By default the test attempts to get the memory size from the
BIOS using the "e820" method. With "e820" the BIOS provides a table of memory
segments and identifies what they will be used for. By default Memtest86
will test all of the ram marked as available and also the area reserved for
the ACPI tables. This is safe since the test does not use the ACPI tables
and the "e820" specifications state that this memory may be reused after the
tables have been copied. Although this is a safe default some memory will
not be tested.

Two additional options are available through online configuration options.
The first option (BIOS-All) also uses the "e820" method to obtain a memory
map. However, when this option is selected all of the reserved memory
segments are tested, regardless of what their intended use is. The only
exception is memory segments that begin above 3gb. Testing has shown that
these segments are typically not safe to test. The BIOS-All option is more
thorough but could be unstable with some motherboards.

The second option for memory sizing is the traditional "Probe" method.
This is a very thorough but not entirely safe method. In the majority of
cases the BIOS-All and Probe methods will return the same memory map.