Can't access all 4GB of ram in Win7 x64

Xaero_toast

Limp Gawd
Joined
Dec 29, 2008
Messages
393
My kids have laptops (so spoiled).

A week or so ago I threw together a couple desktops from salvage, for them to play with. Something I could put 3D cards in. Something to show them what real games look like. Q2, Q3A, UT, all the classics, as opposed to the minecrap they have today.

That was a pretty big hit. The fun part was building them entirely from salvage. I brought 19" flat panel monitors home from work. They were abandoned due to nonfunction. We soldered in replacement capacitors, and had nearly free monitors!

Anyway, the project was enough fun I decided it could stand a little expense, and for about $4, I got a pair of 64 bit capable Pentium4 630's coming from ebay.

The system I've been working on is a Dell Dimension 8400. It's got a 925x chipset, takes DDR2 RAM, and supports "4GB". It had a 32 bit OS on it until the 64 bit capable cpu came in (today). We installed the processor, and booted up to the memtest disk first. Memtest saw just over 3GB (3007MB?). Installing 64 bit Windows 7 shows the same thing.

I expected this behavior with the 32 bit processor, but not so much with the 64 bit. I think I've got the answer, but I'm just looking for someone to "check my work".

I think I'm running into a chipset limitation. I knew the chipset was limited to 4GB of ram (what the spec says), but I think what that actually means is the chipset was limited to 4GB *address space*, and the address space reserved by devices and graphics memory is therefore unavailable to use the RAM at that address, no?

For what they're going to use the machine for, it will be fine without the full 4GB anyway, but if there is something I could have done to get access to all of it, I'd like to know.

I suppose in this situation there's really no advantage to having a 64-bit OS, is there? Besides the fact that just sitting there idling, win7 uses an extra half gig of RAM in 64-bit form...
 
I forgot to include...since I see the same phenomenon in memtest, I have my doubts that the issue could be of software origin. I'm inclined to suspect hardware issue.
 
I'm going to guess lack of driver support. The board itself never had proper 64bit support, although it does accept 64bit CPUs. I would try updating the bios and making sure everything is set to 64bit utilization. For the difference of ~1GB I would throw 32 bit on there and be done with it. For a 10 year old machine I don't think it will matter a whole lot - I doubt you plan on doing serious multitasking with it.
 
I have no serious expectations of the system. It's just a toy for the kids. They have laptops with much more processing power, but even a 10 year old machine with a geforce 8600 can outperform a late model laptop with common integrated graphics.

Further reading online since I posted the question seems to be supporting the theory so far. It seems in order for a board/chipset to use a full 4GB, it must in fact support more than 4GB in order to address all 4GB even after devices have claimed memory addresses for themselves.

I'm just indulging a compulsion of mine here. It's not sufficient to know that it does not work. I often feel driven to also know why.
 
945 and lower chipsets had a 32-bit memory allocation - They support 4GB physical, minus system resources (usually 512MB, but can be 1GB or more) and then minus video RAM.
 
Yeah, the older chipsets didn't support re-mapping I/O addressing space above 4GB. This Intel paper mentions the 915G chipset, but it also applies to the 925 and 945 series chipsets, in the context of why the OS doesn't recognize the full 4GB installed. http://dlsvr01.asus.com/pub/ASUS/mb/4GB_Rev1.pdf
 
Just though I'd ask have you made sure to update to the A09 bios here? Not saying that it would help but couldn't hurt. It has been ages since that era of hardware but I remember a way to change the bios to remap the memory addresses to get the full 4gb, but being a dell the bios is probably locked out.
 
I flashed whatever was the current bios version. A09 sounds right.

I reinstalled Windows 7 on that machine. I'm operating on the assumption I have a hardware limitation holding me to 32-bits of memory address, which effectively negates any benefit potential of a 64 bit OS.

32 bit versions of the same windows product do seem to use less memory to do the same thing, so I think I'm actually better of with 3GB in 32 bit windows than I would have been with 4gb in 64 bit.
 
Some Dells are BIOS-limited to 4 GB. My own Dell Latitude D620 is one such. It has the 945 chipset mentioned above.
 
Some Dells are BIOS-limited to 4 GB. My own Dell Latitude D620 is one such. It has the 945 chipset mentioned above.
It's a chipset limitation on all 945 and lower models. 4GB is the max memory for those 9xx series chipsets. http://www.intel.com/Assets/PDF/datasheet/307502.pdf

Support for 8GB on desktops and mobiles wasn't added until the 955 chipset: http://www.intel.com/Assets/PDF/datasheet/306828.pdf

I'm not disputing artificial locks aren't used sometimes, although on a laptop the limited number of SODIMM slots also becomes a factor. For example if a desktop chipset supports X max memory via 4 DIMM slots, it's not entirely unreasonable for a mobile chipset with only 1/2 as many slots to support less memory. Different situation from an actual chipset limitation like the 945, but just pointing out it's not just companies being evil or whatever.
 
It's a chipset limitation on all 945 and lower models. 4GB is the max memory for those 9xx series chipsets. http://www.intel.com/Assets/PDF/datasheet/307502.pdf

Support for 8GB on desktops and mobiles wasn't added until the 955 chipset: http://www.intel.com/Assets/PDF/datasheet/306828.pdf

I'm not disputing artificial locks aren't used sometimes, although on a laptop the limited number of SODIMM slots also becomes a factor. For example if a desktop chipset supports X max memory via 4 DIMM slots, it's not entirely unreasonable for a mobile chipset with only 1/2 as many slots to support less memory. Different situation from an actual chipset limitation like the 945, but just pointing out it's not just companies being evil or whatever.


That's all exactly right. What I've come to figure out, though, is when all of these pre-955 chipsets say they support "4GB", it means they support a 4GB address space, not a full 4GB of RAM

It's a good thing that 64 bit P4 only cost me $3 ;)
 
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