Check the Memory usage used by Oracle from Operating System
Check the SGA_TARGET and PGA_TARGET size :
SQL> select sum(value)/1024/1024/1024 from v$parameter where name in ('sga_target','pga_aggregate_target');
SUM(VALUE)/1024/1024/1024
-------------------------
1.5
If MEMORY_TARGET is used then check the memory usage form that :
select (value)/1024/1024/1024 from v$parameter where name in ('MEMORY_TARGET');
Check from the real stats view:
select decode( grouping(nm), 1, 'total', nm ) nm, round(sum(val/1024/1024)) mb
from
(
select 'sga' nm, sum(value) val
from v$sga
union all
select 'pga', sum(a.value)
from v$sesstat a, v$statname b
where b.name = 'session pga memory'
and a.statistic# = b.statistic#
)
group by rollup(nm);
NM MB
----- ----------
pga 173
sga 1024
total 1197