Half of memory is not usable - Please help me understand where it is

So here is the short story: I have 4GB of Ram (2x2GB Gskill Ripjaw series) installed. Yet, Ubuntu only recognizes 2GB, Windows recognizes that 4GB is installed, but only 1.99 useable, and the BIOS says that there is 4GB installed. On Windows, running idle, over 1GB of RAM is being used, leaving me with a paltry 1GB left to do anything with. I have an Intel Core i5 750 @2.67Ghz proc and an MSI P55-GD65 mobo, and there is no memory remapping feature. I have an ATI Radeon HD 5750 1GB graphics card.

The RAM is good, I have switched it around, it is in the appropriate channels. Attached are screenshots of performance manager, device manager, and cpu-z. If anyone can help I will sincerely appreciate it! I am VERY bad at converting hex, so am unsure how to read the memory management screenshot, but it should hopefully help identify where all my memory is going.

Additionally, I reran the Windows rating system, and my CPU score dropped from 7.1 to 4.7...is there any way to test if there is now a problem with my CPU? My BIOS settings are Optimized Defaults. I tried generating a system health report, but only the summary would show I could not view any details.

What does BIOS report for total installed memory?
Make sure PAE is enabled in the BIOS (HPET has nothing to do with the issue you have so it can be 32 bit or 64 bit doesn't matter).

Have you tried MemTest86+? Does it indicate 4 GB total installed memory?

Did you check if newer version of BIOS is available?

Each single hex digit is equal to 4 bits (or half byte). It is very clear that the memory address space is 64-bit long in the arrows you pointed out. I can do the conversion for you if you like but not sure if it would help resolve the particular problem you have.

Does your BIOS have a Memory Remap feature?

gpost3 said:

See if you can open up C:\boot.ini and append the following near the end of the file:

Code:

/PAE

Save and restart.

boot.ini is for previous version of Windows below Windows Vista. Nor is PAE going to have any affect on a 64-bit system.

Since no one decided to do the math for you, i'll do it for you. Just follow along and let me know if you have questions.

You have 4 GB of memory, your total address space is: 2^32-1=4294967295. To test that answer, Convert that value back into gigabyte 4294967295/(1024^3)=4 GB

I am looking at the second arrow so let's compute the total number of addresses mapped by the ATI Card. We have 8 hex digits which gives us (8x4)=32 bits.

hex code goes from 0-9 then A, B, C, D and so on. So A is 10, B is 11, C is 12 and D is 13 (all in base 10).

13 (Base 10) is 1101 (Base 2). Computing the bits we get 2^31+2^30+2^28=3489660928.
For the second range we have 2^31+2^30+2^28+2^28-1=3758096383.
Difference between the two is 268435455 which is equal to 256 Megabytes reserved by the ATI card.

You want to know what is using your ram. Open Task Manager/Performance/Resource Monitor. This is easier, Windows key+ R and type in resmon. It will show you what is using your ram.