Microsoft Windows must be the largest, most sophisticated operating system ever written. At its height of success—no, dominance—Windows was installed onto all but a few percent of the computers that it was designed to run on. Even now, talk of its being no longer dominant is less a story of being dethroned by competing operating systems for computers than of a great broadening of what technologists count as a computer.

This website’s interest is in software that runs on computers that derive from the IBM Personal Computer. These are the computers that the mass market in computing was built on. Whether as personal computers or servers, these are still the overwhelming bulk of electronic devices that consumers actually do think of as computers.

What is a Computer?

The day is perhaps not far off that my fridge will be a computer. Perhaps yours already is. Most cars, tractors and even jet engines already are. Above all, most mobile phones are. From a historical perspective, they are rather powerful computers. it has long been a cliché that we now carry in our pockets and bags a vastly more sophisticated computer than took man to the moon. (Well, you do: I don’t have one.)

But do ordinary people really think of their phones as computers? Are any of these devices any more of a computer than is my 25-year-old hi-fi component that has a central processor, hard disk and other peripheral devices, a file system, an operating system with a primitive user interface, and even a serial port through which to update the program that executes from ROM? We might as well say that a Jacquard loom from the early 19th century was a computer—which in some very real sense it was. In another sense, the term computer is nowadays applied so widely that it risks being meaningless.

I tend to think that what makes a computer a computer is the generality of programs it will execute. At one extreme is a machine that has the typical components of a computer but is limited to executing just one program that can only be changed by the manufacturer as a firmware update. That’s my old (and much valued) hi-fi component and may be your fridge or tractor. Technologists may count such a device as a computer, especially if it connects to the Internet, but few of its actual users would think of it as a computer. There’s not enough generality to its purpose.

At the other extreme is a machine that runs an operating system that in turn lets its owner load yet more programs. The owner might write such programs. More likely, the owner obtains programs from software manufacturers. The more that the programs can be arbitrary (subject only to technical requirements of the machine and operating system), the more general is the device’s purpose, and the more widely will the device be perceived more as a computer than as a phone or fridge.

The fortune of Microsoft was built on Windows (and MS-DOS before it) as an operating system for general-purpose computers to execute a generality of other people’s software. Mass-market computing was built on this too. Microsoft is too often not given enough credit for this. An open architecture for the hardware was the gift of IBM, but Microsoft matched this with a progression of operating systems whose support for arbitrary other software was both very general and remarkably stable. That you can realistically be surprised if 64-bit Windows 10 in 2021 doesn’t run a 32-bit Windows program that was written for Windows NT 3.1 in 1993, nearly thirty years ago, is no small achievement from Microsoft’s sustained attention to backwards compatibility.