mem= doesn't control the amount of RAM. It actually just specifies the largest physical address the kernel should try to use. The first 4GB of physical memory

mem= doesn't control the amount of RAM. It actually just specifies the largest physical address the kernel should try to use. The first 4GB of physical memory typically has some non-RAM things in it like the part of the physical address space that's reserved for talking to PCI devices. If you have 2GB of RAM, 2GB of "other stuff" in the first 4GB, and specify mem=4096m, you'll end up with a kernel that sees 2GB of RAM.