I mean O perating S ystem. They are also kind of useful for data aquisition and process control. With a PC, I always felt limited by the software available. Linus Torvald has had a lot to do with the whole attempt to make a unix-like operating system work on microcomputers. Mostly through private email.
Frequent answers are just as boring and uninteresting as frequent questions.
You might want to add a mention of this FAQ list in your email response and a polite suggestion that your correspondent read it before posting future questions. All Macs from the original 128K Thin Mac to the Quadra 950 contain a small amount of battery powered RAM that holds certain settings that belong to the CPU rather than the startup disk, for example the disk to start up from. Even after rethinking their extensions as per step two, most people still have at least half a row of icons march across the bottom of their screen every time they restart. If the System Heap isn't big enough to comfortably accommodate all the programs that want a piece of it, they start playing King of the Mountain on the system heap, knocking each other off to get bigger pieces for themselves and trying to climb back on after they get knocked off. All this fighting amongst the programs severely degrades system performance and almost inevitably crashes the Mac.