Educerie · IB Diploma · Computer Science
Theme A — Topic test · mark scheme
One mark per distinct point. Technical terms shown in bold are required; a correct idea in non-technical language scores at most half the available marks for that point and never the terminology mark.
1Number systems — 5 marks
(a) [2]
| Valid method shown — repeated division by 2 with remainders, or place-value subtraction | 1 |
|---|---|
10101101 | 1 |
Award the second mark alone for a correct answer with no working.
10110101(181) is the expected error, from reading remainders downwards; award the method mark only.
(b) [1]
AD (accept 0xAD, AD₁₆) | 1 ECF from (a) |
|---|
Accept the correct hexadecimal conversion of an incorrect binary answer from (a).
(c) [2]
| The result requires 9 bits, which exceeds the capacity of the 8-bit register — this is overflow | 1 |
|---|---|
| The most significant bit is lost and the value wraps around to 0 | 1 |
The word overflow earns the first mark. "It breaks" or "an error occurs" scores 0. The second mark requires the specific outcome (0), not merely "the data is lost".
2CPU architecture — 5 marks
(a) [2]
| MAR holds the address of the memory location to be accessed | 1 |
|---|---|
| MDR holds the data or instruction at that location, in transit to or from memory | 1 |
Reversing the two scores 0 for both.
(b) [3 max]
| The address in the PC is copied to the MAR | 1 |
|---|---|
| The instruction is fetched into the MDR and the PC is incremented | 1 |
| The control unit decodes the instruction | 1 |
| The ALU executes it, with the result placed in the accumulator | 1 |
Award to a maximum of 3. The steps must be in the correct order; an out-of-sequence account scores a maximum of 1. Register names are required — a correct description in everyday language ("the computer gets the instruction from memory") scores 0.
3Networks — 5 marks
(a) [3 max] — each point must be written as a comparison:
| TCP is connection-oriented, whereas UDP is connectionless | 1 |
|---|---|
| TCP guarantees delivery and ordering through acknowledgements and retransmission, whereas UDP does not | 1 |
| TCP has greater overhead / higher latency, whereas UDP is faster | 1 |
| Appropriate contrasting uses, e.g. file transfer or web pages vs live streaming or gaming | 1 |
Two separate descriptions, one of TCP and one of UDP, cap the question at 1. "TCP is reliable, UDP is fast" is a pair of conclusions with no mechanism: award 1 only.
(b) [2]
| Each packet carries a sequence number in its header | 1 |
|---|---|
| The receiving device uses these to reassemble the packets in the correct order, requesting retransmission of any that are missing | 1 |
An answer claiming packets "arrive in the right order" contradicts the premise of the question and scores 0.
4Databases and machine learning — 5 marks
(a) [2]
| A primary key uniquely identifies each record within its own table | 1 |
|---|---|
| A foreign key is a field that references the primary key of another table, creating the relationship between them | 1 |
(b) [1]
| Learning from labelled data, i.e. training examples paired with their correct outputs | 1 |
|---|
The word labelled (or an unambiguous equivalent such as "the correct answers are provided during training") is required. "The model is taught by a human" scores 0.
(c) [2]
| Identifies overfitting | 1 |
|---|---|
| Explains that the model has learned the noise or specific detail of the training data rather than the general pattern, so it fails to generalise to unseen data | 1 |
Naming overfitting without explanation scores 1. Diagnosing underfitting scores 0 — that would show poor accuracy on both sets, and the training figure of 99 % rules it out.
Mark distribution
| Split | Allocation |
|---|---|
| By sub-topic | A1 Computer fundamentals 10 · A2 Networks 5 · A3 Databases 2 · A4 Machine learning 3 |
| By objective | AO1 knowledge 6 · AO2 application 8 · AO3 analysis and evaluation 6 |
| By command term | Explain 6 · Distinguish 5 · Describe 3 · Convert 3 · State 2 · Define 1 |
Total: 20 marks
Educerie · original mark scheme for original questions, written against the published IB syllabus structure for Computer Science Theme A, first assessment 2027. Last reviewed 5 September 2026.