Spec & Goals 3 min
AQA Spec 3.4 · Computer systems (whole unit)
By the end of this lesson you can:
- Revise and self-check the whole of Unit 4, spotting your weak topics.
- Answer exam questions that span all of spec 3.4, for the marks on offer.
- Make a focused revision plan that targets the topics you got wrong.
Warm-Up — command words & the paper 5 min
This is the last lesson of Unit 4. First, know the exam you are sitting.
Command words tell you what to do
Read the command word first — it sets how much to write.
| Command word | What it demands |
|---|---|
| State | A word or short phrase. No explanation. |
| Describe | Say what something is, or the steps, with detail. |
| Explain | Give reasons — the how or the why. |
| Complete | Fill a table or diagram exactly. |
Key Concept — your Unit 4 spec checklist 14 min
Tick each spec point. If you cannot do one, that is a topic to revise tonight.
| Spec point | You should be able to… |
|---|---|
| Hardware vs software | Define hardware as the physical parts and software as the programs; give examples of each. |
| Boolean logic | Recognise the AND, OR and NOT gates; complete a truth table; read a small logic circuit. |
| Software classification | Sort software into system and application; describe the operating system and utility programs. |
| Languages & translators | Compare low-level and high-level languages; describe the assembler, compiler and interpreter. |
| Systems architecture | Name the CPU components; describe von Neumann and the fetch–decode–execute cycle. |
| Registers | State the role of the PC, MAR, MDR, ACC and the CIR. |
| CPU performance | Explain how clock speed, number of cores and cache affect speed. |
| Memory & storage | Compare RAM and ROM; describe secondary storage (magnetic, optical, solid state). |
The three logic gates
Using the right symbol earns the mark.
Worked Example — a full multi-part question 12 min
Here is one question with two parts. We earn the marks aloud.
Part (a) — Complete the truth table for the circuit Q = (A AND B) OR NOT C. [4 marks]
We work each row left to right: find A AND B, find NOT C, then OR the two together.
| A | B | C | A AND B | NOT C | Q |
|---|---|---|---|---|---|
| 0 | 0 | 0 | 0 | 1 | 1 |
| 0 | 0 | 1 | 0 | 0 | 0 |
| 0 | 1 | 0 | 0 | 1 | 1 |
| 0 | 1 | 1 | 0 | 0 | 0 |
| 1 | 0 | 0 | 0 | 1 | 1 |
| 1 | 0 | 1 | 0 | 0 | 0 |
| 1 | 1 | 0 | 1 | 1 | 1 |
| 1 | 1 | 1 | 1 | 0 | 1 |
Each correct block of values in the Q column earns a mark, up to four.
Part (b) — State the difference between a compiler and an interpreter. [2 marks]
A compiler translates the whole program into machine code in one go (1). An interpreter translates and runs the code one line at a time (1).
Try It Yourself — timed mini-paper 12 min
Give yourself 12 minutes. Write full answers; check them with your teacher.
State one example of hardware and one example of software. [2 marks]
Complete the truth table for Q = A OR NOT B for all four input rows. [3 marks]
Describe the role of the program counter (PC) in the CPU. [2 marks]
Explain how increasing the number of cores can improve CPU performance. [2 marks]
Explain why a games console keeps its start-up program in ROM rather than RAM. [2 marks]
📝 Exam Practice 10 min
These questions span the whole of Unit 4. Match your answer to the command word and the marks.
Complete the missing output values for the gate Q = A AND B.
| A | B | Q |
|---|---|---|
| 0 | 0 | ? |
| 0 | 1 | ? |
| 1 | 0 | ? |
| 1 | 1 | ? |
Mark scheme
- First three rows all output
0(1). - Final row (
1, 1) outputs1(1).
The operating system is one type of software. State the two main categories all software falls into.
Mark scheme
- System software (1).
- Application software (1).
Describe the difference between a compiler and an interpreter.
Mark scheme
- A compiler translates the whole program at once (1).
- It produces an executable / machine-code file (1).
- An interpreter translates and runs the code line by line (1).
- It stops at the first error / produces no separate executable (1).
Describe the three stages of the fetch–decode–execute cycle.
Mark scheme
- Fetch: the instruction at the PC's address is copied from memory (1).
- Decode: the control unit works out what the instruction means (1).
- Execute: the CPU carries out the instruction (1).
Explain one difference between RAM and ROM.
Mark scheme
- RAM is volatile, so it loses its contents when power is removed (1).
- ROM is non-volatile, so it keeps its contents without power (1).
Recap & Key Terms 3 min
Use your mini-paper marks to plan tonight's revision.
- Mixed up hardware and software? Redo CS-L4-01.
- Tripped on a truth table or gate? Redo CS-L4-02 and re-learn AND / OR / NOT.
- Unsure on software types? Redo CS-L4-04 — system vs application.
- Lost marks on translators? Redo CS-L4-06 — compiler vs interpreter.
- Shaky on the CPU or the cycle? Redo CS-L4-07 and learn the registers.
- Confused RAM and ROM or storage? Redo CS-L4-09.
- Hardware
- The physical parts of a computer system that you can touch.
- Software
- The programs and data that tell the hardware what to do.
- Logic gate
- A circuit that gives a Boolean output from its inputs — AND, OR or NOT.
- Truth table
- A table listing the output for every combination of inputs.
- Operating system
- System software that manages hardware, files, memory and the user interface.
- Utility software
- System software that does a maintenance task, such as backup or compression.
- Compiler
- A translator that converts the whole program into machine code at once.
- Interpreter
- A translator that converts and runs the code one line at a time.
- Assembler
- A translator that converts low-level assembly language into machine code.
- Von Neumann architecture
- A design where instructions and data share the same memory.
- Fetch–decode–execute cycle
- The repeating cycle the CPU uses to process each instruction.
- Program counter (PC)
- A register holding the address of the next instruction to fetch.
- RAM
- Volatile main memory that loses its contents when power is off.
- ROM
- Non-volatile memory that keeps its contents without power.
- Secondary storage
- Non-volatile storage for files — magnetic, optical or solid state.
Homework 1 min
Task (≤ 15 min): Revisit your weakest topic from the mini-paper. Open that lesson and redo its Exam Practice section.
Bring one question you still find hard to the next session, so we can work through it together.