The operating system
The master program that manages the machine on behalf of every other program: it shares the processor among competing tasks, hands out and protects memory, organizes files, drives the disks and terminals, and presents all this messy hardware to applications as a clean, uniform set of services. It is the machine that runs the machine.
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IBM's OS/360, the operating system for the System/360 mainframes announced in 1964, became one of the largest programming efforts ever attempted — on the order of 5,000 person-years of work by up to a thousand programmers — and it was chronically, famously late. Its manager, Fred Brooks, distilled the wreckage into a single law in *The Mythical Man-Month* (1975): adding people to a late software project only makes it later. The discipline of software engineering was, in part, the lesson learned from trying to build one operating system.
Ce que c'est
An operating system is the layer of software that stands between raw hardware and everything else. It decides which program gets the processor and for how long, so a dozen tasks appear to run at once on one chip. It parcels out memory and walls each program off from the others, so a crash in one does not corrupt the rest. It turns the physical chaos of disk sectors into named files and folders, and turns keyboards, screens, and networks into simple services a program can call without knowing the hardware's particulars. Every application you run is really asking the operating system to do these things on its behalf.
Pourquoi cela a compté
The first stored-program machines ran one program at a time, loaded by hand, with the processor idle whenever the program paused for slow input or output — an enormous waste on machinery that cost a fortune per hour. The operating system was how that waste was reclaimed: first by batching jobs so the next began the instant the last finished, then by *time-sharing* — slicing the processor so finely that many users at terminals each felt they had the computer to themselves. It also drew the line that made software portable and modular: write to the operating system's interface, not to the bare metal, and your program keeps working when the hardware changes underneath. Without that layer, every program would have to re-solve scheduling, memory, and device handling from scratch.
Ce que cela a débloqué
Everything a modern user takes for granted: running many programs at once, several people sharing one computer, files that outlive the program that made them, and hardware you can upgrade without rewriting your software. The interactive, time-shared systems of the 1960s made programming a conversation rather than a mail-in service, and their descendants — Unix and its lineage, then the systems inside every phone, server, and laptop — are the environment in which essentially all other software lives. The operating system is the reason a computer feels like a general tool rather than a single-purpose appliance: it is the permanent host that makes room for everything else.
Version minimale viable
A resident "monitor": a small program that stays in memory, loads one job after another from a queue, and cleans up between them. Add the ability to suspend one program and resume another — time-sharing, as MIT's CTSS demonstrated from 1961 — and interactive computing becomes possible: many people at terminals, each with the illusion of a machine to themselves.
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A débloqué
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Sources
- — Frederick P. Brooks Jr., *The Mythical Man-Month: Essays on Software Engineering* (1975)
- — F. J. Corbató et al., 'An Experimental Time-Sharing System,' Proceedings of the Spring Joint Computer Conference (1962)
- — Andrew S. Tanenbaum & Herbert Bos, *Modern Operating Systems* (4th ed., 2014)
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