4 GB on 32-bit machine: how much is lost?

[VJ]

Hello,

I'm planning on upgrading the memory on my 32-bit dual Xeon (Intel 7505 chipset). When plugging in 4 GB, how much memory would I loose?

I know the amount of video card memory (in my case 128 MB) is lost in 4 GB, but what else?
(if too much is lost, I'd only add up to 3 GB )


Jörg

[Nowhere]

That might depend on operating system. Since there are no practical implications for me I haven't dug into the issue, but...

There is a mechanism that allows 32-bit machine to adress more than 4 GB - PAE/Physical Adress Extension. Only thing is - in WinXP it was limited/castrated to only allow 4 GB. Same thing with win2k3 standard edition. But...higher editions support much more (not sure how much more, it was something in 32 or 64 GB range). Ofcourse, AFAIK, there's still limit as to how much one process can use (2 GB?), but it's still better than total memory limit of 4 GB... (assuming you can get your hands on student/etc. copy of win2k3, since it's quite expensive...)

[dZeus]

as Nowhere said, XP since SP2 and Vista x86, doesn't allow you to use PAE to see more than 4GB of address space, of which a part is used by bios/addincards/drivers. I've read that generally on a non-PCIe system you get between 3.2 and 3.5 GB of usable RAM, and on a PCIe system less (3.0 - 3.2 GB?). My Intel 875 chipset motherboard sees 3.5GB out of the total 4GB. For support of more than 4GB address space using PAE (needed to see the whole 4GB), you need a fancy MS server OS or linux.

Since 4GB RAM is very cheap, not being able to see all might still be worth getting it.

[az]

http://www.dansdata.com/askdan00015.htm

[VJ]

The operating system is Windows Vista Ultimate; suprisingly, the mainboard allows for configurations up to 12 GB:

http://www.supermicro.com/products/motherboard/Xeon/E7505/X5DA8.cfm

Over the 6 banks, I currently have 2x 512 MB.

The memory has to be registered ECC (more expensive than regular memory), and must be populated in pairs. I could add either 4x512 MB (filling all banks, 3 GB in total), 2x1024 GB (2 banks to spare, 3 GB in total), 2x512 + 2x1024 (no spares 4 GB in total) or maybe even 2x2GB (5 GB in total, 2 spares)

I've read about forcing the PAE to be used, using bcdedit... Just don't how well the system will handle this. But would forcing PAE to be used mean I could put more than 4 GB AND benefit from it?


edit: I came across this:

http://support.microsoft.com/kb/929605

Now I'm totally confused... Does it mean in need to install Vista 64 bit?



Jörg

[dZeus]

as explained in above posts, you need a supported MS SERVER OS or XP <SP2 to get more than 3.x GB of RAM with PAE. I think linux can do PAE too to get more than 4GB address space, but I'm not entirely sure.

[VJ]

In the microsoft link I posted, is this message:
"If a computer has many installed devices, the available memory may be reduced to 3 GB or less. However, the maximum memory available in 32-bit versions of Windows Vista is typically 3.12 GB."
(I have 3 scsi controllers (2 onboard), a quad-controller firewire 800 card, a sata raid controller...)

Combine that with the system requirements on the same site, and I think there is no point for me to install 4 GB.

Jörg

[WxDude]

Ok,

On a typical 32 bit X86 box, you get ~3.5 to 3.8 GB out of 4 GB. The top
256-512 MB are used for PCI addressing/memory mapping. Nothing
can be done on a 32 bit box to get around this.

Unless you are running 64 bit Vista/Linux, it may not make much sense
going past 2 GB on your current system. 32 XP/Vista usually runs in a
mode that splits the 4GB address space into 2 GB Memory/ 2 GB Addressing.

There is a 3 GB/1 GB split mode that you can run by editing the boot args
in boot.ini. The problem is that it is potentially buggy for software as they
have been written with the 2GB/2GB split in mind.

PAE might let you access MORE or close to 4 GB (depending on OS), but
there is a performance hit because it is doing bank-switching to allow access
to the higher portions. There is a performance hit to using this method. But
in the past, it was the only way to get more memory on the 32 bit chips.

And like everyone else has said, you need one of the more advanced (read
higher end/enterprise/expensive) server versions of Windows 2000/2003/2008
to be able to use the PAE mode. Linux will let you use it with the right kernel
(I know most Redhat/Fedora based OSes include a kernel with PAE support).

What chips do you have in this box. I know that the 7505 chipset is for the
Netburst (P4) based Xeons, but is it possible you have one of the Xeon
versions that supports 64 bit? You can check using either CrystalCPUID
or a Linux live cd ( grep lm /proc/cpuinfo ) to find out. If the flags line in
/proc/cpuinfo contains the "lm" (Long Mode) entry, you chips have the 64 Bit
mode. If you are looking for more memory and have 64 bit chips, I'd consider
moving to 64 bit. Makes things much simpler.

Good Luck

-Wx

[VJ]

The processors are Xeon Prestonia, no 64 bit support; moving to 64 bit on this machine is thus impossible.

Guess I'll go to 2 GB or 3 GB (coming from 1 GB)...

Thanks!


Jörg

[Jammrock]

It depends on OS and chipset.

All of the older Intel chipsets will not address the full 4 GB. In my case I get 3.25 GB, so I lose 750 MB, even though I am running Vista x64.

Some are OS limited, such as Vista 32-bit (and that would translate to 2008 32-bit since they use the exact same kernel). The most you'll see there is 3.5 GB.

I would guess your total RAM would be somewhere around there since it is an older server platform.

[Chucky Cheese]

on my system i lost .64GB, windows reported 3.3GB

[degrub]

i think the vid card memory is also subtracted out + the Win OS space.

[Mehen]

i think the vid card memory is also subtracted out + the Win OS space.

Yes it is. So if you decide you want a 1GB card (like say a GTX280), then you are already down to 3GB from 4GB. 32bit can only address 4gb which has to include video, chipset, etc.

edit: wow just noticed he had that in the first post I suck

[VJ]

Thanks...

As far as memory bank layout goes, it is not cheaper for me to add 4 GB. To go to 3 GB I can either add 2x1024 or 4x512; to go to 4 GB, the only option is 2x1024+2x512.
I'll most likely add 2x1024...

Thanks!


Jörg

[VJ]

Just considered that I'm not really using Vista specific stuff, and that the machine is more used as a server anyway. So I'll first try one of the Server OS (Server 2003 or Server 2008), and if that is ok I'll upgrade memory accordingly.


Jörg