But ultimately, there's only two distinctive categories of computers: the ones meant to be operated by organizations vs. by individuals.
Needless to say, supercomputers belong to the former category and most of the others on the list above belong to the latter.
To be clear, if you are a business tycoon or someone like that, you might be able to own a latest supercomputer but will need specialists to "operate" it. And, of course, almost all large organizations in the world owns PCs but operating a personal computer doesn't require more than one person.
The important thing here is, those two categories represent not their capabilities but their suitable applications. Even in today, playing a FPS on a supercomputer is not a good idea - it's capable but not suitable. So, we needed some kind of a personal computer with real-time processing capability to develop and play high-quality video games.
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The invention of the personal computer was not an evolution, but a revolution.
In the industrial revolution, we found a smart way to extend our physical abilities. Industrial machines powered by fossil fuels had multiplied the productivity of individual workers and new kinds of transportation, most notably steam locomotives, had reduced the time required to move goods and people around the world.
Similarly, personal computers extended our mental abilities - I think you don't need clues.
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The history of the revolution began in 1955 at the same place where Whirlwind computer was developed - MIT's Lincoln Laboratory. Until then, practically all digital computers were supercomputers - they were meant to be operated by organizations like governments and giant companies.
The revolutionary, Ken Olsen, was a relatively junior member at the lab. Although he was not deeply involved in the development of Whirlwind, he was very much impressed by its high capability and potential.
His main role there was to evaluate usefulness of new technologies and develop experimental machines using them. One of those works, TX-0, changed his fortune and opened a new frontier in computing. As its extended name "Transistorized eXperimental computer zero" represents, it was the first transistor computer developed at the lab. Although it inherited the core architecture of Whirlwind series, which consisted of thousands of vacuum tubes, it was much smaller and required less maintenance work.
YouTube: TX-0 (MIT Lincoln Laboratory, 1955)
Back then, most research effort was focused on making more and more powerful computers. For example, IBM was also an early adapter of transistor technology and they used it to develop more powerful supercomputers (they preferred to call them "mainframes"). For them, it just meant that they could cram more electronic parts into the same space once occupied by large vacuum tubes.
But Olsen saw the technology from a completely different perspective. He thought he could make a more affordable and easy-to-operate computers by combining it with real-time processing capability and intuitive user interface of Whirlwind series.
To realize the vision, he decided to leave the Lincoln Laboratory for founding his own company, DEC.
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Here, I assert that PDP-1, its first computer released in 1960 and the machine on which Spacewar ran, was the world's first "personal computer" in a practical sense.
Although it cost around a million dollars (in today's dollars) and you couldn't move it alone, it was meant to be operated by individual users and did provide features and I/O devices which later became the industry's standard.
For example, its users would write a program with a typewriter-like keyboard. Then, the machine would load the program and ran it in real-time. They could see the results on a CRT display and could control it with a keyboard or pointing devices. When they found a coding error, they could debug it on the machine itself.
All of those facts are hardly surprising because they're what we usually expect from modern PCs, which in turn supports my assertion.
Therefore, although DEC officially introduced PDP-1 as a "minicomputer", it was undoubtedly a forerunner of today's PCs.
YouTube: PDP-1 (Digital Equipment Corporation, 1960)
But like other innovative products, it didn't become popular overnight. First of all, there was no market for such kind of product. Moreover, it lacked a killer application software to prove the usefulness of real-time computing.
So, it was not just a coincidence that the innovative video game, Spacewar, was developed for the world's first personal computer - they needed each other.
To motivate and assist software developers, PDP-1 provided a simple and useful command set and various kinds of optional I/O devices including "light pens". Also, its hardware architecture was so simple that its users could connect homemade devices - some members of Spacewar team built a gamepad by themselves.
In short, it was a perfect toy for geeks who had enough time and passion to learn new technologies - MIT have never lacked such kind of people.
So, let's resume the development story of Spacewar.
Primary reference:
"Interview with Ken Olsen" on The National Museum of American History
http://americanhistory.si.edu/comphist/olsen.html