Memtest86+

memtest86+ is memory test software based on the (commercially available) memtest86 program.

Prerequisites
Memtest86+ does not support UEFI. You can switch to Memtest86 V5 or newer for UEFI supported memtesting.

Be sure (where the GRUB files are typically installed) is available before installing memtest86+! If is on a separate partition (which it WILL BE if the partitioning scheme in the Gentoo handbook was used for this Gentoo install), then be sure the mount command is run before the emerge section below:

If no errors are return then boot should be successfully mounted. Check by running the command without any arguments and parsing the output with :

Emerge
Ask Portage to install it:

GRUB legacy
Replace the  (question marks) in the file below with correct numbers for the system's boot partition:

GRUB2
For GRUB2 just run. As long as the package has been emerged, a configuration file has already been installed to :

LILO
For LILO, add this to the configuration file:

Then rebuild LILO's MBR entry:

Syslinux
Add this to the configuration file:

Usage
To use memtest86+ the system needs to be rebooted using the newly added boot item

External resources

 * http://www.pcworld.com/article/232640/memtest86.html - A PCWorld article describing the usefulness and features of memtest86+.