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Raju October 18, 2006 09:07

using more than 2GB RAM on a windows PC

My laptop has 3GB (2GB+1GB) RAM. But when I am running my fortran program in compaq visual fortran, it reads on maximum of 2GB memory. If the storage exceed 2GB RAM, the program gives an error. I am not able to access the other 1GB RAM on the other DIMM. Does any body has any suggestion how to get the full 3GB working.

M.Lipinski October 18, 2006 09:57

Re: using more than 2GB RAM on a windows PC

You need to include /3GB switch for the boot sequence of Windows. You can do it in Console->System Properties->Advanced->Startup and Recovery->Settings. Add there in "Default operating system" a /3GB switch at the end of the line (probably after /fastdetect or so). After you reboot your system you will be able to use your 3GB memory (this is maximum for Windows). These settings work for me with Windows XP Professional. If you have different version of Windows then you may need to find out at Microsoft if the /3GB switch will work for you.

To make use of these 3GB memory in your program you need to link it with the /LARGEADDRESSAWARE linker switch. Now your program can use more than 2GB of memory.



Raju October 18, 2006 11:19

Re: using more than 2GB RAM on a windows PC
Hello Mr Lipinksi,

Thank you very much for your response. I have tried it and it is working. Now my program is able to access the extra 1G RAM.

When you mentioned that 3GB is the maximum for windows, does it mean that even if I have 4GB RAM on my machine, windows (on 32 bit machine) will not be able to access it?


M.Lipinski October 18, 2006 14:37

Re: using more than 2GB RAM on a windows PC

Yes, 3GB is maximum for 32-bit Windows XP.



sach October 20, 2006 01:56

Re: using more than 2GB RAM on a windows PC

I think 32 bit windows system has 4 gb ram limitation. I cae to know this when my work station couldnt excess 8gb ram. when I switched to win64 or linux I could access whole 8gb. Could ou please comments on this. Do you have any information to share about faster access to paging memory?

Jonas Holdeman October 20, 2006 08:48

Re: using more than 2GB RAM on a windows PC
My Dell PC owners manual says that Windows XP (32 bit) has a 4GB limit. However, it says that "certain components" use addresses in the 4GB range, making that part of memory (those addresses) unusable.

It would seem then that one could benefit from 4GB, but with limitations.

As an aside, has anyone besides me had problems with Dell PCs (3 GHz Dimension 8400) overheating and crashing on long, compute-bound, computations (using Matlab in my case)?

Patrick October 20, 2006 09:42

Re: using more than 2GB RAM on a windows PC
The 3GB limit is the limit of one process (don't ask why) on windows 32bit. There are similar problems with linux.

Ben October 20, 2006 10:56

Re: using more than 2GB RAM on a windows PC
Its not actually a problem, it is the hard coded limitation of any 32bit CPU/system no matter what OS you are using. A 32 bit processor has, by its very definition, 32 bits to refer to the location of every byte of memory so 2^32 = 4.2 billion so each bit can access 4.2 billion individual locations (or integers) in other words 4GB of RAM, so if you have a 32bit chip you will never be able to access more than this

Renato. October 20, 2006 16:43

Re: using more than 2GB RAM on a windows PC
Taking a ride on this thread I'd like to describe something weird with the machine that I work. This machine is a Dell Precision 370 (Pentium-4 3.6 GHz EM64T with 4Gb of memory). Recently I reinstalled this machine with Windows XP Professional x64 believing that Windows x64 would recognize all the 4Gb of physical memory. The problem is that when I open the "System Properties" window it shows me only 3Gb of memory installed. Where is the rest of my memory? Does anyone know what could be happening?



diaw October 20, 2006 23:56

Re: using more than 2GB RAM on a windows PC
Be a little careful with this switch. If you overspecify at say /3Gb but, for some reason the system registers less by windows - on comes a quick BSOD & you're kicked back to re-boot.

I had to go in via a bootable floppy & safe windows in order to correct the boot.ini file. I thought I'd hosed the system.

Be careful.


M.Lipinski October 22, 2006 03:29

Re: using more than 2GB RAM on a windows PC

The 3GB limitation has to with how the virtual address space (4GB for 32bit systems) is split under Windows in user mode and kernel mode virtual address spaces. In a standard configuration (without the /3GB switch) these address spaces are both equal to 2GB. So the user, at most, can use 2GB of the user mode address space. Adding the /3GB switch means that the user space will be 3GB and the kernel space 1GB.

This is why on 32bit Windows user processes can use at most 3GB of memory. In reality, it will be even less than 3GB because of background processes/services running simultaneously with your application.



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