Classic was not preloaded on new Macs, but was available for installation from the system disks.In 2006, Apple switched from the PowerPC to the Intel platform, and Classic support was dropped in the Intel version of OS X. Legacy SoftwarePrior to Mac OS X 10.5 (Leopard), OS X ran legacy OS 9 and earlier applications via its Classic software, which was a full copy of OS 9 that ran as an OS X process. The heart of OS X is the open source, POSIX-compliant Darwin kernel, which includes an enhanced BSD 4.4 operating system and Mach 2.5 microkernel. For the first time, the rich set of Unix commands became available to all Mac developers (A/UX was an earlier Unix OS for the Mac but was not widely used).OS X added protected memory, pre-emptive multitasking, multithreading and symmetric multiprocessing (SMP) to the Mac world.
Based on Unix and featuring an entirely redesigned user interface, OS X was a major departure from the previous OS 9 system. See macOS and Sierra.OS X Server was introduced in 1999, and the client version came out in 2001. Starting in the fall of 2016, Version 10.12 uses the rebranded 'macOS' moniker and not OS X. (Mac OS 10) The current operating system from Apple for the Mac family.
