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

Porting assembly, there is also the problem of the ABI, that varies from OS to OS. Porting a C program from Unix to Windows (or even from Linux to OpenBSD) may be a straightforward recompilation, but for an assembly program, you may find that some callee-save registers become caller-save, or that the floating-point parameters are passed differently.

And this is not only theoretical, viz. register r2 of the PowerPC versions of Linux and Mac OS X. In practice the problem may not be too bad, for instance AMD published a "recommended" ABI at the same time as its 64-bit instruction set.