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3.7g out of 4g useful in windows 7 x64. What is the reason of such memory gap in my system? Video uses 128M of RAM.

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3.7 out of 4 is not that bad, the problem could be worse depending the other hardware you have.

It basically comes down to your hardware - including capabilities of your BIOS, as well as other hardware devices in your system which make Memory mapped IO reservations in the physical address space you would want to use for RAM.

I'm not sure why hardware devices still do this, but some are much worse than others. My old Dell lost over 700 MB to "Harware reserved", but my new one has less than 14 MB "Harware Reserved." You are right in the middle. Honestly I wouldn't worry about that now that you know why. Any quailty Hardware you buy in a few years will handle this much better.

Here are other SO questions that discuss this. 1,2,3 - (I gave a good answer on this one)

You can cut to the chase reading this blog entry: Memory Mapped IO reservations reduce the usable RAM If you still want more after reading that one, move on to this How windows handles physical memory this one tells you how to spot the offending devices using device manager view "Resources by Connection"

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Check your bios for a "Memory Hole Remapping" setting and enable it.

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While in the bios see if you have memory dedicated to the video card as well. – MrStatic Mar 11 '10 at 21:43
I have no such setting in bios – Artic Mar 12 '10 at 6:03

Read this (includes 64bit instructions towards the bottom).

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Its a pity that i can't down your reputation – Artic Mar 12 '10 at 6:02
@Artic it's a pity you're not more eloquent. What in my response has offended you to a degree you'd want to "down" my reputation? – Traveling Tech Guy Mar 12 '10 at 17:51
Sorry just didnt get answer to my question. What do you mean by not more eloquent? – Artic Mar 13 '10 at 18:46
I mean you didn't elaborate that you felt unanswered. That in itself is no reason to lower someone's reputation. Your response was abrupt and insulting - not the best way to get good will in a community site. – Traveling Tech Guy Mar 14 '10 at 18:13
Oh eya. Will you forgive me. – Artic Mar 16 '10 at 7:11

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