In oracle 9i we have some limitation with reqard to Backing up entire dataabase in one go. First we will have to take the database in mount stage (not open) in order to take the back up, and that too, one tablespace  at a time.
like:-

sql>alter tablespace <tablespace_name> begin backup;

copy all the datafile
querying v$datafile,v$controlfile

after backing up the said tablespace  End the command as:

sql>alter tablespace <tablespace_name> end backup;