When you connect to the database and no connection is established yet,
FB server will allocate cache memory. The size of cache is determined that way:

1) if the database has specified cache size (e.g. using gfix), then that value is used. Period.
otherwise
2) if you specified cache size during connection, that value is used,
otherwise
3) DefaultDbCachePages is used.


When the application connects to the database using isc_attach_database() FB API call,
it can specify desired cache size by isc_dpb_num_buffers value. If you do not use FB API directly,
you have to consult documentation of your connect library or application.
E.g. with ISQL you can specify that value as command line parameter "-cache 12345",
or in FSQL tool you can specify that value as part of Connect command like
CONNECT 'MyDatabase' USER 'TEST' PASSWORD 'test' CACHE 12345;
etc.

Ivan
http://www.volny.cz/iprenosil/interbase/fsql.htm