You probably know intuitively that applications have limited powers in Intel x86 computers and that only operating system code can perform certain tasks, but do you know how this really works? This post takes a look at x86 privilege levels, the mechanism whereby the OS and CPU conspire to restrict what user-mode programs can do.
Gustavo Duarte has written a well researched article explaining the User mode and the Kernel mode operation. In order to fully understand this article it is imperative to read his pervious post about the Memory Translation and Segmentation.
I posed a couple of questions in his blog comments and he clarified them with great detail. Here are the questions and the answers:
Q: You had mentioned that modern x86 kernels use the â€œflat modelâ€ without any segmentation. But won’t that restrict the size of the addressable memory to ~4GB?
A: The segments only affect the translation of â€œlogicalâ€ addresses into â€œlinearâ€ addresses. Flat model means that these addresses coincide, so we can basically ignore segmentation. But all of the linear addresses are still fed to the paging unit (assuming paging is turned on), so there’s more magic that happens there to transform a linear address into a physical address.
Check the first diagram of the previous post, it should make it more clear. Flat model eliminates the first step (logical to linear), but the last step remains and enables addressing of more than 4GB.
Now, regarding maximum memory, there are three issues: the size of the linear address space, the conversion of a linear address into a physical address, and the physical address pins in the processor so it can talk to the bus.
When the CPU is in 32-bit mode, the linear address space is _always_ 32-bits and is therefore limited to 4 GB. However, the physical pins coming out of CPU can address up to 64GB of RAM on the bus since the Pentium Pro.
So the trouble is all in the translation of linear addresses into physical addresses. When the CPU is running in â€œtraditionalâ€ mode, the page tables that transform a linear address into a physical one only work with 32-bit physical addresses, so the computer is confined to 4 GB total RAM.
But then Intel introduced the Physical Address Extension (PAE) to enable servers to use more physical memory. This changes the MECHANISM of translation of a linear address into a physical address. It works by changing the format of the page tables, allowing more bits for the physical address. So at that point, more than 4 GB of physical memory CAN be used.
The problem is that processes are still confined to a 32-bit linear address space. So if you have a database server that wants to address 12 gigs, say, it will have to map different regions of physical memory at a time. It only has a 4 gig linear window into the 12 gigs of physical ram.
Q: If we want to use the PAE, we need changes in the kernel code, right? Is that why we have server versions of Windows?
A: That’s right, the kernel needs to do most of the work, since it’s the one responsible for building the page tables for the processes.
Also, if a single process wants to use more than 4GB, then the process _also_ must be aware of this stuff, because it needs to make system calls into the kernel saying â€œhey, I want to map physical range 5GB to 6GB in my 2GB-3GB linear rangeâ€, or â€œmap 10GB-11GB nowâ€, and so on. (Of course, there would be security checks. Also, these are some nice round numbers, usually it’d probably be done in chunks of X KB of memory, depends on the application).
Regarding Windows, that’s an interesting point. It’s too strong to say that PAE _is_ why we have server versions of Windows, but it’s definitely something Microsoft has used extensively for price discrimination. Not only on the Windows kernel, but also apps like SQL Server have pricier editions that support PAE. The kernel for the server editions of Windows has other tweaks as well though, in the algorithms for process scheduling and also memory allocation. But PAE has definitely been one carrot (or stick?) to get some more money.
Linux has had PAE support since the start of 2.6. To use it one must enable it at kernel compile time. I’m not sure if it’s enabled in the kernels that ship with the various distros. I’ve never looked much into the kernel PAE code to be honest, so I’m ignorant here. My understanding though is that if it’s enabled, it’s once and for all in the machine, for all CPUs and processes.