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2.2: Isolation

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  • One of the most important principles of engineering is isolation: when you are designing a system with multiple components, it is usually a good idea to isolate them from each other so that a change in one component doesn’t have undesired effects on other components.

    One of the most important goals of an operating system is to isolate each running program from the others so that programmers don’t have to think about every possible interaction. The software object that provides this isolation is a process.

    A process is a software object that represents a running program. I mean “software object” in the sense of object-oriented programming; in general, an object contains data and provides methods that operate on the data. A process is an object that contains the following data:

    • The text of the program, usually a sequence of machine language instructions.
    • Data associated with the program, including static data (allocated at compile time) and dynamic data (allocated at run time).
    • The state of any pending input/output operations. For example, if the process is waiting for data to be read from disk or for a packet to arrive on a network, the status of these operations is part of the process.
    • The hardware state of the program, which includes data stored in registers, status information, and the program counter, which indicates which instruction is currently executing.

    Usually one process runs one program, but it is also possible for a process to load and run a new program.

    It is also possible, and common, to run the same program in more than one process. In that case, the processes share the same program text but generally have different data and hardware states.

    Most operating systems provide a fundamental set of capabilities to isolate processes from each other:

    • Multitasking: Most operating systems have the ability to interrupt a running process at almost any time, save its hardware state, and then resume the process later. In general, programmers don’t have to think about these interruptions. The program behaves as if it is running continuously on a dedicated processor, except that the time between instructions is unpredictable.
    • Virtual memory: Most operating systems create the illusion that each process has its own chunk of memory, isolated from all other processes. Again, programmers generally don’t have to think about how virtual memory works; they can proceed as if every program has a dedicated chunk of memory.
    • Device abstraction: Processes running on the same computer share the disk drive, the network interface, the graphics card, and other hardware. If processes interacted with this hardware directly, without coordination, chaos would ensue. For example, network data intended for one process might be read by another. Or multiple processes might try to store data in the same location on a hard drive. It is up to the operating system to maintain order by providing appropriate abstractions.

    As a programmer, you don’t need to know much about how these capabilities are implemented. But if you are curious, you will find a lot of interesting things going on under the metaphorical hood. And if you know what’s going on, it can make you a better programmer.

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