In 9i I change db_cache_size return error

[xiongye]

Hi: In Oracle 9i my machine has enough memory(vmstat 5)
and I execute "alter system set db_cache_size=128M scope=both;" the error is "ERROR at line 1:
ORA-02097: parameter cannot be modified because specified value is invalid
ORA-00384: Insufficient memory to grow cache"
the parameter sga_max_size is very large can not be exceed? what is the error? Thanks !!!