Explanation:
The database must be MOUNTED to perform any updates to the control file. The ARCHIVE LOG mode is initially set in the control file when you issue the ALTER DATABASE ARCHIVE LOG command. When you open the database in NOMOUNT stage, the background processes and SGA are created, but the controlfile is not open.
Reference:
Oracle 9i Net Services Administrator’s Guide
Oracle 9i Net Services Reference Guide
Oracle 9i Recovery Manager Reference
Oracle 9i Backup and Recovery Concepts

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