Also, if anyone is interested in the details of how the hardware
works, just read the chipset docs. Intel chipset docs are easily
available on their website. (The same is not true of some of the
other chipset vendors).
Intel? 4 Series Chipset Family
http://www.intel.com/Assets/PDF/datasheet/319970.pdf
Page 56, Figure 4. System Address Ranges
This figure shows that some of the host system address space below 4GB
is allocated to devices. Some DRAM in that address region is remapped
to host system address space above 4GB. That corresponds to the "OS
Invisible Reclaim" DRAM mapped to the "Main Memory Reclaim Address
Range" in that figure.
If the OS does not make use of host system address space memory
located above 4GB the DRAM corresponding to the "OS Invisible Reclaim"
region in that figure is effectively lost. That would account for the
lost 0.5GB or whatever on a 4GB DRAM system.
-Glen