Linux was originally written as an OS to exercise the features of the Intel 80386. Moving it to other IA-32 processors such as Intel's 80486, Pentium, Pentium Pro, and Pentium II, AMD's 5x86, K5, and K6, and Cyrix's 6x86 series has been relatively straightforward due to backwards compatibility and the wide scale similarities between the processors.
There are now ports to other CPUs such as the Motorola 68030 (including Linux/sun3 homepage and 68K Linux in general), DEC Alpha AXP, IBM/Motorola Power PC, MIPS, including SGI/Linux "Hardhat," ARMRISC, SPARC, and HP PA-RISC. . There is even a project to provide Linux on the IBM 370/390 Architecture, and a commercial implementation, Linux for S/390., AMD is supporting development at x86-64.org on an upcoming 64 bit x86 architecture often called "Sledgehammer."
The Linux Ports page has more detailed and likely more up-to-date information.
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