title: Inability of Linux Kernel to support 1GB physical memory and the HIGHMEM solution
body: <p>I have been reading [1] to have an idea about the HIGHMEM. The article says that "<strong><em>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</strong>"</em> .That is exactly where I have confusion. What do they mean by accessing physical memory directly? Does that mean explicitly stating the physical address?(AFAIK, that is not possible in Linux kernel) Then they go on stating that "<strong><em>it must set up page tables which map that memory into the kernel's part of the address space</em></strong>". That is something you do in the HIGHMEM solution,isn't it? Please help me understand the problem which lead to the HIGHMEM solution.</p>
  
  <p>[1] <a href="http://lwn.net/Articles/75174/" rel="nofollow">http://lwn.net/Articles/75174/</a></p>