I've installed win7 64 bit but it dos'nt use 6 G ram.only support 3.5 G ! how set it to allow 6G ram ?

What is the exact make, model, and series of your computer? Is your BIOS updated? Check your BIOS for a "memory hole" setting. I have 8GB RAM in my system and my Win 7 x64 reports all 8GB available.Please remember to click Mark as Answer on the post that helps you, and to click Unmark as Answer if a marked post does not actually answer your question. This can be beneficial to other community members reading the thread.

Could be your motherboard doesn't support that much ram. My motherboard maxes out at 4 gigs of ram supported. Your computer seeing only that much looks like a 32 bit problem.