Oracle parameter possible value acquisition method

Sometimes it is not clear that all allowable values for 1 parameter can be obtained by the following two methods
1. Enter the wrong value directly and let error prompt

 
SQL> alter system set "_db_block_cache_protect"=what scope=spfile; 
alter system set "_db_block_cache_protect"=what scope=spfile 
* 
ERROR at line 1: 
ORA-00096: invalid value WHAT for parameter _db_block_cache_protect, must be 
from among TRUE, MEDIUM, LOW, FALSE 

All the allowed values of _db_block_cache_protect are TRUE, MEDIUM, LOW, FALSE
The downside is that 9i and previous versions do not support this kind of checking
2. Through scripts
cat a.sql

 
COL pvalid_default HEAD DEFAULT FOR A7 
COL pvalid_value HEAD VALUE FOR A30 
COL pvalid_name HEAD PARAMETER FOR A50 
COL pvalid_par# HEAD PAR# FOR 99999 
BREAK ON pvalid_par# skip 1 
PROMPT Display valid values for multioption parameters matching "&1"... 
SELECT 
-- INST_ID, 
PARNO_KSPVLD_VALUES pvalid_par#, 
NAME_KSPVLD_VALUES pvalid_name, 
ORDINAL_KSPVLD_VALUES ORD, 
VALUE_KSPVLD_VALUES pvalid_value, 
DECODE(ISDEFAULT_KSPVLD_VALUES, 'FALSE', '', 'DEFAULT' ) pvalid_default 
FROM 
X$KSPVLD_VALUES 
WHERE 
LOWER(NAME_KSPVLD_VALUES) LIKE LOWER('%&1%') 
ORDER BY 
pvalid_par#, 
pvalid_default, 
ord, 
pvalid_Value 

Execute with sys user
SQL > @a block
All possible values for the parameters associated with block are fetched.