There are the following registry services:
_GetRegistryKey _GetRegistryPath _RegCreateDynKey _RegCloseKey _RegCreateKey _RegDeleteKey _RegDeleteValue _RegEnumKey _RegEnumValue _RegFlushKey _RegOpenKey _RegQueryInfoKey _RegQueryMultipleValues _RegQueryValue _RegQueryValueEx _RegRemapPreDefKey _RegSetValue _RegSetValueEx
Note that the registry is read-only when the processor is in real mode.