Key Idea: A computer fetches instructions and data from memory, executes them in the CPU, and keeps everything permanently in secondary storage. Every component exists either to feed the processor or to hold what it is not using right now.
Paper 1
- Short answers and structured questions.
- Define, state, describe — one precise line per mark.
Paper 2
- Applied, scenario-based.
- Calculations — conversions, capacities, transfer times.
Both
- Explain wants the mechanism AND its consequence.
- Show every step of a conversion — method marks survive a wrong answer.
⚙️ Inside the CPU
The processor is a small set of named parts, and the exam expects you to name them and say what each one does — not describe a computer in general.
| Part | What it does |
|---|---|
| Control unit | Decodes each instruction and directs the other parts |
| ALU | Performs the arithmetic and the logical operations |
| Registers | Tiny stores inside the CPU holding what it is using now |
| Cache | Small fast memory holding recent data, to avoid a slow main-memory trip |
| Buses | The wires carrying addresses, data and control signals between parts |
PC holds the address of the next instruction · MAR the address being accessed · MDR the data in transit · CIR the instruction being decoded · the accumulator the ALU's result.
🔁 Fetch–decode–execute, in order
Learn this as a fixed sequence. Questions ask you to place one step, or to say which register is involved at a given moment.
The cycle
- The PC holds the address of the next instruction
- That address goes to the MAR; the instruction comes back in the MDR
- The PC is increased, so it points at the following instruction
- The control unit decodes the instruction in the CIR
- The ALU executes it; the result goes to a register or to memory
- The processor checks for interrupts, then the cycle repeats
Important: The program counter is increased during the fetch, before the instruction is executed — not afterwards. That is what lets a jump instruction overwrite it cleanly.
💾 Memory and storage
| RAM | ROM | Secondary storage | |
|---|---|---|---|
| Keeps data without power? | No — volatile | Yes | Yes |
| Holds | What is running now | Start-up instructions | Everything, permanently |
| Speed | Fast | Fast | Slow |
| Cost per GB | High | — | Low |
Secondary storage, compared
- Hard disk — magnetised regions on spinning platters; cheapest per gigabyte
- SSD — flash cells holding a charge; no moving parts, far faster, limited write cycles
- Optical disc — pits and lands read by laser reflection; low capacity, largely obsolete
- All three are non-volatile: they keep their contents without power
RAM is volatile, so at power-on it holds nothing at all. ROM keeps its contents, so its instructions are there the instant the processor starts — and they are what loads the operating system from storage into RAM.
🗜️ Compression and the cloud
| Lossless | Lossy | |
|---|---|---|
| What it removes | Redundancy only | Detail the eye or ear will not miss |
| Original recoverable? | Exactly | Never |
| Used for | Text, programs, archives | Photos, video, audio |
Cloud services — both sides
- For: no up-front cost, capacity scales with demand, the provider maintains the hardware
- Against: depends on the connection, the cost never stops, your data sits on someone else's servers
- Vendor lock-in — keep data in open formats so leaving stays possible
📝 Exam-style questions
A 2.5 GB video file is transferred over a link with a usable rate of 40 Mbps. Determine how long the transfer takes. Take 1 GB as 8000 megabits.
🔒 Model answer plan
See the mark-by-mark plan — for / against / judgement, with marking guidance — in study mode.
A laptop manufacturer fits an SSD rather than a hard disk. Explain two reasons for this choice, and one reason a hard disk might still be chosen for a desktop backup machine.
🔒 Model answer plan
See the mark-by-mark plan — for / against / judgement, with marking guidance — in study mode.
A student saves a 12-megapixel photograph, then saves it again using lossy compression. Describe what changes in the file, and why the difference is hard to see.
🔒 Model answer plan
See the mark-by-mark plan — for / against / judgement, with marking guidance — in study mode.
✅ Quick check
Cover the answers. If one does not come back in a few seconds, that is the part to revise.
Which register holds the address of the NEXT instruction? The program counter (PC).
Why must the start-up instructions be in ROM, not RAM? RAM is volatile — at power-on it is empty, so there would be nothing to run.
Lossless or lossy for a spreadsheet of accounts? Lossless. A changed digit changes the meaning; nothing may be discarded.
A rate is quoted as 40 Mbps. Bits or bytes? Megabits. Multiply a file size in MB or GB by 8 before dividing.
Name one thing an SSD does better than a hard disk, and one thing it does worse. Better: far lower access time (no moving parts). Worse: cost per gigabyte.
Exam tips
- Name the register, not just 'memory' — PC, MAR, MDR, CIR and the accumulator each have one job.
- The program counter is incremented during the FETCH, before execution.
- Volatile means empties at power-off. That single word answers most RAM-versus-ROM questions.
- Convert to one unit before dividing, and remember Mbps is megaBITS.
- Lossy is fine where a human perceives the data; never where a machine reads it exactly.
- Compression ratio = original size ÷ compressed size. Percentage saved uses the ORIGINAL as the denominator.