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2 System 6
3 System 7
4 Mac OS 8
5 Mac OS 9
6 Mac OS X
All of these versions could only run one application at a time, though special application shells such as Switcher (discussed under MultiFinder) could get around this to some extent. System 1.0 and 2.0 were released with the Mac 128K and 512K respectively, and supported a flat filing system called MFS (Macintosh File System). System 3.0 introduced HFS (Hierarchical File System) which had real directories - previously the Finder created the illusion of folders on the flat file system. System 4.0 was introduced with the Mac Plus (adding SCSI support, AppleTalk, etc), and System 5.0 with the Mac SE and Macintosh II.
The other significant change that System 5.x brought to the Mac was Color QuickDraw, which debuted with the Macintosh II in 1986. This significantly altered the extent and design of the underlying graphics architecture (and its APIs), but it is a credit to Apple that most users, and perhaps more importantly existing code, were largely unaware of this.
System 6 added MultiFinder, an add-on replacement for the Finder which could run several programs at once. Time was given to the background applications only when the foreground (or "running") applications gave it up (cooperative multitasking), but in fact most of them did via a clever change on the OS's event handling. MultiFinder had been released with earlier systems, but the 6.x systems were the first to make it official and widely used.
System 6 consolidated the previous releases into a much more complete and stable operating system. It also moved the Mac to true 32-bit memory addressing - necessary with the ever increasing amounts of RAM available. Earlier systems used the lower 24 bits for addressing, and the upper 8 bits for flags. This was a neat solution on the earlier Macs with their very limited amounts of RAM, but became a liability later. Code that assumed the 24 + 8 bit addressing was "not 32-bit clean" in Apple's words, and developers were required to excise such assumptions from their code.
Although the name was changed to 8.x and 9.x over its history, the OS remained basically the same internally.
Mac OS X is the first real replacement for the older Mac OS, based on the OPENSTEP Unix operating system from NeXT. In addition to the original OPENSTEP libraries, OS X adds the Carbon libraries to allow older programming paradigms from the System 7.x core to be run under OS X and gain many of the benefits of this modern OS core. The system also includes Classic, a complete emulator for running older Mac programs.