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Oracle have two Memory spaces:

• SGA memory which is shared by all sessions connected to Oracle Database. See Oracle Database - System Global Area - SGA (Shared Memory)
• and PGA memory which is allocated for each session. See Oracle Database - Process global area (PGA) - Work Area

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Example of configuration changes

With SQLPlus

shutdown immediate
startup nomount

ORACLE instance started.
Total System Global Area | 1570009088 | bytes
Fixed Size               |    2403064 | bytes
Variable Size            |  939525384 | bytes
Database Buffers         |  620756992 | bytes
Redo Buffers             |    7323648 | bytes

alter system set memory_max_target=500m scope=spfile;
alter system set memory_target=500m scope=spfile;
shutdown 
startup

ORACLE instance started.
Total System Global Area |  521936896 | bytes
Fixed Size               |    2404552 | bytes
Variable Size            |  427822904 | bytes
Database Buffers         |   83886080 | bytes
Redo Buffers             |    7823360 | bytes