Cambridge 9618 · International A Level Computer Science · ~14 min read
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What is an Operating System?
An Operating System (OS) is system software that manages computer hardware and software resources and provides common services for application programs. It acts as an intermediary between the user, applications and the hardware.
Cambridge 9618 requires knowledge of the following OS functions:
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Memory Management
Allocates RAM to processes; manages virtual memory and paging
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Processor Scheduling
Decides which process runs on the CPU and when
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I/O Management
Controls communication between CPU and peripheral devices via device drivers
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File Management
Organises and tracks files on storage; handles permissions and directory structure
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Security & Access
Authenticates users; enforces access permissions and protects data
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Interrupt Handling
Responds to hardware/software interrupts; saves state and runs ISR
Memory Management
The OS is responsible for:
Allocating RAM to each process when it starts, and freeing memory when the process ends
Protecting each process's memory space — one process cannot access another's memory
Managing virtual memory: using secondary storage as an extension of RAM by swapping pages in and out (paging)
Tracking which areas of memory are free, in use, or reserved
Processor Scheduling
The scheduler is the part of the OS that decides which process gets CPU time and for how long. Because many processes want to run simultaneously, the scheduler creates the illusion of parallel execution through rapid switching (multitasking).
Process States
Every process moves between three states:
READY
Waiting for CPU
→
RUNNING
Using CPU
→
BLOCKED
Waiting for I/O
When I/O completes, the process moves back to READY. When the scheduler selects a READY process, it becomes RUNNING.
Scheduling Algorithms
Algorithm
Description
Advantage
Round Robin
Each process gets a fixed time slice (quantum); after it, the next process runs
Fair; prevents starvation
First Come First Served (FCFS)
Processes run in arrival order to completion
Simple to implement
Shortest Job First (SJF)
Process with shortest estimated time runs next
Minimises average waiting time
Priority Scheduling
Highest priority process runs next; preempts lower priority
Important processes get fast response
I/O Management
The OS controls all communication between the CPU and peripheral devices (keyboard, disk, printer, network card, etc.). It does this through device drivers — small programs that translate generic OS commands into device-specific instructions. This means application programs don't need to know the details of each device.
Buffering: temporary storage of data during transfer (e.g., print spooler) so the CPU doesn't have to wait for a slow device
Spooling: queuing multiple print jobs; the OS sends them to the printer one at a time
Interrupt-driven I/O: the device sends an interrupt when it's ready; the CPU handles the ISR then returns to its task
File Management
The OS provides a file system that organises data on storage devices:
Creating, reading, writing, copying, moving and deleting files
Organising files into a directory (folder) hierarchy
Setting access permissions (read, write, execute) per user or group
Maintaining file metadata: name, size, creation date, type, location on disk
Providing a consistent interface to application programs regardless of the underlying storage hardware
Security and Access Control
The OS enforces security at multiple levels:
Authentication: verifies user identity at login (username + password, biometrics, MFA)
Authorisation: controls what each authenticated user can do (file permissions, admin rights)
Memory protection: prevents processes from accessing each other's memory regions
Firewall and network controls: some OS functions manage network access rules
Interrupt Handling
An interrupt is a signal to the CPU that an event needs immediate attention. After each FDE cycle, the CPU checks for pending interrupts. If one exists:
The current process state (registers, PC) is saved to a stack
The CPU jumps to the Interrupt Service Routine (ISR)
The ISR handles the event (e.g., read keyboard input, handle disk data ready)
After the ISR completes, the saved state is restored and the original process resumes
Exam tip: Cambridge questions often ask you to describe one or more specific OS functions. Always explain WHAT the OS does and WHY it is necessary. For memory management, mention allocation AND protection. For scheduling, mention the aim (efficient CPU use) AND fairness. For I/O, mention device drivers and buffering.
⚠️ Common Mistakes
Listing functions without explaining them — e.g., writing "memory management" with no detail of what the OS actually does
Confusing the scheduler (part of the OS) with the CPU — the CPU executes instructions; the scheduler decides what the CPU runs
Saying the OS runs programs — the OS MANAGES which programs run and allocates resources; programs themselves run on the CPU
Forgetting that device drivers are part of I/O management — they are key to how the OS interfaces with devices
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Worksheet — 1.5.1 Operating System Functions
8 questions · instantly marked · Cambridge 9618 standard
Q1State four functions of an operating system.[4]
✅ Mark scheme
Mark scheme
Any four from: memory management [1]; processor/CPU scheduling [1]; I/O management [1]; file management [1]; security and access control [1]; interrupt handling [1].
