Skip to content
9618Paper 3 · Advanced Theory§16.1, §16.2

16. System Software

Operating system internals — process management, scheduling algorithms, memory management (paging, segmentation, virtual memory) — and how compilers work (lexical, syntax, code generation, optimisation, RPN).

Read the Smart Notes Watch lectures (22) Ask the corner
Statometer71CoreNext Paper 395%
Marks a paper13 · 17%Rank#4 of 8 · #2 on P3Trend · last 12↘Oct/Nov 24 · 31: 16 · Oct/Nov 24 · 32: 14 · Oct/Nov 24 · 33: 16 · May/Jun 25 · 31: 16 · May/Jun 25 · 32: 17 · May/Jun 25 · 33: 16 · Oct/Nov 25 · 31: 14 · Oct/Nov 25 · 32: 13 · Oct/Nov 25 · 33: 13 · May/Jun 26 · 31: 9 · May/Jun 26 · 32: 9 · May/Jun 26 · 33: 9 marks9 in 10 chance in the next paper

Everything for this topic — study hub

A2 Level · 9618 · Paper 3

This topic is rows 8–10 of the study route.Open it on your study route

Statometer — what 32 real papers say about this topic and each of its 3 syllabus bullets

Core · #4 of 8 in A2 Level · recomputed with every new session

71CORE
Core#4 of 8 in A2 Level#2 on Paper 3↘ Easing

A regular, well-paid topic — you cannot afford a gap here.

Next Paper 3
95%
9 in 10 chance it is set
Marks a paper
13 / 75
17% of Paper 3 · fair share 17%
Appeared in
32 / 32
Paper 3 sittings 2021–2026
Last set
May/Jun 2026
9618/33 · Q7 · 9 marks · 11-series streak
Marks in each of the last 12 Paper 3 sittingsOct/Nov 24 → May/Jun 26
Oct/Nov 24 · 31: 16 marksOct/Nov 24 · 32: 14 marksOct/Nov 24 · 33: 16 marksMay/Jun 25 · 31: 16 marksMay/Jun 25 · 32: 17 marksMay/Jun 25 · 33: 16 marksOct/Nov 25 · 31: 14 marksOct/Nov 25 · 32: 13 marksOct/Nov 25 · 33: 13 marksMay/Jun 26 · 31: 9 marksMay/Jun 26 · 32: 9 marksMay/Jun 26 · 33: 9 marks

What the papers say

  • Set in 32 of 32 Paper 3 sittings on the current syllabus — treat it as certain.
  • Worth about 13 marks a paper (17% of Paper 3).
  • Last set May/Jun 2026 · 9618/33 · Q7 for 9 marks — in the most recent series.
  • Set in each of the last 11 series without a miss.
  • Easing off: about 15.5 marks a paper earlier, 12 in the latest years — still examined, just smaller.
  • Lives on “Explain” and “Complete” — 72% of its questions: full sentences with a reason, not one-word answers.
  • 42% of its questions involve a diagram, table or figure — practise with pen and paper.
  • 42% of its questions are set out as code, pseudocode or a table to complete.
  • Its biggest question so far: 13 marks (May/Jun 2022 · 9618/32 · Q4).
  • Inside the topic, §16.2 Translation software carries the most marks (68%) and §16.1 Memory management the least (10%).
  • It is examined mostly as AO1 (Knowledge & understanding, 59%), the rest AO2 (41%) — definitions and descriptions in syllabus words score.
  • The examiner has commented on 69 of its questions — read “What the examiner said” before you practise.

Command words

Share of questions using the word (a question can use several). What each wants →

Question shapes

  • ≤ 6 mk22
  • 7–9 mk37
  • 10–12 mk24
  • 13–15 mk19
  • 16+ mk3

Average 9.3 marks a question · 42% with a figure or table · 42% with code · biggest 13 marks

Assessment objectives — how it is examined

Every part of every current-syllabus question filed under Cambridge's AO1 / AO2 / AO3 (from its command word and what it asks you to do), so you know whether this topic pays for definitions, for applying, or for judging and building.

