User space remains compatible, although sometimes new instructions are added that would cause a fault if run on an older processor. That is a known and underst

User space remains compatible, although sometimes new instructions are added that would cause a fault if run on an older processor. That is a known and understood issue with things like cpuid and compiler flags being able to choose what to do. Privileged (kernel mode) can change. It is roughly backwards compatible, but sometimes there are changes that when taken into account result in a more optimal system. For example power management keeps getting more and more complex as more processor components are affected, there are interactions and latencies to be understood etc. If you ran older naive kernel code, it could cause considerably higher temperatures and power consumption, possibly even leading to throttling. It may make poor scheduling decisions leading to lower performance. It won't take advantage of newer protection and performance mechanisms. Worst case it may not do context switches properly leading to bizarre crashes or information leakage.