WinXP sees 2.75GB RAM out of 4GB. System with SLI

leesiulung

Member
Nov 30, 2008
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My Windows XP Pro SP3 only sees 2.75GB out of the 4GB available4. I expected it to see about 3.5GB, but was unpleasantly suprised. I searched online and understand how memory there is a limitation in address space.

Here are a few things about the system:

- motherboard is nForce 590 SLI
- configured with 2x Geforce 7600GS in a triple monitor setup with SLI disabled and both cards in use
- Running Windows XP Pro SP3, 32bit
- System only sees 2.75GB RAM of 4GB physically available and detectable by motherboard. I expected 3.5GB deted.
- there is no integrated video chip/card that is using part of that memory

I suspect that the SLI'ed graphics card need some addressing space and thus the amount of memory available is reduced.

1. Anyone know why Windows only detects 2.75GB instead of 3.5GB of RAM?

2. How I can reclaim the missing 0.75GB of RAM, if possible?

Help is appreciated.
 

bsobel

Moderator Emeritus<br>Elite Member
Dec 9, 2001
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This is normal, the video memory for each of your cards is mapped into addressable space...
 

tcsenter

Lifer
Sep 7, 2001
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Originally posted by: leesiulung
I searched online and understand how memory there is a limitation in address space.
Apparently, you don't. You understand the limitation, you just don't want it to be true?

I don't like gravity and my family genetics, either, just to name a couple things that really bug me but am powerless to do a damned thing about.

1. Anyone know why Windows only detects 2.75GB instead of 3.5GB of RAM?
Yes, because 2.75GB is the amount being exposed to Windows, not 3.5GB. Why do you assume you are supposed to be seeing 3.5GB instead of 2.75GB?
 

leesiulung

Member
Nov 30, 2008
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Originally posted by: tcsenter
Apparently, you don't. You understand the limitation, you just don't want it to be true?

I don't like gravity and my family genetics, either, just to name a couple things that really bug me but am powerless to do a damned thing about.

Well, if that is the max I can use then that is the end of that... but I'm not sure, which leads me to the second question that you are asking below.

Yes, because 2.75GB is the amount being exposed to Windows, not 3.5GB. Why do you assume you are supposed to be seeing 3.5GB instead of 2.75GB?

I assumed that I would get 3.5GB assuming that addressing the video memory was not part of that addressing just like a hard drive space addressing is not limited to the 32bit (precluding cluster sizes and etc...)

So my graphics card each has 256MB ram, that is a total of 0.5 GB RAM. So the accounting is:

4GB - 0.5GB (lost automatically by everyone) - 0.5GB (from the two graphics card) - 2.75GB (reported as available by Windows) = 0.25 GB still missing

So why am I missing 0.25GB? Is my assumption wrong? It is still not clear to me that I'm loosing all this memory due to my graphics card....
 

lxskllr

No Lifer
Nov 30, 2004
58,513
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Your MB hardware also takes a chunk of memory. I'm not qualified to explain it in detail, but there really isn't a way you can tell how much memory will be missing until you see for yourself in any particular machine. In my case, I had 3.07gb of 4gb available in Vista32 with my hardware. Someone with different hardware would come up with a different amount(assuming the same gfx card).
 

tcsenter

Lifer
Sep 7, 2001
18,705
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Originally posted by: leesiulung
So why am I missing 0.25GB? Is my assumption wrong? It is still not clear to me that I'm loosing all this memory due to my graphics card....
You aren't losing all or even most memory due to the RAM on your graphics cards. Graphics RAM is not the primary consumer of host address space reserved to hardware resource needs. Your hardware itself is, such as PCI bus and bridges, PCI Express bridges and root hubs, chipset interconnect, storage controllers, LAN, audio, USB controllers, APIC and ACPI BIOS interfaces, so on and so forth. Additionally, RAM on your graphics card is not mapped into host address space on a 1:1 basis. Each graphics card is going to get a chuck of address space, through which the graphics driver must handle ALL the business of the graphics card, not just addressing its RAM.

