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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 in size. 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 that will never run on a machine with more than 1 Gigabyte total physical RAM, answer off here (the 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, 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, and PAE mode is implemented on all recent Intel processors (Pentium Pro and better).


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 autodetected or can be forced by using a kernel command line option such as mem=256M. (See Chapter 10, Kernel boot command-line parameter reference for details about how to pass options to the kernel at boot time, and what options available.)

If unsure, say off.

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