Memtest86 is designed to run as a stand-alone, self-contained program from a
CD-ROM, USB Drive, or from a suitable bootloader without an operating system
present. This is because the program must directly control the hardware being
tested and leave as much of the RAM space as possible for examination. It is
also a quick and convenient way to start the program, and avoids running a
complex operating system/application which could be endangered by hardware
with memory problems. (in System Utilities)