Civil 3D and the 3GB Switch

Increase Performance!One of the biggest struggles facing many Civil 3D users is simply meeting the minimum system requirements to operate the program.  The operating system reserves a certain amount of memory for itself and then allocates whatever is left to all open applications.  The default maximum for 32-bit applications is 2GB of total memory, even when run on 64-bit operating systems.  The application is immediately terminated by the operating system if it accesses more than 2GB of memory. 

Simply adding additional RAM will not solve the problem.  The total amount of memory available to open applications must be increased by employing what is known as the 3GB switch.  This is accomplished by adding an additional memory setting instruction to the Windows boot.ini file.  

Autodesk Technical Solution TS1060353 addresses specific Revit performance issues on the Windows XP operating system but the document applies to Civil 3D as well.  Remember: you must have sufficient memory (4GB) for the the operating system and open applications to take advantage of the 3GB switch.

To enable the 3GB Switch on Windows XP

  1. Right-click My Computer. Click Properties.
  2. In the System Properties dialog box, click the Advanced tab.
  3. On the Advanced tab, under Startup and Recovery, click Settings.
  4. In the Startup and Recovery dialog box, under System startup, click Edit.
    The Windows boot.ini file will be opened in Microsoft Notepad. The contents of this file may vary from computer to computer.  Save a backup copy in case you need to revert to the original version.
  5. Select the following line in the boot.ini file:
    multi(0)disk(0)rdisk(0)partition(2)\WINDOWS=”Microsoft Windows XP Professional” /fastdetect
    Note: Your string may be different from the string shown. Be sure to copy the string from your boot.ini file, not the string shown here.
  6. Copy (CTRL+C) this line and paste (CTRL+V) immediately below the original.  The sequence of these lines determines the system default.  If you prefer the 3GB option as the default, move that line of instruction ahead of the original.
  7. Modify the copied line to include 3GB as shown in the following example. The text within the quotation marks will be displayed during startup.
    multi(0)disk(0)rdisk(0)partition(2)\WINDOWS=”Microsoft Windows XP Professional 3GB” /3GB /fastdetect
  8. Save the boot.ini file and exit Notepad.
  9. Click OK to close each dialog box. 
  10. Restart your computer. 
  11. During startup, select the 3GB option. 

To enable the 3GB switch on Windows Vista or Windows 7

Newer 64-bit operating systems do manage memory more efficiently but the same memory limitations still apply to 32-bit applications.  The process to increase the available memory for Windows Vista and Windows 7 is different than for Windows XP.  Autodesk Technical Solution TS1069947 details the steps to enable the 3GB switch on these operating systems. 

If you experience problems using the 3GB switch, switch back to the original configuration.

  1. Right-click Command Prompt in the Accessories program group of the Start menu.  Click Run as Administrator.
  2. On the command line, enter “bcdedit /set IncreaseUserVa 3072”
  3. Restart the computer.

To disable the 3GB switch

  1. Right-click Command Prompt in the Accessories program group of the Start menu.  Click Run as Administrator.
  2. On the command line, enter “bcdedit /deletevalue IncreaseUserVa” 
  3. Restart the computer.

For more information on the 3GB switch, refer to the following Microsoft MSDN article:

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