[jgeralnik]

Agreed. The second snippet (http://www.xorpd.net/pages/xchg_rax/snip_01.html) is relatively simple to understand and is now my favorite two opcode program.

[kaoD]

Is this the Fibonacci sequence?

EDIT: For anyone curious, being rax and rdx the starting terms, the rcx-th term is found in rax at the end of the loop.

[jgeralnik]

Yup. The xadd opcode adds the two registers together and swaps them so that the previous element is always in the same register.