Hidden and undocumented INIT.ORA parameters

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

the current values of such parameters can also be found by

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;