80486 and Pentium added relatively few instructions to the core instruction set, but there are a couple of pretty important tools which got added. The ones you'

80486 and Pentium added relatively few instructions to the core instruction set, but there are a couple of pretty important tools which got added. The ones you'd miss the most would probably be: * CMPXCHG (486). Central to multiprocessor synchronization and locking. * CPUID (P6). Admittedly, if the instruction set were frozen you wouldn't need this... but if not, it's how you detect what CPU you're running on and what it supports. * RDMSR/WRMSR (P6, kernel only). A general-purpose mechanism for adding extra special-purpose registers to the CPU without having to allocate an instruction to each one. * INVD/WBINVD/INVLPG (486, kernel only). This was the first Intel CPU to support cache; these instructions were used to manage it.