Windows CE Registry

Like desktop Windows, Windows CE uses the registry to save application, driver, and user settings, along with other configuration information. The Windows CE registry also uses a tree structure to manage configuration data. Since its structure and functionality are almost identical to desktop Windows, I won’t详细介绍 (detail it) here — readers can参考 (refer to) other resources on the registry.

Windows CE supports four root keys:

Key	Description
HKEY_LOCAL_MACHINE	Hardware and driver configuration data
HKEY_CURRENT_USER	User configuration data
HKEY_CLASSES_ROOT	OLE and file type association configuration data
HKEY_USERS	Data applicable to all users

Due to embedded system characteristics, some devices have no external storage. Therefore Windows CE provides two registry implementations: RAM-Based Registry and Hive-Based Registry. Either can be used; the type is transparent to users and applications.

RAM-Based Registry

As the name suggests, the RAM-based registry stores the entire registry as an object store heap in system memory. This means all changes are lost on cold boot or power loss.

RAM-based registry read/write access is very efficient, making it suitable for devices without external storage that have battery-backed RAM. For devices with external storage that frequently cold boot, the registry must be saved on power loss and restored on next boot.

Windows CE provides two methods to save the RAM-based registry on power loss:

1. Two system APIs for saving and restoring the entire registry:
   BOOL RegCopyFile(LPCWSTR lpszFile); // File to save registry info BOOL RegRestoreFile(LPCWSTR lpszFile); // File with saved registry info

   Call RegCopyFile on power loss to save the registry to a file, then RegRestoreFile on restart to load it into RAM, followed by a warm boot. The warm boot is necessary because the system only detects RegRestoreFile’s registry data at startup. This method is simple but requires two boots, making it relatively inefficient.

2. OEMs can implement WriteRegistryToOEM and ReadRegistryFromOEM in the BSP’s OAL layer:
   DWORD ReadRegistryFromOEM(DWORD dwFlags, LPBYTE lpData, DWORD cbData); BOOL WriteRegistryToOEM(DWORD dwFlags, LPBYTE lpData, DWORD cbData);

   Windows CE calls these at system startup and shutdown to save/restore the registry. This avoids double booting, but implementing ReadRegistryFromOEM is difficult — at startup, block device and file system drivers aren’t loaded yet, so file system APIs like CreateFile/ReadFile can’t be used; only lower-level operations work.

Hive-Based Registry

Since Windows CE 4.0, the hive-based registry has been available. It stores registry data in file system files called hives. This eliminates the need for save/restore operations on power loss or startup.

A hive is a group of keys, subkeys, and values in the registry, appearing as a single file on the file system. Windows CE has three types of hives:

Type	File	Description
Boot hive	Boot.hv in ROM	Contains HKEY_LOCAL_MACHINE, HKEY_CLASSES_ROOT, HKEY_USERS. Used only at boot.
System hive	Determined by OEM (usually System.hv)	Contains HKEY_LOCAL_MACHINE, HKEY_CLASSES_ROOT, HKEY_USERS. Device-scoped data that doesn’t change with user.
User hive	User.hv	Contains HKEY_CURRENT_USER. User-specific settings; each user has a separate User.hv.

The hive-based registry is suitable for devices with permanent storage that frequently cold boot. It separates system and user data, providing multi-user support — different User.hv files can be loaded when different users log in.