Mission 1 · Spec 1.2.1
Primary storage
RAM, ROM, and what happens when RAM runs out: virtual memory.
- Starter 5 min
- Learn 10 min
- Lab 15 min
- Quiz 10 min
- Exam 10 min
The power cut
The power goes off while you're writing an essay. When the computer restarts, your unsaved work is gone but the computer still knows how to start up. Why?
Reveal
Unsaved work was in RAM, which is volatile. The start-up instructions (the boot-up sequence) are in ROM, which is non-volatile.
Key ideas
Primary storage is memory the CPU can access directly. It's needed because the CPU must be able to fetch programs and data quickly while they are running.
| RAM | ROM | |
|---|---|---|
| Volatile? | Yes: contents lost when the power is off | No: contents kept |
| Read/write? | Read and write | Read only |
| Holds | The operating system, programs and data currently in use | The boot-up instructions (BIOS) |
Virtual memory
When RAM is full, part of secondary storage is used as if it were RAM. Data not currently needed is moved out of RAM to make space.
The downside
Secondary storage is much slower than RAM, so moving data back and forth slows the computer. The fix is to add more RAM.
Running out of RAM
Open all five programs, then switch between them.Exam-style questions
1. Explain why a computer needs both RAM and ROM.
[4 marks]Mark scheme
- RAM stores programs / data currently in use (1) and can be written to / changed as programs run (1)
- ROM stores the boot-up instructions (1) which must not be lost when the power is off / ROM is non-volatile (1)
2. Describe how virtual memory is used, and why it can slow a computer down.
[3 marks]Mark scheme
- When RAM is full, part of secondary storage is used as extra memory (1)
- Data not currently needed is moved from RAM to virtual memory, and back when needed (1)
- Secondary storage is slower to access than RAM, so performance drops (1)
TUTOR NOTES
- Misconception: virtual memory is "extra RAM". It's secondary storage pretending to be RAM.
- Misconception: ROM stores the operating system. It stores the boot-up instructions; the OS is on secondary storage.
- Lab prompt: why does switching to the Game take so long once RAM is full?
Mission 2 · Spec 1.2.2
Secondary storage
Why we need it, the three types (optical, magnetic, solid state), and how to choose the right one.
- Starter 5 min
- Learn 10 min
- Lab 15 min
- Quiz 10 min
- Exam 10 min
Where does it all live?
Where are your photos, games and school work stored when your devices are switched off?
Reveal
On secondary storage: non-volatile storage that keeps data permanently until it is deleted. Without it, everything would vanish when the power went off.
Types and characteristics
Magnetic (HDD)
Spinning platters with a moving read/write head. High capacity, low cost per GB. Slower, and moving parts can be damaged if dropped.
Solid state (SSD, SD card, USB stick)
Flash memory with no moving parts. Fast, durable, portable and silent, but more expensive per GB.
Optical (CD, DVD, Blu-ray)
Read by a laser. Cheap and portable, but low capacity, slow, and easily scratched.
When choosing, consider: capacity, speed, portability, durability, reliability and cost. In the exam, link each characteristic to the scenario.
Pick the storage
Match the characteristic
Exam-style questions
1. Explain why computers need secondary storage.
[2 marks]Mark scheme
- RAM is volatile, so data would be lost when the power is off (1)
- Secondary storage is non-volatile, so files and programs are kept permanently (1)
2. A photographer needs storage to take on location shoots. Recommend a type of secondary storage and justify your choice using three characteristics.
[4 marks]Mark scheme
- Solid state (1)
- Durable: no moving parts, so won't be damaged if knocked while travelling (1)
- Portable: small and light to carry (1)
- Fast: large photo files can be saved and opened quickly (1)
TUTOR NOTES
- Misconception: "SSDs are magnetic".
- Exam habit: a bare list of characteristics scores little; tie each to the scenario.
Mission 3 · Spec 1.2.3 & 1.2.4
Units, binary and hexadecimal
Why computers use binary, the units of data, and converting between denary, binary and hex.
