Linux can use up to 64 Gigabytes of physical memory on x86 systems.
  However, the address space of 32-bit x86 processors is only 4
  Gigabytes large. That means that, if you have a large amount of
  physical memory, not all of it can be "permanently mapped" by the
  kernel. The physical memory that's not permanently mapped is called
  "high memory".
 
  If you are compiling a kernel which will never run on a machine with
  more than 960 megabytes of total physical RAM, answer "off" here (default
  choice and suitable for most users). This will result in a "3GB/1GB"
  split: 3GB are mapped so that each process sees a 3GB virtual memory
  space and the remaining part of the 4GB virtual memory space is used
  by the kernel to permanently map as much physical memory as
  possible.
 
  If the machine has between 1 and 4 Gigabytes physical RAM, then
  answer "4GB" here.
 
  If more than 4 Gigabytes is used then answer "64GB" here. This
  selection turns Intel PAE (Physical Address Extension) mode on.
  PAE implements 3-level paging on IA32 processors. PAE is fully
  supported by Linux, PAE mode is implemented on all recent Intel
  processors (Pentium Pro and better). NOTE: If you say "64GB" here,
  then the kernel will not boot on CPUs that don't support PAE!
 
  The actual amount of total physical memory will either be auto
  detected or can be forced by using a kernel command line option such
  as "mem=256M". (Try "man bootparam" or see the documentation of your
  boot loader (grub, lilo or loadlin) about how to pass options to the
  kernel at boot time.)