ata66 vs ata100

I have been told that since 1999 all ata 100 hardrives are backward compatable with ata66 and ata33 machines.my machine was built in 1999 so I assume it is ata4.question is: what do I need to do to run an ata100 hd in an ata66 machine?It has a Gigabyte GA-5AX (rev 4) mainboard and has an AMD K62-500 processor.Any help would be appreciated.