My PC says it has 4GB of RAM installed but only 3.5GB are usable, i have windows 7 home premium X64.
CPU: Athlon II x3 Unlocked to Phenom II X4 @3Ghz.
Motherboard: Asrock N68-VS3-UCC
RAM: 4GB DDR3 generic brand.
HDD: 1x 320GB SATA, 1x 250GB SATA and 1x 250GB IDE
GPU: XFX ONE 5400 series
PSU EM 500w PSU.
Windows recognized the full 4GB and the whole 4GB were usable but when i checked about 2 days ago it said that only 3.5GB were usable.... i was very confused so i stated to do a little research and i found out that my GPU might be using Hyper Memory so i tuned off my system, removed the GPU and when i wen't to see how much RAM was usable it said 4GB installed 3.25GB usable....so its not my GPU, if you guys need any more data for helping i will provided ASAP.
it how the mb uses ram for the pci bus and other parts. it a hold over from the old pc days. there nothing wrong with your system. the only thing i would check is see if you have your onboard video on and are not using it. if it is on and your not using it turn it off and see if that reserved memory comes back.