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As I outlined in my memory diagnostic primer, memory diagnostic tools that are integrated in modern BIOSes can't guarantee that a memory module is flawless. This also applies to Memtest86, a free standalone memory diagnostic tool. However, the utility is more sophisticated than most BIOS memory diagnostic tools. You can also use Memtest86 if you are uncertain which of the RAM modules has flaws and you just want a second opinion.
For this review, I tested Memtest86 v3.5b. At the time of this writing, Memtest86 v4.0 is still in beta. When I tried the beta version on a virtual VMware Workstation machine, the tool froze. However, I admit that testing virtual memory for physical flaws doesn't make much sense. Memtest86 v4.0 has important additional features, such as support for multiple CPUs and machines with large memory. It is also quite likely that v4.0 works better with most physical main boards. The download of Memtest86 v4.0 also includes v3.5b, so you can change versions easily if problems occur.
To run Memtest86, you have to boot up from a floppy, CD, or USB stick. The download file contains the ISO image that you have to burn on a CD. The floppy disk package can be installed from Windows; for the USB version, you will need a Linux installation to create the bootable USB stick.
After you boot from the Memtest86 medium, you can choose which version you want to run: v3.5b or v4.0. The memory diagnostic tool will then immediately start testing the memory. The tool continues diagnosing until you stop it. Once it finishes testing the whole RAM, it just starts all over again. Thus, you can use the tool to rule out temperature-related problems by running the tool on a cold server until it reaches its normal operation temperature.
Memtest86 has quite a few options that you can access by pressing the C key. The Test Selection allows you to choose different test algorithms. Test Range is for configuring the memory range you want to diagnose. You can use the Memory Size option if Memtest86 doesn't correctly detect the size of your memory. Read the Memory Sizing section under Technical Info on the tool's homepage for information about different detection methods. The Error Report Method lets you configure how many details the utility displays about detected errors. DMI Memory Info gives you information about the memory modules in the computer. Important is the ECC mode setting. If you use ECC memory, you should enable this function. If the computer has multiple CPUs, you can tell Memtest86 to use only a single CPU or to use the different CPUs sequentially or in round robin mode.
Another popular memory testing tool is Microsoft's Windows Memory Diagnostic tool. One advantage of Memtest86 over comparable free tools is that it supports computers wit more than 4 GB memory. Do you know of other good memory diagnostic tools?