What is Invisible Memory?

With cheaper and cheaper memory, nowadays more and more people install 4 GB memory or more for their computers, intending to get higher performance. However, because of the limits of 32-bit IBM PC architecture and Microsoft market policies, you may not be able to access the full installed physical memory, even on 64-bit Windows. To know the maximum physical memory supported by your Windows, please visit Memory Limits for Windows and Windows Server Releases. If your Windows can support all installed physical memory, you may skip this article.

The following figure shows an example often seen in 32-bit Windows operating systems. In this example, the operating system is 32-bit Windows 7, and the computer has 6 GB physical memory installed. But only 3.25 GB memory is supported by Windows. There is 2.75 GB memory cannot be used. This "lost" memory is called Invisible Memory (also known as Hidden Memory or Unmanaged Memory).

In order to fully utilize installed physical memory, we now provide the Unified Invisible Memory Management Interface (UIMMI) to claim and manage the Invisible Memory. Products supporting UIMMI, such as Primo Ramdisk or PrimoCache, can detect and use Invisible Memory to run product specific functions.

Prerequisites for Using Invisible Memory

To use Invisible Memory, it requires the support of both hardware and software. First, your CPU must support Physical Address Extension (PAE), and your motherboard must support Memory Remapping. Almost all computers today support these features. But if your computer is an old one, especially, bought before 2007, you shall check them. The earliest motherboard chipset that supports Invisible Memory is Intel 946 chipset family or NVIDIA nForce 570. Prior chipsets such as Intel 945 chipset family are not supported.

Second, your Windows operating system must enable the PAE feature. By default, Windows automatically enables the PAE feature if Data Execution Prevention (DEP) is enabled on a computer or if the computer is configured for hot-add memory devices in memory ranges beyond 4 GB. Otherwise PAE must be explicitly enabled. Please try the following steps to enable PAE if it is disabled.

  1. For Windows Vista and later, open Command Prompt as administrator and then type the following command.
    bcdedit /set pae ForceEnable
    For Windows Server 2003 and Windows XP/2000, locate and open the Boot.ini file, and then modify the line that corresponds to current Windows OS by appending the /PAE switch. Click here to see how to edit the Boot.ini file.
  2. Restart the computer to apply above changes.


  • Because Invisible Memory is not managed by Windows, in order to avoid memory access violations and resulting unexpected behavior, you should ensure that only one application is using Invisible Memory, unless all applications that use Invisible Memory support UIMMI.
  • If you use Invisible Memory to create a RAM-disk and set the system page file (pagefile.sys) on it, you must disable Windows hibernation. Otherwise errors may happen while the computer resumes from hibernation.
  • Usually there is no fatal effect on system to enable Invisible Memory. But in case that you cannot boot up the system, you can restore the settings by booting into Windows Safe Mode.