How to setup the /3GB switch?

The /3GB switch allows an application (like Inventor) to use 3GB of address space and thus reducing the available address space for the operating system to 1GB. Normally, 2GB is allocated to the operating system and 2GB is allocated to user applications. See page 2 for more information.

  • Make sure you are running Windows XP with the latest patches from Microsoft that address the startup issue with the /3GB switch.
  • Make sure to get latest patches for your computer from the workstation vendor.
  • Right click onMy Computer -> Properties -> Advanced -> Startup & Recovery Settings

Click on Edit button next to “To edit startup options file manually”.

  • This will open the boot.ini file in notepad.
  • Copy the line in YOUR “boot.ini” file that reads something like

multi(0)disk(0)rdisk(0)partition(1)\WINDOWS="Microsoft Windows XP Professional" /fastdetect

and paste it right below the line.

Edit the pasted line to add /3GB before /fastdetect to read likemulti(0)disk(0)rdisk(0)partition(1)\WINDOWS="Microsoft Windows XP Professional (3GB)" /3GB /fastdetect

  • Save and exit notepad

Page file:

It is important that you increase your page file size to 4GB. Right click My Computer -> Properties -> Advanced Tab -> Performance Settings -> Advanced Tab -> click on "Change" button in the virtual memory section. Enter 4096 in Initial size box as well as in Maximum size box and click on Set button. Click OK.

Reboot:

After this change to your boot.ini file, whenever you reboot the machine, you have the option to boot it with 3GB switch. If you do not make any selection, it boots under normal mode. By default, you have only 5 seconds to make the selection. You can change this in My Computer -> Properties -> Advanced Tab -> Click Settings in "StartUp and Recovery" section. Change the Time field in "System startup" section.

More settings:

/3GB takes a parameter called /Userva that can be used to precisely specify the amount of user and kernel address space. For example,

multi(0)disk(0)rdisk(0)partition(1)\WINDOWS="Microsoft Windows XP Professional (3GB-2800)" /3GB /userva=2800 /fastdetect

gives 2800 megabytes to user application and 1200 megabytes to the kernel.

So, you can have multiple lines in your boot.ini file that have different settings and choose the one you want while starting your computer.

Refer to additional information.

More info on /3GB switch:

  • Physical address space: refers to the address space in the RAM
  • Virtual address space: think of it as a 'fake' flat area of memory available to every process. Whenever memory is needed by a program, it is allocated from the virtual address space.
  • The operating system takes care of the mapping between virtual memory and physical memory (RAM) via page files. Even if you have 4GB of RAM, you still need a large page file. Roughly, your page file size should be 1.5 times the size of your RAM.
  • 32 bit operating systems like Windows NT, XP can support (only) 4GB of virtual address space i.e. the processor can only access 4GB of address space at any given time. With 32 bits, only 232(=4GB) values can be represented distinctly. This means thateven if you have 8 GB of RAM or set your page file size to 8GB, processor can only address 4GB of data. This limitation is a big bottleneck as we try to build huge parts and assemblies. It won't be too long before we see 64 bit (264= 16 terabytes) and then 128 bit processors.
  • By default, this 4GB of virtual address space is divided equally between system processes and user applications i.e. 2GB is reserved for user applications and 2GB is reserved for kernel apps. 2GB is not enough in many situations as users trying to push the limits of Inventor with bigger parts and assemblies.
  • Using the /3GB switch, we can enlarge the user-mode virtual address space to 3GB (but reduces the kernel mode address space to 1GB)
  • Applications have to support /3GB switch in order to use the extra space. Inventor is designed to support /3GB switch. If the application does not support /3GB switch and if the /3GB switch is set, the O.S. will still operatewith 1GB and the app will run using 2GB only.
  • To take advantage on the 3GB switch, one has to run on Windows XP. Even on Windows XP, some drivers, especially video adapter drivers with onboard RAM, cannot run with the /3GB parameter because they require more address space than the 1 GB kernel address space permits.
  • /3GB does come with a cost. When the /3GB switch is set, the kernel has to operate in a reduced space of 1GB and this could seriously affect system performance. So, after the job is done, it is recommended to restart the system under normal mode (without the 3GB switch).
  • It is recommended that you have atleast 2GB of RAM if you are going to use the /3GB switch. Otherwise, your system may end up "thrashing" (swapping page files between RAM and hard disk) without doing anything useful.

If you have any questions or comments, feel free to email