Keys and values

The registry contains two basic elements: keys and values.
Registry keys are similar to folders—in addition to values, each key can contain subkeys, which may contain further subkeys, and so on. Keys are referenced with a syntax similar to Windows' path names, using backslashes to indicate levels of hierarchy. Each subkey has a mandatory name, which is a non-empty string that cannot contain any backslash, and whose letter case is insignificant.
The hierarchy of registry keys can only be accessed from a known root key handle (which is anonymous but whose effective value is a constant numeric handle) that is mapped to the content of a registry key preloaded by the kernel from a stored "hive", or to the content of a subkey within another root key, or mapped to a registered service or DLL that provides access to its contained subkeys and values.
E.g. HKEY_LOCAL_MACHINE\Software\Microsoft\Windows refers to the subkey "Windows" of the subkey "Microsoft" of the subkey "Software" of the HKEY_LOCAL_MACHINE root key.
There are seven predefined root keys, traditionally named according to their constant handles defined in the Win32 API, or by synonymous abbreviations (depending on applications):

• HKEY_LOCAL_MACHINE or HKLM
• HKEY_CURRENT_CONFIG or HKCC (only in Windows 9x/Me and NT-based versions of Windows)
• HKEY_CLASSES_ROOT or HKCR
• HKEY_CURRENT_USER or HKCU
• HKEY_USERS or HKU
• HKEY_PERFORMANCE_DATA (only in NT-based versions of Windows, but invisible in the Windows Registry Editor)
• HKEY_DYN_DATA (only in Windows 9x/Me, and visible in the Windows Registry Editor)

Like other files and services in Windows, all registry keys may be restricted by access control lists (ACLs), depending on user privileges, or on security tokens acquired by applications, or on system security policies enforced by the system (these restrictions may be predefined by the system itself, and configured by local system administrators or by domain administrators). Different users, programs, services or remote systems may only see some parts of the hierarchy or distinct hierarchies from the same root keys.
Registry values are name/data pairs stored within keys. Registry values are referenced separately from registry keys. Each registry value stored in a registry key has a unique name whose letter case is not significant. The Windows API functions that query and manipulate registry values take value names separately from the key path and/or handle that identifies the parent key. Registry values may contain backslashes in their names, but doing so makes them difficult to distinguish from their key paths when using some legacy Windows Registry API functions (whose usage is deprecated in Win32).
The terminology is somewhat misleading, as each registry key is similar to an associative array, where standard terminology would refer to the name part of each registry value as a "key". The terms are a holdout from the 16-bit registry in Windows 3, in which registry keys could not contain arbitrary name/data pairs, but rather contained only one unnamed value (which had to be a string). In this sense, the entire registry was like a single associative array where the registry keys (in both the registry sense and dictionary sense) formed a hierarchy, and the registry values were all strings. When the 32-bit registry was created, so was the additional capability of creating multiple named values per key, and the meanings of the names were somewhat distorted.^[4] For compatibility with the previous behavior, each registry key may have a "default" value, whose name is the empty string.
Each value can store arbitrary data with variable length and encoding, but which is associated with a symbolic type (defined as a numeric constant) defining how to parse this data. The standard types are:

List of standard registry value types

Type ID	Symbolic type name	Meaning and encoding of the data stored in the registry value
0	REG_NONE	No type (the stored value, if any)
1	REG_SZ	A string value, normally stored and exposed in UTF-16LE (when using the Unicode version of Win32 API functions), usually terminated by a null character
2	REG_EXPAND_SZ	An "expandable" string value that can contain environment variables, normally stored and exposed in UTF-16LE, usually terminated by a null character
3	REG_BINARY	Binary data (any arbitrary data)
4	REG_DWORD / REG_DWORD_LITTLE_ENDIAN	A DWORD value, a 32-bit unsigned integer (numbers between 0 and 4,294,967,295 [232 – 1]) (little-endian)
5	REG_DWORD_BIG_ENDIAN	A DWORD value, a 32-bit unsigned integer (numbers between 0 and 4,294,967,295 [232 – 1]) (big-endian)
6	REG_LINK	A symbolic link (UNICODE) to another registry key, specifying a root key and the path to the target key
7	REG_MULTI_SZ	A multi-string value, which is an ordered list of non-empty strings, normally stored and exposed in UTF-16LE, each one terminated by a null character, the list being normally terminated by a second null character.
8	REG_RESOURCE_LIST	A resource list (used by the Plug-n-Play hardware enumeration and configuration)
9	REG_FULL_RESOURCE_DESCRIPTOR	A resource descriptor (used by the Plug-n-Play hardware enumeration and configuration)
10	REG_RESOURCE_REQUIREMENTS_LIST	A resource requirements list (used by the Plug-n-Play hardware enumeration and configuration)
11	REG_QWORD / REG_QWORD_LITTLE_ENDIAN	A QWORD value, a 64-bit integer (either big- or little-endian, or unspecified) (Introduced in Windows XP)

