How can Linux run on different microprocessors?

Linux kernel is, as each compiled program, platform dependent. Kernel hackers are very skilled coders, so a huge part of the kernel code is written in a platform-aware way (by the use ho pre-processor macro for example), the remaining part has to be architecture dependent either for performance issues or because is is bounded to some hardware feature not present on other archs, and so on. This is not generally a limit in the ability of compiling a kernel on arch A and produce a kernel that will run on arch B, this process is known as cross-compilation, and is made by means of a http://en.wikipedia.org/wiki/Cross%5FCompiler So the answer is that it has not to be compiled at installation time, because it can be compiled or cross-compiled by your distro vendor. You could re-compile it on your own machine if you need to customize it, of couse.

Usual steps to get linux working on particular platform: Download kernel sources Download specific platform patches (for example, for Atmel's AT91RM9200 ARM CPU) Apply official/non-official/your patches Download crosstool-ng or similar tool & build your crosscompiler/libraries Build your kernel Assembly/update/build your root manually or using scripts/toolies Transfer your kernel & root FS to your platform using either bootloader/JTAG/copying/other methods. Done Repeat for each platform.

It is normally compiled before installing. But yes it's compiled for each platform separately.