Plz explain about wrong size of database buffer cache ?

   Hi all,

    I have been used oracle version 9.2.0.4 32 bit.

    I want change size of database buffer cache, I use:

    SQL> show parameter sga_;

NAME                                 TYPE        VALUE

------------------------------------ ----------- ------------------------------

sga_max_size                         big integer 269554828

   SQL> alter system set db_cache_size=40M;

   System altered.

   I checked again:

   SQL> show parameter db_cache_size;

NAME                                 TYPE        VALUE

------------------------------------ ----------- ------------------------------

db_cache_size                        big integer 50331648    --> why it about 48 MB ?? (I set 40 MB) ??

    And after that, I were set DB_KEEP_CACHE_SIZE is 10MB with SQL> alter system set db_keep_cache_size=10M;

    But when I checked again:

    SQL> show parameter db_keep_cache;