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OCR H446 1.2.1 Scheduling with FCFS and round robin

Part 5 of 12 · H446 1.2.1 · Systems software

Scheduling begins here for H446 1.2.1. The worksheet introduces the ready, running and blocked process states, then has students build schedules by hand: first come first served in arrival order, and round robin rotating unfinished processes to the back of the queue after a fixed quantum.

Students will:

  • describe the ready, running and blocked states and what moves a process between them
  • apply first come first served rules to a queue of arriving jobs
  • construct a round-robin trace for a given time quantum
  • explain the likely effect of making the quantum extremely small
  • compare the two algorithms for one long job followed by several short interactive jobs

Inside: 6 explanation cells, 4 multiple-choice questions, 4 fill-in-the-blanks cells and 3 written answers. 37 marks, about 45 to 55 minutes.

Series: H446 1.2.1 · Systems software, part 5 of 12.

Shared by Coding PathwayVerified teacher

  • 17 cells
  • About 45 minutes
  • CC BY-SA 4.0
  • Shared 31 Aug 2026
  • Updated 3 Sept 2026

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Scheduling with FCFS and round robin

A multitasking operating system may have many processes able to run, but a processor core can execute only one instruction stream at a moment. The scheduler chooses which ready process runs next and how long it may run.

This worksheet introduces process states, first come first served and round robin.

Worked model: a process can be ready, running or blocked

Ready, running and blocked process statesREADYRUNNINGBLOCKEDscheduler selectsquantum expireswaits for I/OI/O completesthe scheduler chooses who runs and for how longA finished process leaves the system. A blocked process cannot usefully run until its event completes.An interrupt can report that I/O has completed, allowing the process to return to ready.

A ready process is able to run and waits for processor time. A running process currently has the processor. A blocked process cannot continue until an event such as input/output completion occurs.

A scheduler prevents the CPU sitting idle when another process is ready, and it helps share processor access according to an algorithm.

Multiple choice1 mark

Which decision belongs to the scheduler?

  • AWhich process should execute next and for how long
  • BWhich files should be permanently deleted
  • CWhich monitor resolution is physically possible
  • DWhich instruction set the CPU was manufactured with
Fill in the blanks4 marks
A process waiting for CPU time is entry 1. While it owns the CPU it is entry 2. If it must wait for disk input it becomes entry 3. When the input completes, it can return to the entry 4 queue.
Multiple choice1 mark

Why should a process waiting for a disk read enter the blocked state?

  • AIt has permanently finished
  • BIt cannot make progress until the data arrives, so another ready process can use the CPU
  • CIt must keep the CPU while doing no work
  • DIt has become a device driver

First come first served (FCFS)

FCFS runs processes in arrival order. If jobs J, K and L arrive in that order, J runs first. A later short job cannot pass an earlier long job.

This non-pre-emptive simplicity suits a fair first-in-first-out queue such as ordinary print jobs with equal priority. Its weakness is the convoy effect: one long job can make every later job wait.

Fill in the blanks4 marks
A printer receives jobs at these times.
JobArrival time
R0
S3
T4
U9
Under FCFS, the order is entry 1, then entry 2, then entry 3, then entry 4.
Written answer3 marks

Explain why FCFS can be suitable for an ordinary printer queue in which all jobs have equal priority.

Answer why the operating system uses it in this context, not why a user personally likes it.

Students type their answer here.

Round robin (RR): rotate after a quantum

Round robin gives the process at the front of the ready queue a fixed time slice, also called a quantum. If it has not finished when the quantum expires, it is pre-empted and moved to the back of the queue.

Worked example with quantum 2:

  • A needs 5 ticks, B needs 3 and C needs 1.
  • A runs 2 (3 remain), B runs 2 (1 remains), C runs 1 and finishes.
  • A runs 2 (1 remains), B runs 1 and finishes, A runs 1 and finishes.
  • Execution sequence: A, B, C, A, B, A.
Fill in the blanks10 marks
Trace round robin with quantum 2 for A=4 ticks, B=3 ticks and C=2 ticks.
TurnProcessRemaining after turn
1entry 1entry 2
2entry 3entry 4
3entry 5entry 6
4entry 7entry 8
5entry 9entry 10
Multiple choice1 mark

What is a likely effect of making the round-robin quantum extremely small?

  • AFCFS automatically replaces round robin
  • BBlocked processes can never become ready
  • CMore frequent context switching, which can waste processor time
  • DEvery process finishes in one turn
Written answer4 marks

Explain how round robin supports multitasking and responsiveness.

Use ready queue, quantum and pre-emption in your answer.

Students type their answer here.

Multiple choice1 mark

Which algorithm is most directly defined by rotating unfinished processes to the back of a queue after equal time slices?

  • AFirst come first served
  • BShortest job first
  • CShortest remaining time
  • DRound robin
Written answer4 marks

Compare FCFS and round robin for a workload containing one long job followed by several short interactive jobs.

Explain what each does and which better supports responsiveness.

Students type their answer here.

Closed-book checkpoint

Complete the scheduler summary without looking back.

Fill in the blanks4 marks
The scheduler selects from processes able to execute. FCFS preserves entry 1 order and normally lets the chosen job continue. Round robin limits one turn with a time entry 2 and then rotates an unfinished process, improving regular entry 3 to the processor at the cost of extra context entry 4.

Review your understanding

Before submitting, check that you can explain the main distinction in your own words, apply it in an unfamiliar context and justify each consequence rather than only naming a feature.