Why is FreeBSD finding the wrong amount of memory?
The reason is the difference between physical memory addresses and virtual addresses. The convention for most PC hardware is to use the memory area between 3.5G and 4G for a special purpose (usually for PCI). This address space is used to access PCI hardware. As a result real, physical memory can not appear in that address space. What happens to the memory that should appear in that location is dependent on your hardware. Unfortunately, some hardware does nothing and the ability to use that last 500M of RAM is entirely lost. Luckily, most hardware remaps the memory to a higher location so that it can still be used. However, this can cause some confusion if you watch the boot messages. On a 32 bit version of FreeBSD, the memory appears lost, since it will be remapped above 4G, which a 32 bit kernel is unable to access. In this case, the solution is to build a PAE enabled kernel. See this FAQ entry for more information. On a 64 bit version of FreeBSD, or when running a PAE-enabled kernel
