How un-portable is assembly language, /really/?

Question

I understand that writing anything in assembly, or adding assembly to any program harms its portability. But, how bad? I mean, basically all PC\'s are x86 or x64 these days,

Answer 1

assembly is writing instruction directly for a specific processor, which means yeaah if the x86 live forever your code is somehow portable.

But even now the arm processor are coming back (i.e. next generation net book) and I am sure if processor won't change in next year.

I would say assembly language is by design not portable.