Yet, as MultiFinder was still optional, such a step had not been taken. If the system were able to support multiple tasks, this one-off solution would no longer be needed - desk accessories could simply be small programs, placed anywhere. For instance, to support a limited form of multitasking, the original Mac OS supported small co-resident programs known as desk accessories which had to be installed into the system using special tools. While additions had been relatively limited, so had fixes to some of the underlying oddities of the system architecture.

Running MultiFinder normally required a larger amount of RAM and a hard drive, but these became more common by the late 1980s. Most notable among these was the single-tasking model, the replacement of which had first been examined in 1986's 'Switcher' and then replaced outright with MultiFinder in System 5. However, many of the assumptions of this model were no longer appropriate. That is, the machine was geared towards a single user and task running on a floppy disk based machine of extremely limited RAM. These limited changes meant that the original Macintosh system remained largely as it was when initially introduced.

This initially shipped in three volumes, adding another to describe the changes introduced with the Mac Plus, and another for the Mac II and Mac SE. Some perspective on the scope of the changes can be seen by examining the official system documentation, Inside Macintosh. The development of the Macintosh system software up to System 6 followed a fairly smooth progression with the addition of new features and relatively small changes and upgrades over time.

Apple system extensions