Cambridge O Level Computer Science · Syllabus 2210 · Software
Operating System
What is Operating System?
An operating system is system software that manages the computer's hardware and resources and provides a platform on which application software can run. Its basic functions are managing files, handling interrupts, providing an interface, managing peripherals and drivers, managing memory, managing multitasking, providing a platform for running applications, providing system security and managing user accounts.
This definition is part of the Software chapter in Cambridge O Level Computer Science.
Operating System in context
A computer is built in layers, and each layer only works because the layer beneath it is already running. Hardware executes machine instructions and nothing else. Firmware, held in non-volatile memory, starts the hardware and its bootloader loads the operating system. The operating system then manages files, memory, peripherals, users and security, and provides the platform on which application software runs. While all of that is happening, interrupts let an urgent event — a key press, a division by zero — reach the CPU without it having to keep asking. And because a CPU can only execute machine code, every program a person writes must first be put through a translator: an assembler, a compiler or an interpreter.
System software provides the services that the computer itself requires. It manages the hardware, keeps the machine usable, and provides the platform that other software runs on. The two categories the syllabus names are the operating system and utility software. System software is not written to do a job for the user; it is written to keep the computer in a state where a job can be done at all.
Application software provides the services that the user requires. It performs the tasks a person actually wants done — writing a letter, editing a photograph, keeping accounts, browsing the web, designing a part, playing a game. Application software cannot reach the hardware on its own: it depends on the system software beneath it, and requests every file, every allocation of memory and every use of a device through the operating system.
An operating system is system software that manages the computer's hardware and resources and provides a platform on which application software can run. It is not one job but nine, carried out at the same time and constantly interacting: managing files, handling interrupts, providing an interface, managing peripherals and drivers, managing memory, managing multitasking, providing a platform for running applications, providing system security, and managing user accounts. Every one of those functions exists because a shared machine with limited resources needs someone to be in charge of them.
Each of the nine functions below is stated as a definition, then unpacked into what the operating system actually does, then tied to a practical consequence — the thing a user would notice if that function stopped. That last link is what turns a memorised list into an answer that can survive an "explain" or a "suggest" command word.
Application software cannot run on its own. Applications run on the operating system; the operating system runs on the firmware; and the bootloader — which is part of the firmware — runs on the hardware. Each layer depends on the one below it being present and working. Firmware is the only software that is already there when the power arrives, which is why it, and not the operating system, is what starts the machine.
Common mistakes with Operating System
- 1. Inverting the stack. Why it fails Writing that "the operating system loads the firmware" or "the operating system starts first" reverses the only ordering the syllabus states. Firmware is already present in non-volatile memory when the power arrives; the operating system is a file on secondary storage that something has to fetch. Fix Rehearse the order upward: hardware → bootloader in firmware → operating system → applications. See section 4.1 E.
- 5. Describing multitasking as literal simultaneity. Why it fails On a single processor core, only one instruction is executed at a time. The operating system allocates processor time to each process in turn and switches between them so quickly that they appear to run at the same time. Fix Use the words "allocates processor time", "saves and restores state" and "appears simultaneous". See OS function 6.
- "Firmware and the operating system are the same thing." Correct model They are different software with different jobs, held in different places. Firmware is in non-volatile memory, is tied to that specific hardware, and starts the machine. The operating system is on secondary storage, is loaded into main memory by the bootloader, and then manages the machine.
- "The operating system starts before the firmware." Correct model Impossible. The operating system has to be loaded from storage, and the thing that loads it is the bootloader, which is part of the firmware. Firmware is already present when the power arrives; the operating system is not.
- "Applications control the hardware directly." Correct model Applications request resources from the operating system, which controls the hardware using device drivers. Direct hardware access without system-software mediation is not how the stack works.
- M1. "The operating system is application software." Why it fails Application software provides the services the user requires. An operating system provides the services the computer requires, and applications run on it. Putting it in the wrong category inverts the whole layer model. Corrected model An operating system is system software. It manages the hardware and resources and provides the platform on which application software runs. Exam-safe sentence "An operating system is system software, because it provides the services the computer requires and gives application software a platform to run on." Show the retrieval checkHide the check Check: Which category does an operating system belong to, and what is the test?Answer: System software. The test is whose requirement it serves — the computer's, not the user's.
- M2. "All system software is an operating system." Why it fails It ignores the second half of the category. Backup software, anti-malware, compression tools, storage-management tools, encryption utilities and device drivers are all system software and none of them is an operating system. Corrected model System software has two parts: the operating system and utility software. Every operating system is system software; not all system software is an operating system. Exam-safe sentence "System software includes the operating system and utility software such as backup, anti-malware, compression, storage-management and encryption programs." Show the retrieval checkHide the check Check: Name three pieces of system software that are not operating systems.Answer: Any three of: backup software, anti-malware, compression tools, storage-management tools, encryption utilities, device drivers.
