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Oracle initialization or INIT.ORA parameters with an underscore in front are hidden or unsupported parameters. One can get a list of all hidden parameters by executing this query:

select *
from SYS.X$KSPPI
where substr(KSPPINM,1,1) = '_';

The following query displays parameter names with their current value:

select a.ksppinm "Parameter", b.ksppstvl "Session Value", c.ksppstvl "Instance Value"
from x$ksppi a, x$ksppcv b, x$ksppsv c
where a.indx = b.indx and a.indx = c.indx
and substr(ksppinm,1,1)='_'
order by a.ksppinm;

Remember: Thou shall not play with undocumented parameters! Using undocumented parameters without the consent of Oracle Support will make your database “un-supported”. You will be on your own if the parameters you’ve set cause problems or data corruption.