"sudo cat /dev/urandom > /dev/mem won't do anything, since sudo will elevate the privilege of cat but not of the redirect. You can either do sudo su and then work in the root shell, or use sudo dd if=/dev/urandom of=/dev/mem
/dev/mem provides access to physical memory, i.e. all of the RAM in the system, however this doesn't mean that it gives you full read/write access to RAM (see CONFIG_STRICT_DEVMEM option in this document). Also note that some regions of physical memory will have other devices like video card memory, etc. mapped onto it.
Writing blindly to /dev/mem will result in an uncertain behaviour, here is a youtube video doing the same."
