It seems like you don't have a real problem, and that you were asking a theoretical question -- like fact-checking GeekSqaud personnel.

Do you or do you know what's eating your top in .5 GB of memory? Can you check if it's the video?

A 32-bit processor can natively and directly address 4 GB of memory. Since the 8086 architecture allowed for a "segment" address combined with an actual memory pointer, the x86 architecture theoretically maintains the ability to address more memory than this.
I doubt it hard to believe that a 64-bit chip would be able to address more memory.

I'm currently eating all of my memory and would like to get the last half-gig. What's more, I'd like to know if I'd be able to double the memory further (buying 4x2GB, since my current setup is full at 4x1GB). So no, it's not theoretical.

I don't understand why we don't understand each other on this point. I have a 64-bit processor and a 64-bit operating system; I should have no trouble addressing 128 GB. Video memory only reduces currently-available memory if you've used up your addressing limit. So when I had 2 GB and was uncontroversially below my addressing limit, I could see all 2 GB (precisely 2048 MB). Video mapping did not reduce the memory at all.

But now 512 MB are not showing up. The possibilities, as I see them:
1. The OS can only address 32 bits, so only 3.5 GB can be used.
2. The processor can only address 32 bits, so only 3.5 GB can be used.
3. The motherboard can only address 32 bits, so only 3.5 GB can be used.
4. The BIOS is misconfigured to address only 32 bits, so only 3.5 GB can be used.
5. The BIOS is misconfigured in some other way that causes only 3.5 GB to show up.
6. The RAM is bad in some bizarre way that causes only 3.5 GB to be usable.
7. 4 GB are available, but for some reason the tools I'm using (ps, gnome-system-monitor, the BIOS) only see 3.3 to 3.5 GB.
8. Something else is wrong.

#1 and #2 clearly don't apply to me: the OS is 64-bit and the processor, a Phenom II, is also 64-bit. I don't think that #4 is possible. I've looked through the BIOS settings pretty carefully and didn't see anything, but I concede that #5 is possible. #6 and #7 seem absurd. #8 is always a possibility.

What do you think?

I know of no way to do this.

As I have an x86_64 (aka AMD64) processor rather than the classic x86, I don't think this is particularly relevant. Of course the microarchitecture is similar, but I don't think there's any disagreement as to the chip's ability to work with more than (say) 8 GB of memory.

I don't follow...?

Your BIOS is reporting 3.5 GB, so it has nothing to do with the OS. Theoretically, you can tell the kernel to see 4 GB of memory, and if it's addressable and physically there, it will find it. You can do this with the mem= parameter (modify lilo/grub.conf).

Where do you get this idea? Most video controllers use system memory. How much they use is usually configured in the BIOS. What BIOS do you have, and what's the version? This is reported on POST. You can also see it in BIOS setup. Changing video memory is almost always an option in BIOS setup. Perhaps this parameter reset itself when you added memory?

These are absurd! 32 bits can address 4 GB.

Check video RAM usage.

This is HIGHLY plausible. Perhaps by handling the sticks you have shorted out one or more chips. Or, perhaps one of your sticks is slower than the others and needs a slower refresh rate. Each stick has an ID, a frequency (or data rate) and a series of numbers after, like 3200-3-2-2-3. All yours should be the same. If not, you have to tell the BIOS to go with the larges value in each slot. For instance, if you have one stick that says (-3-2-2-4) and another that is (-3-3-2-3), you need (-3-3-2-4).

That's not what I'm talking about. This is memory that would show up as used, not memory that would fail to show up.

I'm talking about the "memory barrier":
Ask Dan: What's with the 3Gb memory barrier?
TabBlogger : The 3GB-not-4GB RAM problem
which is caused by addressing space being used, not for memory, but for an external resource. This does *not* affect systems with actual memory + resources <= addressing space.

A 32-bit system with a 512 MB of hardware resources* and 2048 MB of memory can use 2048 MB of memory.

A 32-bit system with a 512 MB of hardware resources* and 4096 MB of memory can use 3584 MB of memory.

* PCI, BIOS, etc.

*sigh* No. This would be memory that doesn't show up. You are confusing memory usage of the OS with BIOS allocation. The BIOS might be allocating 512 MB for hardware, but if it supports a 64-bit OS and a 64-bit address space, then that should not be a problem. You are using a newer (2008 -- his article was in 2007 and referred to "old" motherboards) Motherboard, and it appears it can support this configuration.

Read this article here and you can see how to configure the BIOS for a bunch of things. It does not show video RAM allocation, but I'm sure you'll find it. When you have tuned it down to 128 MB RAM, and checked the memory size again, and if you're STILL missing 512 MB, then I'll eat my words. Ok?

Howdy folks!

This might not relate, but:

2 Years ago I had pretty much the same problem on one of my 8 puters at home:

Ubuntu 6 or 7, 64 bit, showed only 3.5 gb visible. Tried windows with PAE switch, same stuff as with windows 64 bit. Couldn't get my 4GB...

Went to the gigabyte's site, flashed the BIOS, and bang... I've got my 4GB all nice and shiny.
(I don't recommend flashing BIOS unless it fixes stuff you are actually having problem with since many things can go wrong. If you are desperate enough, give it a try.)


Btw - as Otheus said - check the BIOS settings.
As you probably know yourself, there's bunch of stuff to configure, might be there's some for RAM speed, etc that you missed or require manual setup.
Also - check motherboard manual, might be there's some jumper that needs placing on a different spot (as with overclocking the processor in some cases - but I've never seen something like it for RAM before).
Of course - be very, very careful if you do something like that since you might actually cause harm to the hardware (you probably know this, but it should be mentioned for those who don't and might be reading this article).

If you have a non-onboard graphics card I believe that the video-memory sharing allocation is not used since onboard videocard is auto-disabled in that case.

Hope any of this helped

Cheers,
Matt

My BIOS doesn't have an option to change video RAM.

I pulled out 2 GB of RAM so I only have 2 GB in the computer. But it's showing up as 2048 MB, not 1536 MB. So the problem only happens when I'm trying to use 4 GB.

So exactly 0 MB of my RAM is used as video RAM when I have only 2 GB installed.

I hope that's the case. You can understand if I'm a little wary, though.

---------- Post updated at 06:31 PM ---------- Previous update was at 06:26 PM ----------

That sounds promising. I may try it.

The two things that I most wanted to change were RAM speed and video settings, but my BIOS doesn't seem to have options for either! It's strange, because I'm used to seeing both. Maybe the RAM speed is there and I just missed it because it was listed as a multiplier rather than a speed. Hmm...

OK, I'll check that. I had a lot of trouble getting this computer to work -- the original motherboard never worked but conducted enough voltage that it fried the 3 (!) processors that were installed (one at a time, naturally -- two by me and the third by a Microcenter tech), so I've read the manual rather closely. But I haven't consulted it in the months since.