- Starter 5 min
- Learn 15 min
- Lab 15 min
- Quiz 10 min
- Exam 10 min
Birthday in binary
Using only 16, 8, 4, 2 and 1, each at most once, which add up to the day of the month you were born?
Reveal
Born on the 22nd: 16 + 4 + 2. Put a 1 under each number used: 10110. That's 22 in binary. Computers use binary because their circuits are made of switches with two states: on and off.
Key ideas
| Unit | Size |
|---|---|
| Bit | 1 binary digit |
| Nibble | 4 bits |
| Byte | 8 bits |
| Kilobyte | 1,000 bytes |
| Megabyte | 1,000 KB |
| Gigabyte | 1,000 MB |
| Terabyte | 1,000 GB |
| Petabyte | 1,000 TB |
OCR accepts either 1,000 or 1,024 in calculations. Pick one and use it consistently.
Binary → denary
Write the place values 128, 64, 32, 16, 8, 4, 2, 1 above the bits and add those with a 1.
Denary → binary
Work left to right: if the place value fits, write 1 and subtract; otherwise write 0.
Binary ↔ hex
Split into two nibbles; each nibble is one hex digit (0–9, A–F).
Hex ↔ denary
First digit × 16 plus second digit. To go back, divide by 16.
Unit ladder
Bit flipper
Exam-style questions
1. Convert 1011 0110 to denary and to hexadecimal.
Mark scheme
- 182 (1)
- B6 (1)
2. Convert the denary number 75 into 8-bit binary.
[1 mark]Mark scheme
0100 1011(1)
3. Explain why data must be stored in binary in a computer.
[2 marks]Mark scheme
- Computers are made of circuits / transistors / switches (1)
- which only have two states, on and off, represented by 1 and 0 (1)
TUTOR NOTES
- Misconception: dropping leading zeros in 8-bit answers.
- Misconception: writing "11" instead of "B" in hex.
- Check: 1011 0110 = 128 + 32 + 16 + 4 + 2 = 182; nibbles 1011 = B, 0110 = 6.
Mission 4 · Spec 1.2.4
Binary addition and shifts
Adding two 8-bit numbers, spotting overflow, and multiplying or dividing with binary shifts.
- Starter 5 min
- Learn 10 min
- Lab 20 min
- Quiz 10 min
- Exam 10 min
Carry on
In denary you carry when a column reaches 10. When do you carry in binary?
Reveal
Whenever a column reaches 2, because 2 is written 10 in binary. And 1 + 1 + 1 = 3 = 11: write 1, carry 1.
Key ideas
Overflow
When the result of a calculation is too large to fit in the number of bits available, e.g. a 9th bit is needed in an 8-bit register. The extra bit is lost, so the answer is wrong.
Binary shifts
Left shift by n: multiply by 2n. Right shift by n: divide by 2n. Bits shifted off the end are lost, which can cause a loss of accuracy (or overflow on a left shift).
Column adder
Shift machine
Exam-style questions
1. Add 0110 1101 and 0101 0110. Give your answer in 8-bit binary.
Mark scheme
1100 0011(2); 1 mark for correct working with one error- Check: 109 + 86 = 195
2. 1100 1000 + 0100 0001 is calculated in an 8-bit register. Explain what happens.
Mark scheme
- The result (265) needs 9 bits (1)
- so an overflow error occurs / the 9th bit is lost and the stored answer is wrong (1)
3. Perform a 2-place right shift on 0010 1100 and state the effect.
Mark scheme
0000 1011(1)- The number is divided by 4 (44 → 11) (1)
TUTOR NOTES
- Check your working: convert both numbers and the answer to denary.
- Misconception: overflow means "the computer crashes". It means the result can't be stored correctly.
Mission 5 · Spec 1.2.4
Characters and images
Character sets (ASCII and Unicode), and how images are stored as pixels, with colour depth, resolution and metadata.
- Starter 5 min
- Learn 10 min
- Lab 20 min
- Quiz 10 min
- Exam 10 min
Zoom in
Zoom right into a photo until you can't zoom any more. What do you see, and how could you describe it with numbers?
