Editing X86 memory segmentation (section)

== Later developments ==
The [[x86-64]] architecture does not use segmentation in long mode (64-bit mode). Four of the segment registers, CS, SS, DS, and ES, are forced to base address 0, and the limit to 2<sup>64</sup>. The segment registers FS and GS can still have a nonzero base address. This allows operating systems to use these segments for special purposes. Unlike the [[global descriptor table]] mechanism used by legacy modes, the base address of these segments is stored in a [[model-specific register]]. The x86-64 architecture further provides the special ''SWAPGS'' instruction, which allows swapping the [[kernel mode]] and [[user mode]] base addresses.

For instance, [[Microsoft Windows]] on x86-64 uses the GS&nbsp;segment to point to the [[Win32 Thread Information Block|Thread Environment Block]], a small data structure for each [[thread (computer science)|thread]], which contains information about exception handling, thread-local variables, and other per-thread state. Similarly, the [[Linux kernel]] uses the GS&nbsp;segment to store per-CPU data.

GS/FS are also used in [[GNU Compiler Collection|gcc]]'s [[thread-local storage]] and [[Buffer overflow protection#GNU Compiler Collection .28GCC.29|canary-based]] stack protector.