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    Memory structures are allocated to the Oracle instance when the instance is started.
The two major memory structures are known as the System Global Area(Also called the shared
Global Area)and the Program Global Area(also called the Private Global Area or the Process
Global Area)
SGA:
 The System Global Area(SGA) is a shared memory area.All users of the database share the
 information maintained in this area.The SGA and the backgroud processes constitute an Oracle instance.Oracle allocates memory for the SGA when an Oracle instance is started and de-allocates it when the istance is shut down.The information stored in the SGA is divided into multiple memory structures that are allocated fixed space when the instance started.
     DATABASE Buffer Cache:
     The database buffer cache is the area of memory that caches the database area,holding blocks
from the data files that have been read recently.The DB buffer cache is shared among all the users connected to the database.
     There are three type of buffers.
     Dirty buffers:Dirty buffers are the buffer blocks that need to be written to the data files.
The data in these buffers has changed and has not yet been written ti the disk.
      Free buffers :Free buffers do not contain any data or are free to be overwritten.When Oracle reads data from disk,free buffers are used to hold this area.
      Pinned buffers :Pinned buffers are the ones that are currently being accessed or explicityly  retained for futer use.(for example,the KEEP buffer)
      Oracle maintains two lists to manage the buffer cache,The write list(dirty buffer list) has the buffers that are modified and need to be written to the disk(the dirty buffers).
      The least recently used(LRU) list contains fr