Change Database into Archvielog Mode

If you create your Oracle9i database through DBCA, it is created in NOARCHIVELOG mode by default unless you select archivelog option during the creating process.

You can change Archive Mode when database is mounted but now opened.

SQL> alter database archivelog;
SQL> alter database noarchivelog;

After archivelog mode is enabled and archive log destination is properly defined, the database is ready for archiving log files, either manually or automatically.
You can enable/disable automatic archiving by changing parameter archive_log_start. This parameter is static.

SQL> alter system set log_archive_start=TRUE scope=spfile;
SQL> alter system set log_archive_start=FALSE scope=spfile;

As log as automatic archival is enabled, you can see ARCn background processes running.
The following command can show you database archive settings.

SQL> archive log list;
Database log mode              Archive Mode
Automatic archival             Enabled
Archive destination            C:\BACKUP
Oldest online log sequence     34
Current log sequence           37