With the Macintosh 512K, a system extension called the Switcher was developed to use this additional memory to allow multiple programs to remain loaded. Initial versions of the System Software ran one application at a time. The last major release of the system was Mac OS 9 in 1999.

That program ended after the release of Mac OS 8 in 1997. Apple rebranded the system as Mac OS in 1996, starting officially with version 7.6, due in part to its Macintosh clone program. The name Macintosh System Software came into use in 1987 with System 5. This operating system consisted of the Macintosh Toolbox ROM and the 'System Folder', a set of files that were loaded from disk. As part of an agreement allowing Xerox to buy shares in Apple at a favorable price, it also used concepts from the Xerox PARC Alto computer, which former Apple CEO Steve Jobs and other Lisa team members had previewed. The first version of the system software, which had no official name, was partially based on the Lisa OS, which Apple previously released for the Lisa computer in 1983. Monolithic for 68k, nanokernel for PowerPCĬommercial software, proprietary softwareĪpple released the original Macintosh on January 24, 1984.