Table A lists the standard NT 4.0 Registry hives and their
corresponding support files. The hive support files reside in the \config
directory of your NT installation. Files without extensions contain a copy of
the hive. Files with the .log extension contain a machine-readable record of any
changes to a hive. The .sav files contain copies of the hives at the end of
Setup's text-mode step. The .alt file contains the Last Known Good information,
in case anything happens to the system hive. The .dat files contain user profile
information.
Under normal circumstances, your system will reference the files without
extensions. When writing changes to a hive file, the system will note the
changes in the hive's log, so that it can cancel any unfinished changes in case
the hive's updating process is interrupted. If something happens to the critical
system hive, the . alt version replaces it during the boot process; . alt then
makes itself the new system hive and creates another backup . alt file.