How do you decrease available ram

I have a system that I want to perform some tests on with less memory. The problem is that I don't have a physical RAM module that is small enough, so I was wondering if there was some way I could configure Linux to only use a portion of the physical ram that is installed. I am also running the system off of U-Boot, so if you know a way to do it in there that would be sufficient as well. Thanks.

If your distro has grub, you can pass the "mem=512M" option at boot. Your menu.lst kernel line should look something like

kernel /boot/vmlinuz-2.6.** ro quiet splash mem=512M

EDIT: Turns out the mem= parameter is a kernel option, so you just have to figure out where to stick it so it's passed when the kernel boots. Sorry, not familiar with Uboot or embedded stuff...

Thanks for the reply. I actually just tried that, but it cause a kernel panic during boot, not sure why. I'm currently just creating a ramdisk take up some of the available memory, but I would like to find a better method.