2. Computing as a system

Decide what counts as “the system” in examples like a calculator, a phone app, or a browser talking to a server. Trace how one system can contain smaller subsystems and also connect to larger ones.

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Separate the physical parts of a computer from the instructions they run and the app instance currently doing work. Reason through why the same phone can behave like a camera, game console, calculator, or browser.

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Recognize the operating system as the coordinator between apps and shared resources like the screen, keyboard, files, sensors, and network. Follow why apps usually ask the OS for access instead of controlling every device directly.

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Classify inputs such as keystrokes, taps, microphone sounds, sensor readings, file contents, and network messages. Trace how a user action becomes an event or piece of data that a program can respond to.

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Connect stored instructions to the processor that runs them step by step. Reason through how changing software can change a computer’s behavior without replacing the physical machine.

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Distinguish temporary memory, long-term storage, and the current state of a running program. Trace what must be remembered only while an app is open versus what must still exist after the device restarts.

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Identify outputs such as screen changes, sounds, printed pages, saved files, and network replies. Follow how a program’s result leaves the processing step and becomes something a person or another system can use.

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Trace a simple calculator from button presses to arithmetic instructions, temporary stored values, and the displayed answer. Notice how even a tiny app uses input, processing, storage, and output together.

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Follow a phone app as it receives a touch or sensor event, uses the operating system, updates state, and changes the screen or sends a notification. Compare local work on the device with work that depends on a remote service.

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Trace a browser action from typing or tapping a link to a request sent to a server and a response returned to the browser. Use the client-server model to see how one user action can involve multiple computers.

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Use the system model to locate likely causes when something goes wrong, such as bad input, incorrect instructions, missing saved data, unavailable network access, or a broken output path. Practice asking which part of the trace failed first.

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Review this chapter with practice based on your mistakes.