  • AO1 Knowledge & understanding
  • AO2 Apply & analyse
  • AO3 Design, program & evaluate

Paper 3 as a whole

Paper 3SyllabusMeasured
AO1 Knowledge & understanding60%54%
AO2 Apply & analyse40%46%
AO3 Design, program & evaluate0%0%

Syllabus = Cambridge's grid; measured = the bank's current-syllabus papers.

Inside the topic — every syllabus bullet, measured

Each part of each question is filed under the bullet it examines; the numbers are per Paper 3 sitting, exactly like the topic's. Open a bullet for its own Statometer.

  • 16.1Purposes of an operating system#12 of 14 on Paper 3Regular · 3644% next paper2.4 marks13/32 sittings→ Oct/Nov 2025

    Set most sessions for a few marks; know the definition and one example.Syllabus: maximising resource use; hiding hardware complexity; multi-tasking and process states; scheduling (round robin, SJF, FCFS, SRT); the kernel as interrupt handler

    Next Paper 3
    44%
    2 in 4
    Marks a paper
    2.4
    3% of the paper · 23% of the topic
    Asked in
    13 / 32
    Paper 3 sittings · 27 questions
    Last asked
    Oct/Nov 2025
    9618/33 · Q5 · 4 marks · 1 series ago
    • Asked in 13 of 32 Paper 3 sittings — roughly one paper in 2.
    • About 2.4 marks a paper (3% of Paper 3; 23% of the topic's marks across its 3 bullets).
    • Last asked Oct/Nov 2025 · 9618/33 · Q5 (4 marks), 1 series ago.
    • Usually “Explain” or “Describe”: full sentences with a reason, not one-word answers.
    • Biggest chunk of marks so far: 9 in Oct/Nov 2022 · 9618/32 · Q9.
    • It is examined almost entirely as AO1 (Knowledge & understanding, 100%) — definitions and descriptions in syllabus words score.
    Last 12 sittings→ Steady
    Oct/Nov 24 · 31: 0 marksOct/Nov 24 · 32: 5 marksOct/Nov 24 · 33: 0 marksMay/Jun 25 · 31: 6 marksMay/Jun 25 · 32: 7 marksMay/Jun 25 · 33: 6 marksOct/Nov 25 · 31: 6 marksOct/Nov 25 · 32: 5 marksOct/Nov 25 · 33: 4 marksMay/Jun 26 · 31: 0 marksMay/Jun 26 · 32: 0 marksMay/Jun 26 · 33: 0 marks

    Assessment objectives

    • AO1 Knowledge & understanding
    • AO2 Apply & analyse
    • AO3 Design, program & evaluate
    • Explain74%
    • Describe56%
    • State37%
    • Identify30%
    Practise its 27 questionsBiggest: 9 marks · Oct/Nov 2022
  • 16.1Memory management#14 of 14 on Paper 3Occasional · 1420% next paper0.8 marks6/32 sittings→ Oct/Nov 2024

    Rotated in occasionally — the bullet students skip and then meet.Syllabus: paging, segmentation and virtual memory; the difference between paging and segmentation; page replacement; disk thrashing

    Next Paper 3
    20%
    1 in 5 or less
    Marks a paper
    0.8
    1% of the paper · 10% of the topic
    Asked in
    6 / 32
    Paper 3 sittings · 13 questions
    Last asked
    Oct/Nov 2024
    9618/33 · Q8 · 7 marks · 3 series ago
    • Asked in only 6 of 32 Paper 3 sittings — rare, which is exactly why it catches candidates out.
    • About 0.8 marks a paper (1% of Paper 3; 10% of the topic's marks across its 3 bullets).
    • Last asked Oct/Nov 2024 · 9618/33 · Q8 (7 marks), 3 seriess ago.
    • Usually “Explain” or “State”: full sentences with a reason, not one-word answers.
    • Biggest chunk of marks so far: 7 in Oct/Nov 2024 · 9618/31 · Q8.
    • It is examined almost entirely as AO1 (Knowledge & understanding, 100%) — definitions and descriptions in syllabus words score.
    Last 12 sittings→ Steady
    Oct/Nov 24 · 31: 7 marksOct/Nov 24 · 32: 0 marksOct/Nov 24 · 33: 7 marksMay/Jun 25 · 31: 0 marksMay/Jun 25 · 32: 0 marksMay/Jun 25 · 33: 0 marksOct/Nov 25 · 31: 0 marksOct/Nov 25 · 32: 0 marksOct/Nov 25 · 33: 0 marksMay/Jun 26 · 31: 0 marksMay/Jun 26 · 32: 0 marksMay/Jun 26 · 33: 0 marks

