A home computer is used to stream music, edit a video and download files at the same time.
Discuss how the operating system manages the processes running on the computer.
Your answer should include:
- how the CPU is shared between processes
- the states a process can be in
- how the operating system responds when a process needs attention
Mark scheme
Indicative content
How the CPU is shared between processes
- The operating system uses multitasking to give each process a short time slice on the CPU, so the music, video editor and download appear to run at the same time.
- The operating system decides which ready process will run next, so one program cannot keep the CPU to itself.
- The operating system can stop a running process and move it back to ready when its time slice ends, so all processes make progress.
The states a process can be in
- When a program is opened, the operating system creates a process and moves its instructions into RAM so it is ready.
- The operating system moves a ready process to running when the CPU is free.
- A running process becomes blocked when it is waiting for input or data, such as the download waiting for data to arrive.
- When the process finishes, the operating system terminates it and releases the resources it was using.
How the operating system responds when a process needs attention
- An interrupt is a signal to the CPU that needs immediate attention, so the operating system suspends the current process and runs the interrupt handler.
- A hardware interrupt could be a printer out of paper, and a software interrupt could be an error in a program.
- After the interrupt has been handled, the operating system can return the interrupted process to the CPU.
Levels
- Level 0 (0 marks): No rewardable content.
- Level 1 (1–2 marks): Basic, independent points are made, showing elements of understanding of key concepts/principles of computer science. (AO1) The discussion will contain basic information with little linkage between points made or application to the context. (AO2)
- Level 2 (3–4 marks): Demonstrates adequate understanding of key concepts/principles of computer science. (AO1) The discussion shows some linkages and lines of reasoning with some structure and application to the context. (AO2)
- Level 3 (5–6 marks): Demonstrates comprehensive understanding of key concepts/principles of computer science to support the discussion being presented. (AO1) The discussion is well developed, with sustained lines of reasoning that are coherent and logically structured, and which clearly apply to the context. (AO2)
Additional guidance
- Level 3 responses must cover at least two of scheduling, process states and interrupts.
- A response that only states that the OS 'manages processes' or gives a program a turn, without expansion, is limited to Level 1.
Example answers and how they were marked
The OS uses multitasking to share the CPU. It gives each process, such as the music player and the video editor, a small time slice so they appear to run at the same time. It decides which ready process is next and how long it runs. When the video editor is opened, the OS creates a process and moves its instructions into RAM so it is ready; when the CPU is free it moves it to running. If the download is waiting for data, the OS marks it blocked so the CPU is not wasted. If the printer runs out of paper, a hardware interrupt tells the CPU to stop and run the interrupt handler; afterwards the OS returns to the process.
Why: Covers scheduling with time slices, process states (ready, running, blocked) and interrupts, with sustained application to the music, video editor and download. Level 3.
The operating system uses multitasking so the music and video editor can run at the same time. It gives each program a small amount of time on the CPU. When a program is opened it creates a process and puts it in RAM. If a process is waiting for input it is blocked.
Why: Covers CPU sharing and process states, with some application, but says nothing about interrupts. Level 2.
The operating system manages processes by giving each program a turn on the CPU. This stops the computer crashing when too many programs are open.
Why: Gives only a basic, undeveloped point about giving each program a turn, with no linkage or application. Level 1.