From the end user perspective: you could only use one application at a time. In most cases, switching between programs meant quitting the current program and l

From the end user perspective: you could only use one application at a time. In most cases, switching between programs meant quitting the current program and launching another program from the Finder. There were a couple of exceptions to this: * Some programs let you launch another program without quitting. If I recall correctly, those programs would remain in memory but would not run in the background. If you wanted to return to the original program, you would have to exit the program it launched. This was typically available in development tools. * Desk accessories were device drivers that looked like programs. This allowed the end user to run small programs, like a calculator or clipboard manager, while continuing to use another piece of software. The more interesting question is how did the system behave with multitasking turned on, since the original system software did not support multitasking. For a while multitasking support could be added with a separate piece of software. System 6 was the first version of the Macintosh system software to incorporate multitasking and it was turned off by default (if I recall correctly). System 7 was the first version where multitasking could not be disabled. I suspect that multitasking could be added in a relatively seamless fashion to the system software since developers were supposed to use the Macintosh Toolbox (API), so things like memory management were always mediated by the operating system. The main user-visible quirk was the ability to partition memory, which ensured that properly written software would not overwrite the memory of other applications. (Granted, there was no memory protection, so it could happen.)