v$memory_dynamic_components shows the current sizes of all dynamically tuned memory components, including the total size of the SGA and the instance PGA.
v$memory_dynamic_components shows exactly the same information as v$sga_dynamic_components except that v$sga_dynamic_components does not include the two rows for SGA Target and PGA Target).
Determine diff between SGA target and actual SGA size:
select
(select current_size from v$memory_dynamic_components where component = 'SGA Target') -
(select sum(value) from v$sga ) diff_sga_target_to_sga,
--
(select current_size from v$memory_dynamic_components where component = 'SGA Target') -
(select value from v$parameter where name = 'sga_target') diff_to_param
from
dual;