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IB Computer Science revision notes

Group 4 · Sciences · SL / HL · first exams 2027

A1 explains what's inside a computer and how it represents everything as 0s and 1s. Number conversion and logic gates are quick marks if you practise them.

Course topics

A1 · Computer fundamentals · CPU, memory, binary and hex, logic gates, operating systems · 4 subtopics
A2 · Networks · Network types, protocols, layers, security · 3 subtopics
A3 · Databases · Relational databases, SQL, normalisation · 2 subtopics
A4 · Machine learning · Types of ML, training, ethics · 2 subtopics
B1 · Computational thinking · Abstraction, decomposition, algorithms · 1 subtopics
B2 · Programming · Data types, control structures, files, searching and sorting · 5 subtopics
B3 · Object-oriented programming · Classes, objects, inheritance, encapsulation · 2 subtopics
B4 · Abstract data types (HL) · Stacks, queues, linked lists, trees · 1 subtopics

Free lesson: A1 · Computer fundamentals

CPU: Fetches, decodes and executes instructions
Binary: Base 2: every value is made of 0s and 1s
Logic gate: A circuit that gives an output from one or more binary inputs

CPU parts: ALU (arithmetic and logic), CU (control unit, directs operations), registers (tiny fast stores) and cache.
Key registers: PC holds the address of the next instruction; MAR holds the address being accessed; MDR holds the data being moved.
Primary memory: RAM (volatile, current programs) and ROM (non-volatile, startup instructions).

Binary place values: 128 64 32 16 8 4 2 1 Hex digits: 0–9 then A=10 B=11 C=12 D=13 E=14 F=15 1 hex digit = 4 bits

A	B	AND	OR	XOR
0	0	0	0	0
0	1	0	1	1
1	0	0	1	1
1	1	1	1	0

Worked example

Convert 1011 0110 to denary and hexadecimal.

Add the place values where there is a 1: 128 + 32 + 16 + 4 + 2.
Denary = 182.
Split into nibbles: 1011 = 11 = B, 0110 = 6.
Hex = B6. Check: 11 × 16 + 6 = 182.

Common mistakes

Mixing up the PC and the MAR.
Saying ROM is used for running programs.
Forgetting that XOR gives 0 when both inputs are 1.
Reading binary from the wrong end.

Quick check

What is 13 in binary?

1101. 8 + 4 + 1 = 13, so 1101.

What is hex 2F in denary?

47. 2 × 16 + 15 = 47.

An AND gate outputs 1 when…

both inputs are 1. AND needs both inputs to be 1.

Which register holds the address of the next instruction?

Program counter. The program counter (PC) points to the next instruction to fetch.

Practise this in the app. Interactive diagrams, quizzes, exam-style papers with mark schemes and a personal study planner. Open Computer Science in FortyFive →

FAQ

What topics are in IB Computer Science?

IB Computer Science (SL / HL · first exams 2027) is organised into: A1 · Computer fundamentals; A2 · Networks; A3 · Databases; A4 · Machine learning; B1 · Computational thinking; B2 · Programming; B3 · Object-oriented programming; B4 · Abstract data types (HL).

Are these IB Computer Science notes free?

Yes. The topic pages, key terms and the first lesson are free. The full lessons, interactive diagrams and exam-style practice papers with mark schemes are in the FortyFive app, which you can start for free.

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