Q2Describe two things the OS does as part of memory management.[4]
✅ Mark scheme
Mark scheme
Any two from: allocates RAM to processes when they start [1+1 per point]; protects each process's memory space from other processes [1]; manages virtual memory / swaps pages between RAM and secondary storage [1]; frees memory when a process ends [1].
Q3Name three process states and describe the transition from BLOCKED back to READY.[4]
✅ Mark scheme
Mark scheme
Three states: READY, RUNNING, BLOCKED [1 each, max 2]; transition: a BLOCKED process is waiting for I/O to complete; when the I/O operation finishes (signalled by interrupt), the OS moves the process back to the READY queue [1]; it does not go directly to RUNNING — it must wait for the scheduler [1].
Q4Explain the role of a device driver in I/O management.[3]
✅ Mark scheme
Mark scheme
A device driver translates generic OS commands into device-specific instructions [1]; this allows the OS (and application programs) to communicate with hardware devices without needing to know the specific details of each device [1]; different devices need different drivers but the OS interface remains consistent [1].
Q5Describe what happens when an interrupt is received by the CPU, in the correct sequence.[4]
✅ Mark scheme
Mark scheme
Current process state (registers, PC) is saved to a stack [1]; CPU jumps to the Interrupt Service Routine (ISR) for that interrupt type [1]; the ISR handles the interrupt (e.g., reads keyboard data, processes disk transfer complete) [1]; after ISR completes, the saved state is restored and the original process resumes [1].
Q6Describe what spooling is and why it is needed.[3]
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Spooling is the queuing of output jobs (e.g., print jobs) [1]; multiple programs can send print requests simultaneously; the OS queues them and sends them to the printer one at a time [1]; this frees programs from waiting for a slow device — they can continue executing while the OS manages the queue [1].
Q7Compare Round Robin and Shortest Job First scheduling algorithms — include one advantage of each.[4]
✅ Mark scheme
Mark scheme
Round Robin: each process gets a fixed time slice (quantum), cycling through all processes [1]; advantage: fair — every process gets CPU time; no starvation [1]; SJF: process with shortest estimated execution time runs next [1]; advantage: minimises average waiting time [1]. Accept other valid comparisons.
Q8Explain the difference between authentication and authorisation in OS security.[2]
✅ Mark scheme
Mark scheme
Authentication: verifies who the user IS (e.g., username/password, biometrics) — confirms identity before granting access [1]; authorisation: determines what an authenticated user is ALLOWED to do — controls permissions, file access, admin rights [1].
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Mini Test — 1.5.1 Operating System
10 questions · 10 marks · 10 minutes
⏱ 10:00
10 marks
Section A — Multiple Choice [5 marks]
Q1Which OS function decides which process gets CPU time?
Q2A device driver's role is to:
Q3A process is BLOCKED when:
Q4Which scheduling algorithm gives each process a fixed time slice in turn?
Q5Authentication verifies:
Section B — Short Answer [5 marks]
Q6Describe two tasks the OS performs as part of file management.
Mark schemeAny two from: creating/reading/writing/deleting files [1]; organising files in a directory hierarchy [1]; setting access permissions per user [1]; maintaining file metadata (name, size, date, location) [1].
Q7Explain why an OS needs to protect each process's memory space.
Mark schemeWithout memory protection, one process could read or overwrite data belonging to another process [1]; this could corrupt another process's data, crash the system, or be exploited as a security vulnerability [1].
Q8What is buffering and why is it used in I/O management?
Mark schemeBuffering stores data temporarily in memory during transfer between the CPU and a peripheral [1]; it is used because peripherals are much slower than the CPU — buffering allows the CPU to carry on working rather than waiting for the device [1].
Q9State one advantage and one disadvantage of Shortest Job First (SJF) scheduling.
Mark schemeAdvantage: minimises average waiting/turnaround time [1]; Disadvantage: longer processes may be starved (never get CPU time if shorter jobs keep arriving); also requires knowing job length in advance [1].
Q10Describe the sequence of events that occurs when a hardware interrupt is received by the CPU.
Mark schemeCPU finishes current FDE cycle [1]; current process state (registers, PC) saved to stack [1]; CPU executes the Interrupt Service Routine (ISR) for that interrupt [1]; ISR completes, state is restored, original process resumes [1]. (Award max 3 for this question.)