The latest version of IA-32 Intel Architecture Software Developers Manual Volume 2B: Instruction Set Reference, N-Z
(ftp://download.intel.com/design/Pentium4/manuals/25366719.pdf) contains the opcode for a multi-byte NOP instruction. The opcode is
0F 1F mod-000-rm
The multi-byte NOP can have any length up to 9 bytes. Quite useful for alignment.
The manual says that this multi-byte NOP works on all processors with family number 6 or F, which is all Intel processors back to Pentium Pro (except Pentium MMX). I have verified that it works. I was surprised to discover that it works also on an AMD64 processor, although it is not documented in AMD manuals. I didn't find it on any website of undocumented opcodes.
How come that this opcode has been kept secret for so many years? Why is it made official now? How come it works on AMD processors when noone else has discovered it, and AMD recommends the opcode 66 66 66 90 for multibyte NOP?
I guess this is not the right place to ask about AMD processors, but how do I safely detect whether the multi-byte NOP is supported on an AMD processor? There is no use for this opcode unless you have an absolutely safe method of detecting whether it is supported, and this detection method works on all brands.