All of which is determined by the hardware, firmware, and BIOS before Windows gets up in the morning, which is why many BIOS can show you the amount of RAM that will be "useable" or "available" whether or not an OS is even installed. The BIOS exposes to the OS whatever is available for addressing RAM after giving the hardware what it needs.
 

dguy6789

Diamond Member
Dec 9, 2002
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Explanations above are accurate. To fix this issue, you need either Windows XP Pro x64 Edition or a 64 bit version of Vista.(I recommend this choice)
 

aatf510

Golden Member
Nov 13, 2004
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It is almost totally dependant on the motherboard hardware itself.
I used to have 2.75GB avaliable to me in XP-32bit as well, but once I switched to a different motherboard (from eVGA 680i to ASUS P5Q) while keeping the same CPU/Memory/Video card, I immediately jumped up to 3.25GB.
I didn't even format/re-install Windows either!
 

leesiulung

Member
Nov 30, 2008
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Thanks everyone!

I wanted to point out this though from the article linked by 0roo0roo:

"Power users with a hankerin' for dual graphics cards may be experiencing something of a sinking feeling, at this juncture. Yes, the 256Mb reserved for my little old graphics card means exactly what you think it means: Those two 768Mb graphics cards you can totally justify buying will eat one point five gigabytes of your 32-bit memory map all by themselves, cutting you down to a 2.5Gb ceiling before you even take the other reservations into account."

Basicallly, your video memory addressing mapping is taking it from your system memory mapping and hence for every bit of memory taken by your dedicated card, it is skimmed off the top of your 4GB RAM barrier....

That sucks!

Does 64-bit Vista have compatibility issues with 32-bit software or does it run smooth as cake? I have a few Vista Ultimate licenses laying around, but no 64bit disc....
 

dguy6789

Diamond Member
Dec 9, 2002
8,558
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I have used Vista 64 for 3 months now. Everything works perfectly and smooth as glass :) Drivers are available for just about everything, AVG Free antivirus supports it, and all my games run just fine!
 

0roo0roo

No Lifer
Sep 21, 2002
64,795
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Originally posted by: leesiulung
Thanks everyone!

I wanted to point out this though from the article linked by 0roo0roo:

"Power users with a hankerin' for dual graphics cards may be experiencing something of a sinking feeling, at this juncture. Yes, the 256Mb reserved for my little old graphics card means exactly what you think it means: Those two 768Mb graphics cards you can totally justify buying will eat one point five gigabytes of your 32-bit memory map all by themselves, cutting you down to a 2.5Gb ceiling before you even take the other reservations into account."

Basicallly, your video memory addressing mapping is taking it from your system memory mapping and hence for every bit of memory taken by your dedicated card, it is skimmed off the top of your 4GB RAM barrier....

That sucks!

Does 64-bit Vista have compatibility issues with 32-bit software or does it run smooth as cake? I have a few Vista Ultimate licenses laying around, but no 64bit disc....

yup 32bit only has so many memory addresses. i'm sure memory makers are a bit pissed ms didn't push 64bit more forcefully withthe new os. now they are stuck with tons of people who can't really buy more than 4gb.
 

zerocool84

Lifer
Nov 11, 2004
36,041
472
126
Originally posted by: AdamK47
All on Vista x64. Vista has such horrible gaming compatibility, right?

ahaahaha yea Vista64 is a great OS. There are a couple of things that suck cus some msnufactures won't take the time to create Vista64 stuff aka Adobe/Macromedia and all the pop-ups from the beginning but it runs fast, looks great, and is very stable. I have never noticed a difference in the performance of games.
 

tcsenter

Lifer
Sep 7, 2001
18,705
430
126
Originally posted by: leesiulung
Basicallly, your video memory addressing mapping is taking it from your system memory mapping and hence for every bit of memory taken by your dedicated card, it is skimmed off the top of your 4GB RAM barrier....
Not quite. Dan's article is a work-in-progress. Compare Dan's memory address allocation using GF 7800GT 256MB:

C0000000 - CFFFFFFF GeForce 7800 GT (256MB)
C0000000 - CFFFFFFF PCI Bridge (256MB)


With my memory address allocation using Radeon HD 3650 512MB:

D0000000 - DFFFFFFF Radeon HD 3650 (256MB)
D0000000 - DFFFFFFF PCI Bridge (256MB)

Twice the graphics RAM, same amount of address space. My graphics card is actually using less address space than Dan's example, when you add-up all ranges related to the graphics card (and PCI bridge).

Certain allocations must be aligned on 256MB boundaries and/or in 256MB blocks, per relevant industry specs (PCI, PCI Express, PCI Bridge, ACPI, et. al.). It is merely a coincidence in Dan's case that 256MB happens to correspond with the amount of RAM on the graphics card. If the graphics card had 1GB RAM, you would not see the graphics card allocated more than 512MB address space.