The shape of the lesson
By the end of the lesson, learners can
- Primary storage: When you open an app, it is copied from secondary storage (such as an SSD) into primary storage so the CPU can fetch its instructions.
- RAM and ROM: Learn the comparison as pairs.
- Why a computer needs both: Neither kind can do the other's job, so every computer, from a laptop to a washing machine, has some of each.
- Primary: directly accessed by the CPU.
- RAM: volatile, read and write, programs and data in use.
- ROM: non-volatile, read only, start-up instructions.
- Differences: always give both sides of each difference.
One picture every idea returns to
Primary storage is storage that is directly accessed by the CPU. RAM (random access memory) holds the programs and data currently in use; it can be read and written, and it is volatile, so its contents are lost when the power is off. ROM (read only memory) holds instructions that must never be lost, such as the start-up (bootstrap) instructions; it can only be read, and it is non-volatile. A computer needs both: ROM to start up, RAM to run programs.
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, Memory the CPU can reach.. Primary storage is the memory the CPU can use directly. Put the figure on the board: it is the one picture every idea in the lesson returns to. | Whole class, then pairs |
| 5 to 13 min | Teach and check: Primary storage | When you open an app, it is copied from secondary storage (such as an SSD) into primary storage so the CPU can fetch its instructions. | Whole class, then pairs |
| 13 to 21 min | Teach and check: RAM and ROM | Learn the comparison as pairs. Your turn: Give two differences between RAM and ROM. | Pairs |
| 21 to 30 min | Teach and check: Why a computer needs both | Neither kind can do the other's job, so every computer, from a laptop to a washing machine, has some of each. Model the worked example Where is it stored? on the board, one line at a time. | Connect-Extend-Challenge |
| 30 to 36 min | Check | The four Stage 1 questions from the student lesson, answered on mini whiteboards or by show of fingers. Re-teach the one most get wrong. | Individual, then whole class |
| 36 to 43 min | Plenary | The exam-style questions from Stage 2, one at a time: thirty seconds alone, then a random call. The feedback lines in the key give the examiner's wording. | Individual, random call |
| 43 to 45 min | Exit and homework | Set the worksheet below (6 questions, 16 marks). Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in Why a computer needs both, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Connect-Extend-Challenge
After the new idea, each student writes three lines: how it connects to what they knew, how it extends their thinking, and what still challenges them. Pairs compare, then the challenges become the plenary.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
| Trap learners fall into | Teaching move that pre-empts it |
|---|---|
| Cache and registers are also inside or close to the CPU, but in this topic the two named types of primary storage are RAM and ROM. | Return to Primary storage: ask a pair to state the correct version and show it on the lab before moving on. |
| "RAM is temporary" on its own is vague. | Say volatile: its contents are lost when the power is turned off. |
| ROM does not store the operating system in a PC. | ROM holds the start-up instructions, which load the operating system into RAM. |
Primary storage: RAM and ROM: worksheet
6 questions, 16 marks. Written in the style of the papers; no past paper question is reproduced.
State what is meant by primary storage.
Describe the role of RAM in a computer.
Give two differences between RAM and ROM.
Explain why a computer needs both RAM and ROM.
State whether RAM or ROM would store each item: (a) the instructions that start a games console; (b) a game level being played; (c) the player's current score.
A student says: "ROM is used because it is faster than RAM." Explain why this is not the reason ROM is used.
Answer key and marking notes
Storage that is directly accessed by the CPU.
- It stores the programs/instructions and data currently in use (including the operating system).
- So the CPU can access them directly and quickly.
- Its contents can be read and written and are lost when the power is turned off (volatile).
- RAM is volatile; ROM is non-volatile.
- RAM can be read and written; ROM can only be read.
- RAM stores programs and data in use; ROM stores the start-up instructions.
- (Any two differences, each with both sides, for 2 marks each.)
- ROM is non-volatile, so it keeps the start-up instructions when the power is off, ready for when the computer is turned on.
- These instructions load the operating system into RAM.
- RAM is needed to hold programs and data that are in use and must be changed (read and written) quickly.
- (a) ROM.
- (b) RAM.
- (c) RAM.
- ROM is used because it is non-volatile / its contents are kept without power.
- And it cannot be changed, so the start-up instructions are not lost or altered (speed is not the reason).
6 multiple-choice questions on this lesson
From the topic question bank. Assign them in the app for a marked score in your gradebook, or run the topic as a class game.
Which type of primary storage is volatile?
- ARAM
- BROM
- Csolid-state drive
- Dhard disk drive
Where are a computer's start-up (bootstrap) instructions stored?
- AROM
- BRAM
- Ccache
- Dvirtual memory
A student is editing an essay in a word processor. Where are the word processor program and the essay being edited held while she works?
- Aboth in RAM
- Bboth in ROM
- Cthe program in ROM and the essay in RAM
- Dthe program in RAM and the essay in ROM
Why can the bootstrap instructions not be stored in RAM?
- ARAM is volatile, so it is empty when the computer is switched on
- BRAM is read only, so the instructions could not be loaded into it
- CRAM is too small to hold the start-up instructions
- DRAM is not directly accessed by the CPU
A student writes: "ROM stores the operating system so that the computer can run it." Which correction is accurate for a PC?
- AROM holds the start-up instructions, which load the operating system from secondary storage into RAM
- BROM holds the operating system, but it is copied into the cache to run it
- CRAM holds the operating system permanently, even when the power is off
- DThe operating system runs directly from secondary storage without using RAM
In a washing machine's embedded system, where is the fixed control program kept, and where is the user's current wash setting (for example 40 °C, quick wash) kept?
- Acontrol program in ROM; current setting in RAM
- Bcontrol program in RAM; current setting in ROM
- Cboth in ROM
- Dboth in RAM
Quiz key
RAM is volatile: when the power is turned off, its contents are lost. ROM is non-volatile, keeping its contents without power. Solid-state drives and hard disk drives are also non-volatile, and they are secondary storage, not primary storage.
The bootstrap program is kept in ROM because ROM is non-volatile and read only, so it is present at power-on and cannot be accidentally changed. RAM and cache are empty when the power comes on, and virtual memory is part of secondary storage that the operating system sets up only after it is running.
Programs and data currently in use are held in RAM, which can be read from and written to, so both the word processor and the changing essay are in RAM. ROM is read only and holds only fixed start-up instructions, so it cannot hold an essay that is being edited and does not hold application programs.
RAM loses its contents when the power is off, so at start-up there would be no instructions for the CPU to run. RAM can be written to, so 'read only' describes ROM instead. RAM is large (for example 8 GiB), far bigger than the start-up instructions, and RAM is primary storage, so the CPU does access it directly.
In a PC, ROM holds only the bootstrap instructions; these load the operating system from secondary storage (HDD or SSD) into RAM, where the CPU runs it. The OS is not kept in ROM. RAM is volatile, so it cannot keep anything permanently, and the CPU cannot run a program directly from secondary storage.
The control program is fixed and must be there whenever the machine is switched on, so it is in ROM. The current setting changes each wash and only needs to last while the machine runs, so it is in RAM. Swapping them would lose the program at power-off and stop settings being changed. ROM cannot be written to for each wash, and RAM would lose the program.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.