ONCE UPON A TIME, when you booted a computer, it gave you the ability to run a single program. This doesn’t mean there was only one program you could run. It means you could only run one program at a time. When that program was finished, you could run another or run the same one all over again—whichever you chose. The operating system was unable to switch between programs, so all you had was the operating system, which allows programs to interface with hardware, and the one program you were running.
You could select which program you wanted to run, of course. You weren’t limited to just ...
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