Windows only recognizes 3GB of memory when I installed 4GB.

30 pts.
Microsoft Windows XP
Windows errors
I’m working on a Dell Precision 670 desktop for one of my users. The desktop originally had 4 512MB DDR2 memory sticks which have been removed and I installed 2 2GB DDR2 into the first 2 slots on the motherboard (there are 6 slots total). The desktop is currently running Windows XP SP3 and when I checked the system properties it shows that it only has 3GB of memory installed when I have 4GB. I tried changing the positions of the 2 memory modules but I would either get a “System Halted” message or the desktop would beep at me when powered on. I checked the Boot.ini file but the /3GB switch is not present. How could I go about getting the OS to recognize the correct amount of memory installed? I know this is an old Windows problem but just out of curiosity. I would like to know. Thanks in advance!

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The issue is that Windows XP does not report physical RAM but reports addressable RAM.

Windows XP is a 32-bit OS. A 32-bit OS has 4 GB of address space. This is for all addressable items which include RAM, devices and system resources. Address space for system resources and devices is created from the top of addressable space down. The amount of address space used by system resources and devices varies by system configuration including options and devices installed. RAM is addressed from the bottom of the addressable space up to where the address space reservation for system resources and devices ends.

The address space available for RAM can be anywhere from ~ 3GB to ~ 3.3 GB on most modern computers under a 32-bit OS. Windows XP will show the amount of addressable memory if more memory than available address space is installed. Windows Vista reported the same way until SP1 where Microsoft changed the report to be physical RAM installed.

There are a number of KB articles that talk about this limitation and how Windows reports RAM. KB929605, KB935268, KB946003

If you have a specific need for more RAM, there is a “trick” that can gain some of that make some of the additional RAM addressable. This involves the use of PAE mode. Unless you are running hardware that allows the use of PAE mode (most current hardware does) and an application that is PAE aware (only a few are) there is no point in doing so as PAE mode has other limitations and issues.

Now, as to how Windows uses the RAM it can see…

The 4GB address space offered by the operating system is split in two areas
of 2GB each: the first 2GB is for user mode, and the other 2GB isg reserved
by the kernel. On Windows XP, it is possible to increase the default
allocation capabilities up to 3.0GB (3GB for user mode, 1GB reserved for
kernel). Such capability requires additional tunings in order to be

A modification of boot.ini file is needed to activate this capability at the
system level. The boot.ini switch /3GB needs to be added in order to make
3GB available for user mode applications. For example:

[boot loader]
[operating systems]
multi(0)disk(0)rdisk(0)partition(1)\WINDOWS=”Microsoft Windows XP
Professional 3GB” /fastdetect /3GB
multi(0)disk(0)rdisk(0)partition(1)\WINDOWS=”Microsoft Windows XP
Professional” /fastdetect

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  • OParada
    Thanks Karl for the info!! I'll go ahead and modify the boot.ini file to see how the system responds. Thanks again!
    30 pointsBadges:

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