script Host lets you run VBScript and JScript scripts directly in the operating system. You can create VBScript and JScript files that use Windows Script Host methods to delete, to read, and to write registry keys and values. For more information about these methods, visit the following Microsoft Web sites:
RegDelete method
http://msdn2.microsoft.com/en-us/library/293bt9hh.aspxp (http://msdn2.microsoft.com/en-us/library/293bt9hh.aspx)
RegRead method
http://msdn2.microsoft.com/en-us/library/x05fawxd.aspx (http://msdn2.microsoft.com/en-us/library/x05fawxd.aspx)
RegWrite method
http://msdn2.microsoft.com/en-us/library/yfdfhz1b (http://msdn2.microsoft.com/en-us/library/yfdfhz1b)
Use Windows Management Instrumentation
Windows Management Instrumentation (WMI) is a component of the Microsoft Windows operating system and is the Microsoft implementation of Web-Based Enterprise Management (WBEM). WBEM is an industry initiative to develop a standard technology for accessing management information in an enterprise environment. You can use WMI to automate administrative tasks (such as editing the registry) in an enterprise environment. You can use WMI in scripting languages that have an engine on Windows and that handle Microsoft ActiveX objects. You can also use the WMI Command-Line utility (Wmic.exe) to modify the Windows registry.
For more information about WMI, visit the following Microsoft Web site:
http://msdn2.microsoft.com/en-us/library/aa394582.aspx (http://msdn2.microsoft.com/en-us/library/aa394582.aspx)
For more information about the WMI Command-Line utility, click the following article number to view the article in the Microsoft Knowledge Base:
290216 (http://support.microsoft.com/kb/290216/ ) A description of the Windows Management Instrumentation (WMI) command-line utility (Wmic.exe)
Use Console Registry Tool for Windows
You can use the Console Registry Tool for Windows (Reg.exe) to edit the registry. For help with the Reg.exe tool, type reg /? at the Command Prompt, and then click OK.
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Restore the registry
To restore the registry, use the appropriate method.
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Restore the registry keys
To restore registry subkeys that you exported, double-click the Registration Entries (.reg) file that you saved in the Export registry subkeys section. Or, you can restore the whole registry from a backup. For more information about how to restore the whole registry, see the “Restore the whole registry” section later in this article.
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Restore the whole registry
To restore the whole registry, restore the system state from a backup. For more information about how to restore the system state from a backup, click the following article number to view the article in the Microsoft Knowledge Base:
309340 (http://support.microsoft.com/kb/309340/ ) How to use Backup to restore files and folders on your computer in Windows XP
Note Backing up the system state also creates updated copies of the registry files in the %SystemRoot%\Repair folder. If you cannot start Windows XP after you edit the registry, you can replace the registry files manually by using the steps in the "Part One" section of the following Microsoft Knowledge Base article:
307545 (http://support.microsoft.com/kb/307545/ ) How to recover from a corrupted registry that prevents Windows XP from starting