    Assessment objectives

    • AO1 Knowledge & understanding
    • AO2 Apply & analyse
    • AO3 Design, program & evaluate
    • Explain85%
    • State69%
    • Complete46%
    • Give31%
    Practise its 13 questionsBiggest: 7 marks · Oct/Nov 2024
  • 16.2Translation software#1 of 14 on Paper 3Banker · 9895% next paper9.2 marks32/32 sittings↘ May/Jun 2026

    Asked in nearly every paper — the bullet to know cold.Syllabus: how an interpreter executes code; compilation stages (lexical analysis, syntax analysis, code generation, optimisation); BNF and syntax diagrams; Reverse Polish Notation

    Next Paper 3
    95%
    9 in 10
    Marks a paper
    9.2
    12% of the paper · 68% of the topic
    Asked in
    32 / 32
    Paper 3 sittings · 72 questions
    Last asked
    May/Jun 2026
    9618/33 · Q7 · 9 marks · 11-series streak
    • Asked in 32 of 32 Paper 3 sittings — nearly every paper.
    • About 9.2 marks a paper (12% of Paper 3; 68% of the topic's marks across its 3 bullets).
    • Last asked May/Jun 2026 · 9618/33 · Q7 (9 marks) — in the most recent series.
    • Asked in each of the last 11 series.
    • Easing: 11.7 → 8.7 marks a paper.
    • Usually “Write” or “Complete”: you must produce something — code, a diagram, a table — practise doing it, not reading it.
    • Biggest chunk of marks so far: 12 in May/Jun 2022 · 9618/31 · Q5.
    • It is examined mostly as AO2 (Apply & analyse, 59%), the rest AO1 (41%) — you must apply it to the given data or scenario — work it out, trace it, explain it in context.
    Last 12 sittings↘ Easing
    Oct/Nov 24 · 31: 9 marksOct/Nov 24 · 32: 7 marksOct/Nov 24 · 33: 9 marksMay/Jun 25 · 31: 8 marksMay/Jun 25 · 32: 9 marksMay/Jun 25 · 33: 9 marksOct/Nov 25 · 31: 8 marksOct/Nov 25 · 32: 8 marksOct/Nov 25 · 33: 9 marksMay/Jun 26 · 31: 9 marksMay/Jun 26 · 32: 9 marksMay/Jun 26 · 33: 9 marks

    Assessment objectives

    • AO1 Knowledge & understanding
    • AO2 Apply & analyse
    • AO3 Design, program & evaluate
    • Write46%
    • Complete42%
    • Show38%
    • Explain32%
    Practise its 72 questionsBiggest: 12 marks · May/Jun 2022

11% of the topic's marks sit in question parts that belong to another topic (scenario questions cross sections) or that no bullet claims; they count for the topic, not for a bullet.

Marks a paper, year by year

151617181920212223242526

Grey years are the previous syllabus (2015–2021, 50 questions) — history, not counted in the rates.

By exam series

  • May/Jun18/18 · 14.2 mk
  • Oct/Nov14/14 · 14.3 mk
Practise 105 real questions Smart Notes Quiz Flashcards Every topic on the Statometer

What you need to know3syllabus §16.1, §16.2

  1. 16.1Purposes of an operating system — maximising resource use; hiding hardware complexity; multi-tasking and process states; scheduling (round robin, SJF, FCFS, SRT); the kernel as interrupt handler
  2. 16.1Memory management — paging, segmentation and virtual memory; the difference between paging and segmentation; page replacement; disk thrashing
  3. 16.2Translation software — how an interpreter executes code; compilation stages (lexical analysis, syntax analysis, code generation, optimisation); BNF and syntax diagrams; Reverse Polish Notation

