It is different from other operating systems , because Mac OS X is supposed to run only on Macs and not on other computers . PowerPC versions of Mac OS X prior to Leopard retain compatibility with older Mac OS applications by providing an emulation environment called Classic , which allows users to run Mac OS 9 as a process within Mac OS X , so that most older applications run as they would under the older operating system . Classic is not supported on Intel-based Macs or in Mac OS X v10 .5 `` Leopard '' , but users still requiring Classic applications on Intel Macs can use the SheepShaver emulator to run Mac OS 9 on top of Leopard .