The Windows Registry 2






Registry Storage Space


Although there are few technical limits to the type and size of data an application can store in the registry, certain practical guidelines exist to promote system efficiency. An application should store configuration and initialization data in the registry, and store other kinds of data elsewhere. Generally, data consisting of more than one or two kilobytes (K) should be stored as a file and referred to by using a key in the registry rather than being stored as a value. Instead of duplicating large pieces of data in the registry, an application should save the data as a file and refer to the file. Executable binary code should never be stored in the registry. A value entry uses much less registry space than a key. To save space, an application should group similar data together as a structure and store the structure as a value rather than storing each of the structure members as a separate key. (Storing the data in binary form allows an application to store data in one value that would otherwise be made up of several incompatible types.) Views of the registry files are mapped in the computer cache address space. Therefore, regardless of the size of the registry data, it is not charged more than 4 megabytes (MB). There are no longer any explicit limits on the total amount of space that may be consumed by hives in paged pool memory, and in disk space. The size of the system hive is limited only by physical memory.


Windows 2000


Registry data is stored in the paged pool, an area of physical memory used for system data that can be written to disk when not in use. The RegistrySizeLimit value establishes the maximum amount of paged pool that can be consumed by registry data from all applications. This value is located in the following registry key:







By default, the registry size limit is 25 percent of the paged pool. (The default size of the paged pool is 32 MB, so this is 8 MB.) The system ensures that the minimum value of RegistrySizeLimit is 4 MB and the maximum is approximately 80 percent of the PagedPoolSize value. If the value of this entry is greater than 80 percent of the size of the paged pool, the system sets the maximum size of the registry to 80 percent of the size of the paged pool. This prevents the registry from consuming space needed by processes. Note that setting this value does not allocate space in the paged pool, nor does it assure that the space will be available if needed. The paged pool size is determined by the PagedPoolSize value in the following registry key:









The maximum paged pool is approximately 300,470 MB so the registry size limit is 240-376 MB. However, if the /3GB switch is used, the maximum paged pool size is 192 MB, so the registry can be a maximum of 153.6 MB.


Predefined Keys


An application must open a key before it can add data to the registry. To open a key, an application must supply a handle to another key in the registry that is already open. The system defines predefined keys that are always open. Predefined keys help an application navigate in the registry and make it possible to develop tools that allow a system administrator to manipulate categories of data. Applications that add data to the registry should always work within the framework of predefined keys, so administrative tools can find and use the new data. An application can use handles to these keys as entry points to the registry. These handles are valid for all implementations of the registry, although the use of the handles may vary from platform to platform. In addition, other predefined handles have been defined for specific platforms. The following are handles to the predefined keys.





Registry entries subordinate to this key define types (or classes) of documents and the properties associated with those types. Shell and COM applications use the information stored under this key.

This key also provides backward compatibility with the Windows 3.1 registration database by storing information for DDE and OLE support. File viewers and user interface extensions store their OLE class identifiers in HKEY_CLASSES_ROOT, and in-process servers are registered in this key. This handle should not be used in a service or an application that impersonates different users.


Contains information about the current hardware profile of the local computer system. The information under HKEY_CURRENT_CONFIG describes only the differences between the current hardware configuration and the standard configuration. Information about the standard hardware configuration is stored under the Software and System keys of HKEY_LOCAL_MACHINE.

HKEY_CURRENT_CONFIG is an alias for HKEY_LOCAL_MACHINE\System\CurrentControlSet\Hardware Profiles\Current.


Registry entries subordinate to this key define the preferences of the current user. These preferences include the settings of environment variables, data about program groups, colors, printers, network connections, and application preferences. This key makes it easier to establish the current user's settings; the key maps to the current user's branch in HKEY_USERS. In HKEY_CURRENT_USER, software vendors store the current user-specific preferences to be used within their applications. Microsoft, for example, creates the HKEY_CURRENT_USER\Software\Microsoft key for its applications to use, with each application creating its own subkey under the Microsoft key.

The mapping between HKEY_CURRENT_USER and HKEY_USERS is per process and is established the first time the process references HKEY_CURRENT_USER. The mapping is based on the security context of the first thread to reference HKEY_CURRENT_USER. If this security context does not have a registry hive loaded in HKEY_USERS, the mapping is established with HKEY_USERS\.Default. After this mapping is established it persists, even if the security context of the thread changes. This handle should not be used in a service or an application that impersonates different users. Instead, call the RegOpenCurrentUser() function.


Registry entries subordinate to this key define the physical state of the computer, including data about the bus type, system memory, and installed hardware and software. It contains subkeys that hold current configuration data, including Plug and Play information (the Enum branch, which includes a complete list of all hardware that has ever been on the system), network logon preferences, network security information, software-related information (such as server names and the location of the server), and other system information.


Registry entries subordinate to this key allow you to access performance data. The data is not actually stored in the registry; the registry functions cause the system to collect the data from its source.


Registry entries subordinate to this key reference the text strings that describe counters in the local language of the area in which the computer system is running. These entries are not available to Regedit.exe and Regedt32.exe. For Windows 2000:  This key is not supported.


Registry entries subordinate to this key reference the text strings that describe counters in US English. These entries are not available to Regedit.exe and Regedt32.exe. For Windows 2000:  This key is not supported.


Registry entries subordinate to this key define the default user configuration for new users on the local computer and the user configuration for the current user.


The RegOverridePredefKey() function enables you to map a predefined registry key to a specified key in the registry. For instance, a software installation program could remap a predefined key before installing a DLL component. This enables the installation program to easily examine the information that the DLL's installation procedure writes to the predefined key. The RegDisablePredefinedCache() and RegDisablePredefinedCacheEx() functions disable handle caching for predefined registry handles. Services that use impersonation should call RegDisablePredefinedCacheEx() before using predefined registry handles. The predefined handles are not thread safe. Closing a predefined handle in one thread affects any other threads that are using the handle.





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