IB Computer Science A1.3: Operating systems and control systems
New IB Computer Science syllabus · first exams May 2027 · Theme A · SL & HL (A1.3.5, A1.3.6 and A1.3.7 HL only)
A1.3 bridges hardware and software. The operating system sits between users and applications and the hardware. Control systems (HL) connect computing to the physical world through sensors and actuators.
| Statement | Topic | Command term |
|---|---|---|
| A1.3.1 | Role of operating systems | Describe |
| A1.3.2 | Functions of an operating system | Describe |
| A1.3.3 | Approaches to scheduling | Compare |
| A1.3.4 | Polling and interrupt handling | Evaluate |
| A1.3.5 (HL) | Multitasking and resource allocation | Explain |
| A1.3.6 (HL) | Control system components | Describe |
| A1.3.7 (HL) | Control systems in real-world applications | Explain |
Key points
A1.3.1 The role of an operating system
An operating system is system software that abstracts the hardware and manages resources, acting as the intermediary between users or applications and the hardware. It hides hardware complexity, shares out CPU time, memory, storage and I/O, and provides the user interface (GUI, command line or touch).
A1.3.2 Functions of an operating system
- Memory management: allocating and reclaiming RAM, using paging, segmentation and virtual memory.
- File system: organising files and folders, and controlling access to them.
- Device management: working with hardware through device drivers.
- Scheduling: deciding which process gets CPU time.
- Security: authenticating users and setting permissions.
- Other functions: accounting (tracking resource use), the GUI, virtualisation and networking.
A1.3.3 Scheduling algorithms (Compare)
| Algorithm | How it works | Strength | Weakness |
|---|---|---|---|
| First-come first-served | Runs processes in order of arrival | Simple; no starvation | Convoy effect: short jobs wait behind long ones |
| Round robin | Each process gets a fixed time slice in turn | Fair and responsive | Context-switching overhead |
| Priority | The highest priority runs first | Urgent tasks run first | Starvation, fixed by "aging" |
| Multilevel queue | Separate queues for system, interactive and batch processes | Best algorithm for each type | Complex; lower queues can starve |
Compare using fairness, throughput, waiting time, response time and starvation.
A1.3.4 Polling vs interrupts (Evaluate)
- Polling: the CPU repeatedly checks each device. It's simple and predictable, but wastes CPU cycles and power, and can miss fast events.
- Interrupts: the device signals the CPU, which runs an interrupt service routine. It's efficient and responds almost immediately, but it's more complex and can be overwhelmed by "interrupt storms".
- Rule of thumb: use interrupts for infrequent, unpredictable events such as keyboard input, network packets and disk I/O. Use polling for frequent, predictable events or strict timing, such as a sensor checked at fixed intervals. Battery-powered devices strongly favour interrupts.
A1.3.5 Multitasking and resource allocation (HL only)
- Multitasking: the OS rapidly switches the CPU between processes (context switching), saving and loading each process's registers and program counter.
- Resource contention: processes compete for the same resources. The OS manages this with mutual exclusion, semaphores and locks, and critical sections.
- Deadlock: two or more processes each wait for a resource the other holds, so none can continue. It needs all four conditions at once:
- mutual exclusion
- hold and wait
- no preemption
- circular wait
Prevent it by removing one of these conditions.
A1.3.6 Control system components (HL only)
- Sensors (input transducers) measure the environment.
- The controller runs the control algorithm and makes decisions.
- Actuators (output transducers) carry out physical actions.
- Open-loop systems have no feedback, such as a basic toaster on a timer.
- Closed-loop systems feed the output back through a sensor so the controller can adjust, such as a thermostat.
A1.3.7 Real-world control systems (HL only)
Examples include autonomous vehicles, thermostats, lifts, washing machines, traffic lights, irrigation, security systems and automatic doors.
For any example, state:
- The sensors and what they measure.
- The controller and the decision it makes.
- The actuators and the action they perform.
- Whether it is open or closed loop, and why.
- The feedback mechanism, if it is closed loop.
Exam-style question
Evaluate the use of polling versus interrupt handling for a keyboard input device. [6]
Mark scheme
- Polling [2]: the CPU would repeatedly check the keyboard even when no key is pressed, wasting CPU cycles and power on an input that is infrequent and unpredictable.
- Interrupts [2]: the keyboard signals the CPU only when a key is pressed, so the CPU is free for other work and still responds almost instantly.
- Justified conclusion [2]: because keystrokes are infrequent and unpredictable, interrupts are the more appropriate choice. Polling would only be justified if the system needed strict, predictable timing rather than low overhead.
Common mistakes that cost marks
- Listing features for an "Evaluate" question without weighing them up or reaching a conclusion for the given scenario.
- Describing each scheduling algorithm separately for "Compare". Make direct comparisons using the same criteria.
- Naming only two of the four deadlock conditions.
- Forgetting the feedback step when explaining a closed-loop system.
Frequently asked questions
Which parts of A1.3 are HL only?
A1.3.5 (multitasking and resource allocation), A1.3.6 (control system components) and A1.3.7 (control systems in real-world applications). Everything else is SL and HL.
Which scheduling algorithms should I know?
First-come first-served, round robin, priority and multilevel queue, with their advantages and disadvantages.
Go further
- Practise A1.3 with instant marking on ExamHaus. → examhaus.com
- Full A1.3 presentation and question bank with mark scheme on CS Exam Ready. → csexamready.com/resources/ib-diploma
- IB CS mock exams: Paper 1 & 2, SL & HL. → examhaus.com/mock-exams