Root keys

The keys at the root level of the hierarchical database are generally named by their Windows API definitions, which all begin "HKEY". They are frequently abbreviated to a three- or four-letter short name starting with "HK" (e.g. HKCU and HKLM). Technically, they are predefined handles (with known constant values) to specific keys that are either maintained in memory, or stored in hive files stored in the local filesystem and loaded by the system kernel at boot time and then shared (with various access rights) between all processes running on the local system, or loaded and mapped in all processes started in a user session when the user logs on the system.
The HKEY_LOCAL_MACHINE (local machine-specific configuration data) and HKEY_CURRENT_USER (user-specific configuration data) nodes have a similar structure to each other; user applications typically look up their settings by first checking for them in "HKEY_CURRENT_USER\Software\Vendor's name\Application's name\Version\Setting name", and if the setting is not found, look instead in the same location under the HKEY_LOCAL_MACHINE key^{[citation needed]}. However, the converse may apply for administrator-enforced policy settings where HKLM may take precedence over HKCU. The Windows Logo Program has specific requirements for where different types of user data may be stored, and that the concept of least privilege be followed so that administrator-level access is not required to use an application.^{[Note 1][5]}

HKEY_LOCAL_MACHINE (HKLM)

Abbreviated HKLM, HKEY_LOCAL_MACHINE stores settings that are specific to the local computer.^[6]

The key located by HKLM is actually not stored on disk, but maintained in memory by the system kernel in order to map there all other subkeys. Applications cannot create any additional subkeys. On NT-based versions of Windows, this key contains four subkeys, "SAM", "SECURITY", "SYSTEM", and "SOFTWARE", that are loaded at boot time within their respective files located in the %SystemRoot%\System32\config folder. A fifth subkey, "HARDWARE", is volatile and is created dynamically, and as such is not stored in a file (it exposes a view of all the currently detected Plug-and-Play devices). On Windows Vista and above, a sixth subkey is mapped in memory by the kernel and populated from boot configuration data (BCD).

• The "HKLM\SAM" key usually appears as empty for most users (unless they are granted access by administrators of the local system or administrators of domains managing the local system). It is used to reference all "Security Accounts Manager" (SAM) databases for all domains into which the local system has been administratively authorized or configured (including the local domain of the running system, whose SAM database is stored a subkey also named "SAM": other subkeys will be created as needed, one for each supplementary domain). Each SAM database contains all builtin accounts (mostly group aliases) and configured accounts (users, groups and their aliases, including guest accounts and administrator accounts) created and configured on the respective domain, for each account in that domain, it notably contains the user name which can be used to log on that domain, the internal unique user identifier in the domain, a cryptographic hash of each user's password for each enabled authentication protocol, the location of storage of their user registry hive, various status flags (for example if the account can be enumerated and be visible in the logon prompt screen), and the list of domains (including the local domain) into which the account was configured.
• The "HKLM\SECURITY" key usually appears empty for most users (unless they are granted access by users with administrative privileges) and is linked to the Security database of the domain into which the current user is logged on (if the user is logged on the local system domain, this key will be linked to the registry hive stored by the local machine and managed by local system administrators or by the builtin "System" account and Windows installers). The kernel will access it to read and enforce the security policy applicable to the current user and all applications or operations executed by this user. It also contains a "SAM" subkey which is dynamically linked to the SAM database of the domain onto which the current user is logged on.
• The "HKLM\SYSTEM" key is normally only writable by users with administrative privileges on the local system. It contains information about the Windows system setup, data for the secure random number generator (RNG), the list of currently mounted devices containing a filesystem, several numbered "HKLM\SYSTEM\Control Sets" containing alternative configurations for system hardware drivers and services running on the local system (including the currently used one and a backup), a "HKLM\SYSTEM\Select" subkey containing the status of these Control Sets, and a "HKLM\SYSTEM\CurrentControlSet" which is dynamically linked at boot time to the Control Set which is currently used on the local system. Each configured Control Set contains:
  • an "Enum" subkey enumerating all known Plug-and-Play devices and associating them with installed system drivers (and storing the device-specific configurations of these drivers),
  • a "Services" subkey listing all installed system drivers (with non device-specific configuration, and the enumeration of devices for which they are instantiated) and all programs running as services (how and when they can be automatically started),
  • a "Control" subkey organizing the various hardware drivers and programs running as services and all other system-wide configuration,
  • a "Hardware Profiles" subkey enumerating the various profiles that have been tuned (each one with "System" or "Software" settings used to modify the default profile, either in system drivers and services or in the applications) as well as the "Hardware Profiles\Current" subkey which is dynamically linked to one of these profiles.

