Memory usage limit on AWS CentOS VM

I deployed a CentOS image using a vmdk file on AWS. The type of image is r3.4xlarge, with which the default memory comes to 120GB, where as I want the memory only to be 64G, I went to /boot/grub/menu.lst and added the option "mem=64G" at the end of the default Kernel entry.

After reboot, my instance never came up, I am pretty sure I screwed up something. In other case, I deployed the instance, changed the menu.lst to try and add the option mem=64G, but the system still came up 120G as total memory.

How can I technically limit the memory on AWS instance?