- M4. "Firmware and the operating system are identical." Why it fails They are stored in different places, run at different times and do different jobs. Merging them makes the start-up sequence impossible to describe. Corrected model Firmware is in non-volatile memory, is tied to that specific hardware, and provides the initial control. The operating system is on secondary storage, is loaded into main memory by the bootloader, and then manages the whole machine. Exam-safe sentence "Firmware provides the initial control of the hardware and its bootloader loads the operating system; the operating system then manages the computer's resources." Show the retrieval checkHide the check Check: Where is firmware stored, and where is the operating system stored before it runs?Answer: Firmware is in non-volatile memory on the device; the operating system is on secondary storage until the bootloader loads it into main memory.
- M5. "The operating system starts before firmware." Why it fails It is not merely conventional but impossible. The operating system has to be loaded from storage, and loading requires a program that is already running. The only software already present when the power arrives is the firmware. Corrected model Hardware → firmware (including the bootloader) → operating system → applications. The bootloader in firmware runs on the hardware, the operating system runs on the firmware, and applications run on the operating system. Exam-safe sentence "The bootloader, which is part of the firmware, runs on the hardware and loads the operating system into main memory; applications then run on the operating system." Show the retrieval checkHide the check Check: Why can the operating system not be the first software to run?Answer: Because it is on secondary storage and must be loaded into main memory, and loading it requires a program that is already running — the bootloader in firmware.
- M6. "A device driver is a physical connector." Why it fails A cable, a port and a plug are hardware. A driver is a program, and the usual symptom of a missing one is a device that is physically connected and still not recognised. Corrected model A device driver is software that allows the operating system to communicate with and control a particular hardware device, translating general instructions into the signals that device understands. Exam-safe sentence "A device driver is software that allows the operating system to communicate with and control a particular hardware device." Show the retrieval checkHide the check Check: A device is plugged in, powered on and not recognised. What is most likely missing, and what kind of thing is it?Answer: The device driver — and it is software, not a cable or connector.
- M7. "Multitasking means a single core executes everything simultaneously." Why it fails A single processor core executes one instruction at a time. Describing genuine simultaneity removes the mechanism the question is actually testing — the switching. Corrected model The operating system allocates processor time to each process in turn, saves and restores state at each switch, and switches so rapidly that the processes appear to run at the same time. Exam-safe sentence "The operating system allocates processor time to each process in turn, saving and restoring their states, and switches between them so quickly that they appear to run simultaneously." Show the retrieval checkHide the check Check: Which single word keeps a multitasking answer honest?Answer: "Appear". The processes appear to run at the same time; a single core executes one instruction at a time.
- M8. "An interrupt always closes the current program." Why it fails If it were true, moving the mouse would close your work. It also makes the whole save-and-restore mechanism pointless — why save a state you will never return to? Corrected model The current instruction completes, the state and return location are saved, the ISR runs, the state is restored, and the interrupted program resumes from where it stopped. A process only ends if the operating system decides that an unrecoverable fault requires it. Exam-safe sentence "After the interrupt service routine has finished, the saved state is restored and the interrupted program continues from the point at which it was interrupted." Show the retrieval checkHide the check Check: Why is the state saved at step 3 of interrupt handling?Answer: So that it can be restored at step 8 and the interrupted program resumed at step 9 exactly where it stopped.
- M19. "Compiled programs never need supporting software." Why it fails It overstates a genuine advantage. It is true that a compiled program does not need its source code or a translator present in order to run — but it still needs an operating system to load it, allocate memory to it and give it access to files and devices, and it is still built for a particular platform. Corrected model A compiled executable runs without a translator. It does not run without an operating system, and it runs only on the platform it was compiled for. Exam-safe sentence "A compiled program can be run without the source code or a translator, but it still runs on an operating system and only on the platform it was compiled for." Show the retrieval checkHide the check Check: Name one thing a compiled executable still needs in order to run.Answer: An operating system — to load it, allocate memory to it and give it controlled access to hardware and storage. (It also needs the platform it was compiled for.)
Questions students ask about Operating System
What is the difference between system software and application software?
System software provides the services the computer itself requires: it manages the hardware and provides the platform other software runs on, and includes the operating system and utility software. Application software provides the services the user requires, performing tasks such as word processing, browsing or games, and it depends on the system software beneath it for every file, memory allocation and device it uses. Who installed the software, or where it came from, decides nothing — only what it is for.
In what order do hardware, firmware, the operating system and applications start up?
Applications run on the operating system, the operating system runs on the firmware, and the bootloader, which is part of the firmware, runs on the hardware. Firmware is held in non-volatile memory and is already present when the power arrives, so its bootloader is what loads the operating system into main memory from secondary storage. The operating system cannot start first, because it has to be loaded by a program — the bootloader — that is already running.
Does an interrupt close the program that was running?
No. An interrupt pauses a program rather than ending it. The current instruction completes, the processor's state is saved, an interrupt service routine executes to service the event, and then the saved state is restored so the interrupted program resumes from exactly the point at which it was interrupted. A process ends only if the operating system decides that an unrecoverable fault requires it, not simply because an interrupt occurred.