There are portable (or not) software applications, portable algorithms code, non-portable algorithm code.

Assembly based software for the Motorola 68000 /and/ the Amiga will not run on a 68000 Mac. A "polynomial based packed fastSin()" subroutine written using the XMM x64 registers will work on most x86 CPUs. The same written for the ZMM registers will not work on a number of x86 CPUs.

Surely 40 years ago we could easily assume talking about specific implementations of software applications; clearly today we see problems as being optimizable on some classes of platforms (e.g. the vectors in ARM work differently from the vectors in x64 CPUs).