A computer has a single CPU.
Explain how the operating system allows a user to run several programs at the same time.
Mark scheme
Award up to two marks for a linked explanation such as:
- The operating system switches rapidly between processes (1) so each process runs for a short time slice before the next one starts (1)
- Each process is given a small amount of CPU time (1) so the CPU moves on to another process when that time is up (1)
- The operating system chooses which process to run next (1) so the CPU is shared between the programs (1)
- Processes take turns to use the CPU (1) because the CPU is so fast the user does not notice the switching (1)
- Only one process can run on the CPU at a time (1) so the operating system must swap processes in and out quickly (1)
For both marks, the expansion must follow/associate with the statement.
Allow 'time slice' / 'time quantum'.
Do not accept 'the computer has more than one CPU' / 'each program runs on its own CPU'.
Do not accept 'the CPU does two things at once' without saying processes take turns/are switched.
Example answers and how they were marked
The operating system switches rapidly between processes, giving each one a small time slice, so the CPU is shared and the programs appear to run together.
Why: Linked point (1) and reason (1).
Each process is given a small amount of CPU time.
Why: A correct point (1) but no linked reason.
The computer has two CPUs so each program can run on its own processor.
Why: The scenario states a single CPU, so this misconception earns no credit.