The AMM (Automatic Memory Management) now in 11g manages both SGA and PGA together by MMAN process.

The MEMORY_TARGET parameter in 11g comes for (SGA_TARGET+PGA_AGGREGATE_TARGET) which was in 10g.
The amount of shared memory available for Oracle to use when dynamically controlling the SGA and PGA. This parameter is dynamic, so the total amount of memory available to Oracle can be increased or decreased, provided it does not exceed the MEMORY_MAX_TARGET limit. The default value is "0".

The MEMORY_MAX_TARGET parameter in 11g comes instead of SGA_MAX_TARGET parameter which was in 10g. This defines the maximum size the MEMORY_TARGET can be increased to without an instance restart. If the MEMORY_MAX_TARGET is not specified, it defaults to MEMORY_TARGET setting.

[ORA-00845: MEMORY_TARGET not supported on this system] is a very common error message you will get when you try to install Oracle 11gR1 or Oracle 11gR2 on Unix platforms.

This Problem can be solved using one of the following :

A) To adjust the shared memory file system size issue the following commands, specifying the required size of shared memory.

# umount tmpfs

# mount -t tmpfs shmfs -o size=1200m /dev/shm
Make the setting permanent by amending the "tmpfs" setting of the "/etc/fstab" file to look like this.

tmpfs /dev/shm tmpfs size=1200m 0 0

B) Reduce the MEMORY_TARGET in the init parameters.