How to determine free memory and used memory in Oracle

How to determine free memory and used memory in Oracle
The summary information of the SGA is determined by,
SQL> select * from V$SGA;

NAME VALUE
------------ --------- --------- --------- --------- --------- - ----------
Fixed Size 2073376
Variable Size 1056967904
Database Buffers 1509949440
Redo Buffers 14700544

To know the current settings of the sga dynamic components ( can be set in memory by ALTER SYSTEM)
SQL> col COMPONENT format a30
SQL> select COMPONENT, CURRENT_SIZE from V$SGA_DYNAMIC_ COMPONENTS;