What does an operating system do, and how does it manage the processor, processes and interrupts?
The operating system: its role as the interface between the user, applications and hardware, process management and CPU scheduling, the handling of interrupts, and the management of input/output and the file system.
An Eduqas Component 2 answer on the operating system: its role as the interface between user, applications and hardware, process management and CPU scheduling algorithms, the handling of interrupts, and the management of input/output and the file system.
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What this dot point is asking
Eduqas wants you to describe the role of the operating system as the interface between user, applications and hardware, explain process management and CPU scheduling, describe how interrupts are handled, and outline the management of input/output and the file system.
The answer
The role and functions of the operating system
Process management and CPU scheduling
Interrupts and input/output
Examples in context
Windows, macOS, Linux, Android and iOS are all operating systems doing exactly this work, scheduling apps, handling the keyboard and touch interrupts, managing files and enforcing permissions. Round robin scheduling is why dozens of apps feel like they run at once on one or a few cores. Interrupts are fundamental: every key press, network packet and timer tick is one, and they connect directly to the fetch-decode-execute cycle (the state saved is the registers from section 3.1). The next dot point details how the OS manages memory, including virtual memory.
Try this
Q1. State two functions of an operating system. [2 marks]
- Cue. Any two of: process management/CPU scheduling, memory management, input/output (device) management, file system management, security, providing a user interface.
Q2. Why is CPU scheduling needed in a multi-tasking system? [1 mark]
- Cue. Many processes are ready but cores are limited, so the OS must share CPU time among them, deciding which runs next and for how long.
Q3. State the first two things the processor does when an interrupt occurs. [2 marks]
- Cue. It finishes the current instruction, then saves its current state (registers and program counter) onto a stack.
Exam-style practice questions
Practice questions written in the style of WJEC Eduqas exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Eduqas 20206 marksDescribe the main functions of an operating system, and explain why CPU scheduling is needed in a multi-tasking system.Show worked answer →
Functions of an OS (up to 4 marks, roughly one each): provides an interface between the user/applications and the hardware; manages processes and schedules the CPU; manages memory (allocation and protection); manages input/output devices and their drivers; manages secondary storage and the file system; provides security (user accounts, access control).
Why scheduling is needed (up to 2 marks): in a multi-tasking system many processes are ready to run but there is a limited number of processor cores, so the OS must decide which process runs next and for how long, sharing CPU time fairly and efficiently so that the system stays responsive and no process is starved.
Markers reward several distinct OS functions and the share-limited-CPU-among-many-processes reason for scheduling.
Eduqas 20225 marksExplain what an interrupt is, describe how the processor responds to one, and give two examples of events that could raise an interrupt.Show worked answer →
What an interrupt is (up to 1 mark): a signal sent to the processor indicating that an event needs immediate attention, causing the processor to temporarily suspend its current task.
How the processor responds (up to 2 marks): it finishes the current instruction, saves the current state (the contents of the registers and the program counter) onto a stack, then runs the appropriate interrupt service routine (ISR) to handle the event; afterwards it restores the saved state and resumes the interrupted task.
Examples (up to 2 marks, one each): a key press or mouse movement (input device ready); a printer signalling it is ready or out of paper; a hardware timer; a divide-by-zero or other error; data arriving on a network.
Markers reward the signal-needing-attention definition, the save-state, run-ISR, restore-state sequence, and two valid interrupt sources.
Related dot points
- Memory management: how the operating system allocates and protects memory between processes, paging and segmentation, and virtual memory using secondary storage to extend the apparent size of main memory.
An Eduqas Component 2 answer on memory management: how the operating system allocates and protects memory between processes, the difference between paging and segmentation, and how virtual memory uses secondary storage to extend the apparent size of main memory.
- Software applications: the distinction between system software and application software, generic, special-purpose and bespoke applications, the role of utility programs (compression, defragmentation, backup, antivirus), and how software is licensed.
An Eduqas Component 2 answer on software applications: the distinction between system and application software, generic, special-purpose and bespoke applications, the role of utility programs such as compression, defragmentation, backup and antivirus, and software licensing models.
- The fetch-decode-execute cycle: the special-purpose registers (PC, MAR, MDR, CIR, ACC) and their use in fetching, decoding and executing an instruction, and the effect of the cycle on the registers.
An Eduqas Component 2 answer on the fetch-decode-execute cycle: the special-purpose registers (program counter, MAR, MDR, current instruction register, accumulator) and exactly how each is used to fetch, decode and execute an instruction.
- Input, output and storage: the role of input and output devices, the memory hierarchy from registers and cache to RAM and secondary storage, and the operating principles, advantages and uses of magnetic, optical and solid-state storage.
An Eduqas Component 2 answer on input, output and storage: the role of input and output devices, the memory hierarchy from registers and cache to RAM and secondary storage, and how magnetic, optical and solid-state (flash) storage work with their advantages and uses.
- The functional paradigm: functions as first-class values, pure functions and referential transparency, immutability and the avoidance of side effects, the use of recursion instead of iteration, and how functional differs from imperative programming.
An Eduqas Component 2 answer on the functional paradigm: functions as first-class values, pure functions and referential transparency, immutability and avoiding side effects, recursion in place of iteration, and how functional programming differs from the imperative approach.
Sources & how we know this
- WJEC Eduqas GCE AS/A Level Computer Science specification (from 2015) — Eduqas (2015)