/3GB Switch problems with Boot.ini file

At work, all our PCs are loaded from a standard image (only modified slightly to cope with different hardware specs).

I need to change the boot.ini file to include the /3GB switch ... I found on some machines it worked, and on other machines it didn't* - even though the machine hardware/software is supposed to be identical.

* i.e. the machine failed to boot - it got to the stage where you are prompted which OS to load (i.e. the list of OS given in the boot.ini file), but then when selecting the OS with the 3GB switch it went straight to an error screen where only choice was to boot into Safe Mode, etc. (none of these choices worked - it consistently failed to boot).

Can anyone suggest why the 3GB switch might cause such behaviour?


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LeeTutorConnect With a Mentor retiredCommented:
And as far as your problem about some pcs failing to boot, are those updated only to SP1?  I found this article:

Windows XP SP1 May Not Start with the /3GB or /USERVA Switch
First question would be, do all machines have in excess of 3GB of RAM?

Chris B
JimbobWAuthor Commented:
They should all have 3GB exactly ...
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burrcmConnect With a Mentor Commented:
Then the switch should not be used. XP can handle 4GB unassisted.

Chris B
From this article:

Available switch options for the Windows XP and the Windows Server 2003 Boot.ini files

Here is a description of the /3gb switch:

This switch forces x86-based systems to allocate 3 GB of virtual address space to programs and 1 GB to the kernel and to executive components. A program must be designed to take advantage of the additional memory address space. With this switch, user mode programs can access 3 GB of memory instead of the usual 2 GB that Windows allocates to user mode programs. The switch moves the starting point of kernel memory to 3 GB. Some configurations of Microsoft Exchange Server 2003 and Microsoft Windows Server 2003 may require this switch.

There is further information in this article:

A description of the 4 GB RAM Tuning feature and the Physical Address Extension switch

For example, this quotation leads me to concur with burrcm's conclusion above:

When the /3GB switch is used with Windows XP Professional, with Windows Server 2003, Datacenter Edition, with Windows Server 2003, Enterprise Edition, the /3GB switch works identically across versions. This functionality lets device-driver developers test their drivers in this configuration without having to install one of the Windows Server 2003 products just listed. The user-mode memory space is now limited to 3 GB.

Caution Microsoft supports using the /3GB switch in Windows Server 2003, Standard Edition in a production environment for use by Active Directory. For other applications, Microsoft supports using the /3GB switch in Windows Server 2003, Standard Edition only in a production environment if the application vendor has tested in this environment and if the vendor is willing to support the customer who is using this functionality. Microsoft Exchange Server 2003 and Microsoft SQL Server 2000 are supported in production using this functionality. Contact your application vendor regarding their application. The /3GB switch can cause some applications to have problems that are related to address dependencies or to a reduction in kernel space. Except in the cases described here, the /3GB switch in Windows Server 2003, Standard Edition is only for development and testing purposes.
JimbobWAuthor Commented:

Interesting information - thanks all.  Yes - we are only at SP1, so this might be the cause of our inconsistent startup behaviour.  However, it now looks like I have a problem in that my PCs are underspec'd and really need an upgrade to 4GB each!

N.B.  My application software vendor does support use of the 3GB switch, so at least that isn't a problem ...

JimbobWAuthor Commented:

Thanks for your help people!  I think that LeeTutor probably has hit the nail on the head with SP1, but burrcm's comment on needing more than 3GB is useful information as well.  Hope the point split is acceptable ...

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