Video lectures22ZAK's YouTube channel · play here

0 of 22 watched

  • ASA220231.0K views

  • A22023231 views

  • A22023277 views

  • A220231.5K views

  • A22023472 views

  • A220213.4K views

Open in the video library ZAK's channel

Infographics4draw these the way the examiner expects · download as PNG

How a compiler translates your codeFour stages — exam questions ask what each stage does and what it produces.Lexical analysissource → tokensstrip whitespace & commentsbuild symbol tableSyntax analysistokens checked against grammar (BNF)parse treesyntax errorsCode generationparse tree → object / machine codeOptimisationremove redundant code, reorder for speed / sizeInterpreter: translates and runs one statement at a time, stops at the first error. Compiler: whole program first.cswithzak.com

Compiler pipeline

A2
Process states & CPU schedulingA process = a program being executed, with its PCB (process control block). The scheduler decides whichREADY process runs next.READYwaiting for the CPURUNNINGusing the CPUBLOCKEDwaiting for I/O / eventdispatched by schedulertime slice ends / pre-emptedwaits for I/OI/O completesnew process admitted ↓finishes → terminatedA RUNNING process cannot go straight to BLOCKED→RUNNING: it must become READY first.Low-level scheduler: READY → RUNNING. High-level scheduler: which jobs are admitted to READY.AlgorithmIdeaFCFSfirst come first served; simpleSJFshortest job first; needs estimatesSRTshortest remaining time; pre-emptiveRound robintime slice each, in turn; fairPriorityhighest first; low may starvePre-emptive = the OS can interrupt a runningprocess (timer interrupt). Non-pre-emptive = itruns until it blocks or finishes.Why multitasking needs interruptsA clock interrupt ends the time slice; the kernel saves the registers in the PCB, picks the next READY process, restores its state.I/O interrupts move BLOCKED processes back to READY. Deadlock: two processes each wait for a resource the other holds.OS purposes: hide hardware complexity, manage resources (CPU, memory, I/O), provide a user interface, run applications.cswithzak.com

Process states & scheduling

A2
Memory management: paging, segmentation & virtual memoryRAM is shared by many processes. The OS maps each process's logical addresses onto physical frames — ordisk when RAM is full.Process (logical pages)page 0page 1page 2page 3Page tablepageframe05122disk37Physical RAM (frames)frame 0: otherframe 1: otherframe 2: P page 1frame 3: otherframe 4: OSframe 5: P page 0frame 6: otherframe 7: P page 3page 2 (swapped out)Disk — page filepage in on demandPagingMemory and process split into fixed-size pages/frames.Any frame will do → no external fragmentation, butthe last page is partly wasted (internal fragmentation).Pages are physical divisions, invisible to the programmer.SegmentationProcess split into variable-size logical segments (code,stack, data). Segment table holds base + limit. Naturalfor sharing/protection, but leaves gaps between segments(external fragmentation).Virtual memoryOnly the pages in use sit in RAM; the rest live in the page file on disk. A page fault loads the missing page,evicting another (page replacement: FIFO, LRU…). Benefit: programs bigger than RAM, more processes at once.Cost: disk is slow — disk thrashing when the OS spends its time swapping pages in and out instead of running code.“Describe how paging works” = pages, frames, page table, logical → physical mapping, swapped to disk when needed.cswithzak.com

Paging, segmentation & virtual memory

A2
BNF, syntax diagrams & Reverse Polish NotationBNF and syntax diagrams define what is legal (syntax analysis). RPN lets a compiler evaluate expressionswith a stack.Backus-Naur Form<digit> ::= 0|1|2|3|4|5|6|7|8|9<unsigned> ::= <digit>|<digit><unsigned><integer> ::= <unsigned>|-<unsigned>::= “is defined as” · | “or” · <…> = non-terminalRecursion (<unsigned> inside itself) allows any length.Syntax diagram for <unsigned>digitloop = one or more digitsCircles = terminals (literal symbols). Rectangles = non-terminals.Follow the arrows; any path through the diagram is valid.Reverse Polish Notation (postfix)(A + B) * C → A B + C *A + B * C → A B C * +Operator after its operands → no brackets or precedence needed.Evaluate 3 4 + 5 * with a stackreadstackaction33push43 4push+7pop 2, add, push57 5push*35pop 2, multiply, pushResult = the one value left on the stack. Convert infix → RPNwith the shunting-yard method or a binary tree (post-order walk).Where this sits in the compilerLexical analysis → tokens · Syntax analysis → parse tree, checked against BNF · Code generation → object code (RPN helps) · Optimisation.Optimisation: remove code that never runs, move loop-invariant calculations out of the loop, reuse computed values, use registers.“Is 123 a valid <integer>?” — walk the rules: 1 is a digit, 23 is an unsigned, so 123 is an unsigned → yes.cswithzak.com

BNF, syntax diagrams & RPN

A2

Browse all infographics →

Key terms12use these exact words in the exam

processschedulinground robinpagingsegmentationvirtual memorylexical analysissyntax analysisBNFsyntax diagramRPNoptimisation

Dotted terms are defined in the glossary.

