The RDTSC instruction is used for cycle timing on Pentiums and other compatible CPUs such as the Cyrix chip set. Note that this instruction does not exist on earlier CPUs in the series.
We could of course try to discover the CPU type at compile or run time and then use the appropriate instruction. This has all sorts of problems, e.g. if we compile on a i586 does that mean it will be run on the same CPU (no of course not....).
For now we use the ‘--enable-rdtsc’ option for ‘./configure’.