Reveal
Squares of solid colour called pixels. Give each colour a binary code and list every pixel's code, row by row. Text works the same way: every character has a binary code from a character set.
Key ideas
Character set
A defined list of characters recognised by the computer, each with a unique binary code. More bits per character means more characters can be represented.
ASCII vs Unicode
ASCII uses 7 bits (128 characters; 8 bits in extended ASCII). Unicode uses more bits, so it can represent characters from every language, plus emoji, but files are larger.
Colour depth and resolution
Colour depth is the number of bits per pixel. Resolution is the number of pixels (width × height). Increasing either improves quality and increases file size.
Metadata
Data about the image stored in the file, e.g. width, height, colour depth, date taken, camera, location. The computer needs it to display the image correctly.
text size (bits) = bits per character × number of characters
Text x-ray
Pixel studio
Exam-style questions
1. An image is 400 × 300 pixels with a colour depth of 8 bits. Calculate its size in kilobytes. Show your working.
[3 marks]Mark scheme
- 400 × 300 × 8 = 960,000 bits (1)
- ÷ 8 = 120,000 bytes (1)
- ÷ 1,000 = 120 KB (accept 117.19 KB using 1,024) (1)
2. Explain why Unicode is needed as well as ASCII.
[2 marks]Mark scheme
- ASCII can only represent a limited number of characters (128 / 256) (1)
- Unicode uses more bits so it can represent characters from many more languages / symbols / emoji (1)
3. State two items of metadata stored with an image.
[2 marks]Mark scheme
- Any two: width, height, colour depth, resolution, date / time taken, camera settings, location, file type (1 each)
TUTOR NOTES
- Misconception: "metadata is the image data". It's data about the image.
- Misconception: confusing colour depth (bits) with number of colours (2n).
Mission 6 · Spec 1.2.4 & 1.2.5
Sound and compression
Sampling analogue sound, calculating sound file sizes, and lossy versus lossless compression.
- Starter 5 min
- Learn 10 min
- Lab 20 min
- Quiz 10 min
- Exam 10 min
Flip-book sound
A flip-book shows movement using separate drawings. How could you record a smooth sound using separate numbers?
Reveal
Measure the amplitude of the sound wave at regular intervals (samples) and store each as a binary number. More samples, stored more precisely, gives a closer copy.
Key ideas
Sample rate
The number of samples taken per second, in hertz.
Bit depth
The number of bits used to store each sample.
Duration
The length of the recording in seconds.
The effect
Increasing sample rate or bit depth makes playback closer to the original, and makes the file bigger.
Compression
Why compress?
Smaller files take up less storage, transfer faster over networks and use less bandwidth.
Lossy
Permanently removes data (e.g. sounds people can't hear, fine detail). Much smaller files, but lower quality and the original can't be recovered. Used for photos (JPEG), music (MP3), video streaming.
Lossless
Reduces size without losing any data; the original can be recreated exactly. Files are larger than lossy. Needed for text, program files and spreadsheets.
Sampling scope
Listen to the difference
File size calculator
Lossy or lossless?
Exam-style questions
1. A sound clip lasts 20 seconds, with a sample rate of 10,000 Hz and a bit depth of 16 bits. Calculate its size in kilobytes.
[3 marks]Mark scheme
- 10,000 × 20 × 16 = 3,200,000 bits (1)
- ÷ 8 = 400,000 bytes (1)
- ÷ 1,000 = 400 KB (accept 390.6 KB using 1,024) (1)
2. Explain why lossless compression must be used for a program file.
[2 marks]Mark scheme
- Lossy compression permanently removes data (1)
- so the program would be missing instructions and would not run correctly; lossless lets it be restored exactly (1)
TUTOR NOTES
- Misconception: lossy files can be "uncompressed" back to the original.
- Misconception: forgetting to convert minutes to seconds.
- Terminology: OCR uses "bit depth" for sound; the lab also calls it sample resolution.