- Call Gates
- Paged Segments
- User Segments
- Memory Ordering and Atomicity
- Backward Compatibility
Compatibility is a two-edged sword. One reason that ARM does better in low-power contexts is that its simpler decoder doesn't have to be compatible with large accumulations of legacy cruft. The downside is that ARM operating systems need to be modified for every new chip version.
In contrast, the latest 64-bit chips from AMD and Intel are still able to boot PC DOS, the 16-bit operating system that came with the original IBM PC. Other hardware in the system might not be supported, but the CPUs have retained backward compatibility with every version since 1978.
Many of the bad things about x86 are due to this backward compatibility, but it's worth remembering the benefit that we've had as a result: New PCs have always been able to run old software.