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Full Discussion: Built in ram limit for 64 bit
Operating Systems Linux Built in ram limit for 64 bit Post 302118101 by Perderabo on Friday 18th of May 2007 11:25:33 AM
Old 05-18-2007
I don't have any experience with x86_64 cpu's and I have never used more than 2 GB Ram on any pc. But after reviewing the memory management sections of my copy "Understanding the Linux Kernel", I may have a handle on this. The Linux kernel needs to use a CPU feature called PAE to access more than 4GB of memory with a 32 bit kernel. This pushes the paging system into a 3 level model (instead of a 2 level model) resulting in the performance hit you mentioned. A 64 bit kernel needs a 3 level model up to kernel version 2.6.10. But this imposed constraints on how memory could be allocated. Starting with version 2.6.11, a four level paging model is used with x86_64 cpu's. It is not clear from the book if it would be possible to not use PAE on a x86_64 with 4 GB or less of memory. But reading between the lines a bit, I gather that the answer is "no". Support for non-PAE kernels seems to be intended for very old cpu's and they won't do 64 bit. Besides large RAM configurations, PAE is required to use the new NX flag which marks some memory regions as non-executable. This closes a security problem. Linux 2.6.11 supports the NX feature.

So a 64-bit kernel may have less decisions for you to make thus "taking care of things".
 

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LEARN ABOUT CENTOS

grub2-file

GRUB-FILE(1)						      General Commands Manual						      GRUB-FILE(1)

NAME

       grub-file -- Check if FILE is of specified type.

SYNOPSIS

       grub-file (--is-i386-xen-pae-domu | --is-x86_64-xen-domu |
		  --is-x86-xen-dom0 | --is-x86-multiboot |
		  --is-x86-multiboot2 | --is-arm-linux | --is-arm64-linux |
		  --is-ia64-linux | --is-mips-linux | --is-mipsel-linux |
		  --is-sparc64-linux | --is-powerpc-linux | --is-x86-linux |
		  --is-x86-linux32 | --is-x86-kfreebsd | --is-i386-kfreebsd |
		  --is-x86_64-kfreebsd | --is-x86-knetbsd |
		  --is-i386-knetbsd | --is-x86_64-knetbsd | --is-i386-efi |
		  --is-x86_64-efi | --is-ia64-efi | --is-arm64-efi |
		  --is-arm-efi | --is-hibernated-hiberfil | --is-x86_64-xnu |
		  --is-i386-xnu | --is-xnu-hibr | --is-x86-bios-bootsector)
		  FILE

DESCRIPTION

       grub-file is used to check if FILE is of a specified type.

OPTIONS

       --is-i386-xen-pae-domu
	      Check if FILE can be booted as i386 PAE Xen unprivileged guest kernel

       --is-x86_64-xen-domu
	      Check if FILE can be booted as x86_64 Xen unprivileged guest kernel

       --is-x86-xen-dom0
	      Check if FILE can be used as Xen x86 privileged guest kernel

       --is-x86-multiboot
	      Check if FILE can be used as x86 multiboot kernel

       --is-x86-multiboot2
	      Check if FILE can be used as x86 multiboot2 kernel

       --is-arm-linux
	      Check if FILE is ARM Linux

       --is-arm64-linux
	      Check if FILE is ARM64 Linux

       --is-ia64-linux
	      Check if FILE is IA64 Linux

       --is-mips-linux
	      Check if FILE is MIPS Linux

       --is-mipsel-linux
	      Check if FILE is MIPSEL Linux

       --is-sparc64-linux
	      Check if FILE is SPARC64 Linux

       --is-powerpc-linux
	      Check if FILE is POWERPC Linux

       --is-x86-linux
	      Check if FILE is x86 Linux

       --is-x86-linux32
	      Check if FILE is x86 Linux supporting 32-bit protocol

       --is-x86-kfreebsd
	      Check if FILE is x86 kFreeBSD

       --is-i386-kfreebsd
	      Check if FILE is i386 kFreeBSD

       --is-x86_64-kfreebsd
	      Check if FILE is x86_64 kFreeBSD

       --is-x86-knetbsd
	      Check if FILE is x86 kNetBSD

       --is-i386-knetbsd
	      Check if FILE is i386 kNetBSD

       --is-x86_64-knetbsd
	      Check if FILE is x86_64 kNetBSD

       --is-i386-efi
	      Check if FILE is i386 EFI file

       --is-x86_64-efi
	      Check if FILE is x86_64 EFI file

       --is-ia64-efi
	      Check if FILE is IA64 EFI file

       --is-arm64-efi
	      Check if FILE is ARM64 EFI file

       --is-arm-efi
	      Check if FILE is ARM EFI file

       --is-hibernated-hiberfil
	      Check if FILE is hiberfil.sys in hibernated state

       --is-x86_64-xnu
	      Check if FILE is x86_64 XNU (Mac OS X kernel)

       --is-i386-xnu
	      Check if FILE is i386 XNU (Mac OS X kernel)

       --is-xnu-hibr
	      Check if FILE is XNU (Mac OS X kernel) hibernated image

       --is-x86-bios-bootsector
	      Check if FILE is BIOS bootsector

SEE ALSO

       info grub

								  Web Feb 26 2014						      GRUB-FILE(1)
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