Code help3referenced to the Cambridge pseudocode guide

Evaluate Reverse Polish Notation with a stack

pseudocode §3, §6.2 Run in Playground
DECLARE Tokens : ARRAY[1:5] OF STRING
DECLARE Stack : ARRAY[1:10] OF REAL
DECLARE Top, i : INTEGER
DECLARE A, B : REAL
Tokens[1] ← "3"
Tokens[2] ← "4"
Tokens[3] ← "2"
Tokens[4] ← "*"
Tokens[5] ← "+"      // 3 4 2 * +  ==  3 + 4 * 2
Top ← 0
FOR i ← 1 TO 5
   CASE OF Tokens[i]
      "+", "-", "*", "/" :
         B ← Stack[Top]
         A ← Stack[Top - 1]
         Top ← Top - 1
         CASE OF Tokens[i]
            "+" : Stack[Top] ← A + B
            "-" : Stack[Top] ← A - B
            "*" : Stack[Top] ← A * B
            "/" : Stack[Top] ← A / B
         ENDCASE
      OTHERWISE :
         Top ← Top + 1
         Stack[Top] ← STR_TO_NUM(Tokens[i])
   ENDCASE
NEXT i
OUTPUT "Result = ", Stack[1]

Round-robin scheduling simulation

pseudocode Run in Playground
DECLARE Burst : ARRAY[1:3] OF INTEGER
DECLARE i, Quantum, Clock : INTEGER
DECLARE Done : BOOLEAN
Burst[1] ← 5
Burst[2] ← 2
Burst[3] ← 4
Quantum ← 2
Clock ← 0
REPEAT
   Done ← TRUE
   FOR i ← 1 TO 3
      IF Burst[i] > 0 THEN
         Done ← FALSE
         IF Burst[i] >= Quantum THEN
            Clock ← Clock + Quantum
            Burst[i] ← Burst[i] - Quantum
         ELSE
            Clock ← Clock + Burst[i]
            Burst[i] ← 0
         ENDIF
         OUTPUT "t=", Clock, " ran P", i, " left ", Burst[i]
      ENDIF
   NEXT i
UNTIL Done

BNF for an integer

text
<digit>   ::= 0|1|2|3|4|5|6|7|8|9
<integer> ::= <digit> | <digit><integer>
<signed>  ::= <integer> | -<integer>

Playground examples1runnable program for this topic

  • Evaluate Reverse Polish Notation with a stack

    Operands are pushed; an operator pops two, applies itself and pushes the result — how a compiler evaluates RPN.

    A2PseudocodeStacks, queues, lists & trees 9618 §16.3
    Run

Test yourself

Ready to check you know it?

Every round is a fresh random draw, weak cards come back until you get them right, and past-paper questions come with their mark schemes. Marks earn XP on your dashboard.

Quiz: System Software (A2) (52)🃏 Flashcards: mix this topic (64)🖥️ OS — processes, scheduling & interrupts (25)🧠 OS — memory management (16)🔤 Compilation stages, BNF & RPN (23)Past-paper questions (105)⚙️ Reverse Polish & BNF (Compiler Lab)🗓️ Scheduling & paging simulator🪤 Examiner's traps🎮 Play ArenaZ🎮 Play RoyaleZ🎮 Play SpireZ🎮 Play ImposterZ

NewCS Games · 16 to play

Revise this topic by playing

Quiz battles, a daily CS word, SQL whodunnits, logic-gate puzzles and arcade — every game drills the syllabus. Solo against AskZ bots, with friends, or with your whole class. Free, no sign-up.

Play now
ArenaZRoyaleZSpireZImposterZCodleZTraceZGolfZ+ all 16 →
Enroll nowOnline classes