32-bit Ubuntu Physical Address Extension (PAE) paging mechanism and how to open PAE to support more than 4G memory (sometimes grub inside will let you choose)

1, first say Ubuntu (PAE) principle, the following say how to open PAE and some need to pay attention to place

By increasing the number of pins from 32 to 36 on the processor to increase the processor's addressing capability to reach 2^36=64GB, Intel introduces a new paging mechanism.

64GB of RAM is divided into 2^24 page boxes, the Physical Address field of the page table entry expands from 20 bits to 24 bits, each page table entry must contain 12 flag bits (fixed) and 24 physical

Address bit (36-12), total 36 bits, so each page table entry must expand from 32 to 64 bits (36-bit >32 bit, so that the page table entries should be enlarged by one to 64 bits, taking into account alignment).

in the case of regular paging in 4KB, because each page table item size is 64, the original 210 size page table can contain only 512 table entries, which takes up

bit linear address in the same way, because the page Directory entries and page table entries have the same structure, the higher level of the page catalog table can only contain 512 page table entries, also occupy

32-bit linear address of 9 bits, at this point, the number of linear address remaining digits are: 32 bits (total digits)-12 bits (page offset)-9 bit (indicates the index in the page table)-9 bits

(indicates the index in the page catalog table) = 2 bits, while Linux introduces a new page table level of page Catalog pointer table (PDPT), consisting of 4 64-bit table entries, with the remaining

2 bits are used to point to one of the 4 items in the PDPT.

The following 4 graphs detail the page table structure in 4 cases (from Wikipedia)

page table structure with 4K paging not enabled for PAE

Page table structure with 2M paging not enabled for PAE

Enable page table structure with PAE under 4K paging

2, open PAE

Need to install a kernel that supports PAE
sudo apt-get install Linux-generic-pae

sudo apt-get install Linux-image-generic-pae

sudo apt-get install Linux-headers-generic-pae

Make sure you don't miss the installation linux-headers-generic-pae, or it will cause missing kernel modules, such as the inability to start the VirtualBox virtual machine

Note: Some postings on the web say that you need to install a graphics driver for example, sudo apt-get install nvidia-current nvidia-settings# apply to n cards, but I suggest that there is no need to think, Linux graphics driver has been criticized, inadvertently led to Destop only see the background can not see the desktop, this time can only enter the character interface to save.