Published at LXer:

The arch directory in the kernel source tree contains all of the architecture-specific code. There is a lot of code there, despite years of work by the development community to make things generic whenever possible. There are currently 26 different top-level architectures supported by Linux, many of which contain a number of sub-architectures. Two of those top-level architectures are i386 (the original Linux architecture) and x86_64, which is the 64-bit big brother to i386. There is quite a bit of commonality between those two architectures, and some efforts have been made to share code between them whenever possible. Even so, the source trees for the two architectures remain distinct from each other.

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