I'm not at all sure that the best answer is "source code". For x86 CPUs the compatible layer has been the ISA and that has turned out pretty damn well, suggesti

I'm not at all sure that the best answer is "source code". For x86 CPUs the compatible layer has been the ISA and that has turned out pretty damn well, suggesting that source compatibility is not the only and may not be the most effective choice. I disagree, though I see your point. If the x86 instruction set had been designed to be implemented in various ways depending on the available semiconductor technology, then that would be a much better situation. However, the basics of x86 were designed in the 1980s to be directly implemented in the hardware of the time. If you were designing an ISA expressly to insulate software from the hardware, I'm sure there are dozens of design choices that would be made differently. Ditto for amd-64.