Authored By: Mark Nicholson
Use the /3GB switch
Question: My computer has 4GB of RAM, but Zemax doesn't seem to use it all. Why?
You are using a 32-bit version of the operating system. 32-bit versions of Windows can address up to 232 memory addresses, or 4GB of memory. By default, Windows splits this memory up so that 2 GB is available to applications, and 2 GB is available to the operating system. That means Zemax can only access half the memory in your machine.
If your machine has more than 2GB RAM, the correct thing is to use a 64-bit version of Windows. We recommend 64-bit Windows 7, although Vista and XP were both available in 64-bit versions. There is a 64-bit version of Zemax available that will work on 64-bit operating systems and make complete use of all resources on your machine.
If you really MUST use a 32bit version of Windows, then by changing a variable in the boot configuration, you can make 3 GB available to applications, and 'only' 1 GB to Windows itself. Note that this can cause stability issues on your machine, particularly if your graphics card approaches 1 GB of memory.
The method used to set 3GB depends on the version of Windows:
Windows 2000 - XP
Set the /3GB switch in the boot.ini file. See here for details.
Vista
Use BCDEdit /Set IncreaseUserVa 3072 to set the 3GB switch. This is a command line tool, see here and here for details.
Note that Zemax support cannot help you diagnose boot-related problems on your computer.
The better solution is to use a 64-bit version of Windows, and the 64-bit version of Zemax, as this can then address all the resources of your computer.