I don't know offhand what instructions were introduced in Pentium or later. Doing some googling... 486 had cmpxchg, that's good. Seems it didn't have rdtsc. I

I don't know offhand what instructions were introduced in Pentium or later. Doing some googling... 486 had cmpxchg, that's good. Seems it didn't have rdtsc. It's not out of the question, I might say even pretty common, for user mode code to use that one via inline asm to implement a timer. Obviously no SIMD instructions. If they say they're supporting 686 or better they might have some compiler flags that depend on mmx or similar? SSE came late to P6.