windows vista - Why is usable RAM less than total RAM?

My girlfriend bought a laptop last week. It's a core 2 duo with 4 GB We installed vista 64bit, and one of the first things we did was right click on "My computer" to see gthe properties. Immediately we noticed something strange about her RAM, the line said: Installed memory (RAM): 4,00 GB (3,68 GB usable)

I told her not to worry, thinking it must be something about the laptop hardware (considering her vista installation came from the same DVD as mine, and I never noticed anything like that on my 4 GB desktop). One hour ago, it got worse. We looked at Properties again, and it now says: Installed memory (RAM): 4,00 GB (2,98 GB usable)

What does that mean? Are those 1,02 GB missing or being used by the system?

EDIT: There is a possibility that the sytem information is wrong. I just noticed that it reports an intel T6500 processor, when it's actually a T6400. How can I find out how much RAM is really available to the system?

EDIT2: Checking the resource monitors, it says 1003 MB are reserved for the hardware. Is that good or bad? Thanks

Answer

Two possible reasons:

1. the video card sharing RAM (using motherboard RAM instead of having its own, or using motherboard RAM to supplement its own)


2. the motherboard chipset does not support remapping (the PCI architecture traditionally "owns" a chunk of the top Gb of the bottom 4Gb of physical memory, the remapping moves this above where your RAM is actually sitting so the two areas don't overlap). This is common with chipsets that only support 4Gb of physical RAM in total.