In Windows 95, data is written to the Registry only when a flush occurs — that is, when something happens after changed data has aged more than a few seconds or when an application intentionally flushes the data to the hard disk.
Each time Windows 95 successfully starts, the operating system backs up the Registry by copying the current SYSTEM.DAT and USER.DAT files to SYSTEM.DA0 and USER.DA0, respectively. If Windows 95 fails to start, the backed-up Registry from the last successful startup can be copied over the current Registry. This method recovers the last successful settings after a system failure.