Ch 3 · 3.3 Data Storage · O Level Computer Science 2210
Topic 3 of 4 in Hardware — O Level Computer Science 2210 — 49 lessons.
Introduction
Your computer remembers things in several places: some memory is lightning-fast but forgets everything when the power goes off; some keeps files for years. This lesson explains each kind and why we need them all. (Syllabus section 3.3.)
Key ideas — Primary storage
Primary storage is directly accessed by the CPU.
🧑🏫 Teacher's explanation
Imagine you are studying at a desk. Your desk surface is like RAM; it is where you keep the textbook, your notebook, and your pen so you can reach them instantly. However, as soon as you leave the room and turn off the lights, the desk is cleared. Your backpack, which is much larger and keeps everything safe even when you aren't there, is like Secondary Storage.
📖 Definitions
🔍 Worked example — Easy
A computer is turned on. Identify whether the following are stored in RAM or ROM: (a) The BIOS/Bootloader instructions, (b) The web browser currently being used.
🔍 Worked example — Standard
A user is running a video editing application. The application requires 8 ext GB of space to operate. The computer has 4 ext GB of RAM. Explain why the computer might slow down if the user tries to load a 6 ext GB video file.
🔍 Worked example — Exam-style
A technician needs to update the firmware of a device. The firmware is stored in ROM. Explain why the technician cannot simply use a standard 'write' command to change the ROM contents like they would with a file on a hard drive.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) State the main difference between RAM and ROM in terms of volatility. [1] (b) Explain why the CPU requires RAM to run an application, such as a web browser. [2] [3 marks]
Key ideas — Secondary storage
Secondary storage is not directly accessed by the CPU. It is non-volatile, so it is needed for more permanent storage of programs and files.
🧑🏫 Teacher's explanation
Think of your computer like a student studying for an exam. Primary storage (RAM) is like your desk surface; it is where you keep the books and papers you are currently working on so you can reach them instantly. However, your desk is small. Secondary storage is like your bookshelf; it is much larger and can hold everything you own, but you have to physically get up and walk to it to find something, which takes more time.
📖 Definitions
🔍 Worked example — Easy
Identify the most appropriate secondary storage medium for a photographer who needs to store 10TB of raw images cheaply and needs to carry them in a backpack.
🔍 Worked example — Standard
A Blu-ray disc stores more data than a DVD. Explain why, in terms of the physical properties of the laser used.
🔍 Worked example — Exam-style
Convert the binary value 101101_2 to decimal to simulate a small sector address.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) State one advantage of using an SSD over an HDD. [1] (b) Describe how data is stored on a magnetic hard disk. [3] [4 marks]
Key ideas — Virtual memory
When RAM is full, the computer can use part of secondary storage (a hard disk or SSD) as if it were extra RAM. This is virtual memory.
🧑🏫 Teacher's explanation
Imagine you are a student studying at a small desk (this is your RAM). You have all your textbooks and notebooks spread out on the desk so you can reach them instantly. However, your desk has limited space. If you need to study a new subject but your desk is already covered in books, you don't just stop studying. Instead, you take the books you aren't currently using and put them on a shelf in the corner of the room (this is your Secondary Storage). When you need those books again, you swap them with the ones currently on your desk.
📖 Definitions
🔍 Worked example — Easy
A computer has 8 ext GB of RAM. A user opens several applications that require a total of 10 ext GB of space. How much space must be allocated to virtual memory to prevent the system from crashing?
🔍 Worked example — Standard
A system is moving data in blocks. If each page is 4 ext KB and the system needs to move 64 ext KB of data from RAM to virtual memory, how many pages are being transferred?
🔍 Worked example — Exam-style
A computer is experiencing 'disk thrashing'. Explain the relationship between RAM usage, secondary storage, and CPU performance.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) Identify the term used to describe this state of constant swapping. [1] (b) Explain why this process causes the computer to slow down. [2] [3 marks]
Key ideas — Cloud storage
Cloud storage keeps data on remote physical servers, owned by a provider and accessed over the internet. Data can be accessed remotely, from anywhere, compared with storing data locally (on the device's own drive).
🧑🏫 Teacher's explanation
Imagine you have a physical notebook (local storage) where you write your homework. It is right there on your desk, and you can access it instantly without needing anything else. However, if you lose that notebook or your desk breaks, your work is gone. Now, imagine instead that you write your notes and upload them to a giant, shared library in the middle of the city (cloud storage). You can access those notes from any desk in any library in the world, as long as you have a way to travel there (the internet).
📖 Definitions
🔍 Worked example — Easy
A student uses local storage for a 5GB project. They decide to move it to cloud storage. List two advantages of this move.
🔍 Worked example — Standard
Compare the accessibility of a file stored on a local SSD versus a file stored in the cloud.
🔍 Worked example — Exam-style
A company is deciding between local servers and cloud storage. They cite 'security' as a concern. Explain why cloud storage might be considered higher risk in this context.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) State one advantage of using cloud storage for these images. [1] (b) Explain one disadvantage of using cloud storage for these images. [2] [3 marks]
Worked example
A user's computer slows down badly when many programs are open. Explain why, and suggest a solution.
🔍 Worked example — Easy
A computer has 8 GB of RAM. A user opens several applications that collectively require 10 GB of space. Explain what happens to the system performance.
🔍 Worked example — Standard
Compare the access speed of RAM and a Hard Disk Drive (HDD) in a system using virtual memory.
🔍 Worked example — Exam-style
A system is experiencing 'thrashing' because the OS is constantly moving data between RAM and the hard drive. Suggest two hardware-based solutions to resolve this.
⚠️ Common errors
Common mistakes
Practice
Answers: (1) Any two: RAM is volatile, ROM non-volatile; RAM can be written to, ROM is read only; RAM holds current data and programs, ROM holds start-up instructions. (2) RAM is empty when the computer starts, so ROM must hold the instructions to start up and load the operating system. (3) Data is stored as pits and lands on a spiral track; a laser reads the different reflections as binary. (4) Any two: faster access; no moving parts so more robust; silent; uses less power. (5) Part of secondary storage used as extra RAM when RAM is full; pages are swapped between RAM and virtual memory so more or larger programs can run. (6) Advantages: access anywhere, sharing, provider handles backups. Disadvantages: needs internet, security/privacy risks, ongoing cost.