• The "HKLM\SOFTWARE" subkey contains software and Windows settings (in the default hardware profile). It is mostly modified by application and system installers. It is organized by software vendor (with a subkey for each), but also contains a "Windows" subkey for some settings of the Windows user interface, a "Classes" subkey containing all registered associations from file extensions, MIME types, Object Classes IDs and interfaces IDs (for OLE, COM/DCOM and ActiveX), to the installed applications or DLLs that may be handling these types on the local machine (however these associations are configurable for each user, see below), and a "Policies" subkey (also organized by vendor) for enforcing general usage policies on applications and system services (including the central certificates store used for authentifying, authorizing or disallowing remote systems or services running outside of the local network domain).
• The "HKLM\SOFTWARE\Wow6432Node" key is used by 32-bit applications on a 64-bit Windows OS, and is equivalent but separate to "HKLM\SOFTWARE". The key path is transparently presented to 32-bit applications by WoW64 as HKLM\SOFTWARE^[7] (in a similar way that 32-bit applications see %SystemRoot%\Syswow64 as %SystemRoot%\System32)

HKEY_CURRENT_CONFIG (HKCC)

Abbreviated HKCC, HKEY_CURRENT_CONFIG contains information gathered at runtime; information stored in this key is not permanently stored on disk, but rather regenerated at boot time. It is a handle to the key "HKEY_LOCAL_MACHINE\System\CurrentControlSet\Hardware Profiles\Current", which is initially empty but populated at boot time by loading one of the other subkeys stored in "HKEY_LOCAL_MACHINE\System\CurrentControlSet\Hardware Profiles".

HKEY_CLASSES_ROOT (HKCR)

Abbreviated HKCR, HKEY_CLASSES_ROOT contains information about registered applications, such as file associations and OLE Object Class IDs, tying them to the applications used to handle these items. On Windows 2000 and above, HKCR is a compilation of user-based HKCU\Software\Classes and machine-based HKLM\Software\Classes. If a given value exists in both of the subkeys above, the one in HKCU\Software\Classes takes precedence.^[8] The design allows for either machine- or user-specific registration of COM objects. The user-specific classes hive, unlike the HKCU hive, does not form part of a roaming user profile.

HKEY_USERS (HKU)

Abbreviated HKU, HKEY_USERS contains subkeys corresponding to the HKEY_CURRENT_USER keys for each user profile actively loaded on the machine, though user hives are usually only loaded for currently logged-in users.

HKEY_CURRENT_USER (HKCU)

Abbreviated HKCU, HKEY_CURRENT_USER stores settings that are specific to the currently logged-in user.^[9] The HKEY_CURRENT_USER key is a link to the subkey of HKEY_USERS that corresponds to the user; the same information is accessible in both locations. On Windows NT-based systems, each user's settings are stored in their own files called NTUSER.DAT and USRCLASS.DAT inside their own Documents and Settings subfolder (or their own Users sub folder in Windows Vista and above). Settings in this hive follow users with a roaming profile from machine to machine.

HKEY_PERFORMANCE_DATA

This key provides runtime information into performance data provided by either the NT kernel itself, or running system drivers, programs and services that provide performance data. This key is not stored in any hive and not displayed in the Registry Editor, but it is visible through the registry functions in the Windows API, or in a simplified view via the Performance tab of the Task Manager (only for a few performance data on the local system) or via more advanced control panels (such as the Performances Monitor or the Performances Analyzer which allows collecting and logging these data, including from remote systems).

HKEY_DYN_DATA

This key is used only on Windows 95, Windows 98 and Windows Me.^[10] It contains information about hardware devices, including Plug and Play and network performance statistics. The information in this hive is also not stored on the hard drive. The Plug and Play information is gathered and configured at startup and is stored in memory.^[11]

Hives

Even though the registry presents itself as an integrated hierarchical database, branches of the registry are actually stored in a number of disk files called hives.^[12] (the word hive constitutes an in-joke).^[13]
Some hives are volatile and are not stored on disk at all. An example of this is the hive of branch starting at HKLM\HARDWARE. This hive records information about system hardware and is created each time the system boots and performs hardware detection.
Individual settings for users on a system is stored in a hive (disk file) per user. During user login the system loads the user hive under the HKEY_USERS key, but it also sets the HKCU (HKEY_CURRENT_USER) symbolic reference to point to the current user. This allows applications to store/retrieve settings for the current user implicitly under the HKCU key.
Not all hives are loaded at any one time. At boot time only a minimal set of hives are loaded and after that hives are loaded as the operating system initializes and as users log in or whenever a hive is explicitly loaded by an application.

Editing

Registry editors