If the kernel wishes to be able to access the system's physical memory directly, however, it must set up page tables which map that memory into the kernel's part of the address space. With the default 3GB/1GB mapping, the amount of physical memory which can be addressed in this way is somewhat less than 1GB - part of the kernel's space must be set aside for the kernel itself, for memory allocated with vmalloc(), and various other purposes.Honestly, I don't understand here why only 1GB is accessible under these premises.
PS Great article, Jon. In fact, great articles, both part I and part II.
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