Why Do We Need Secondary Storage?
RAM has fast read/write speeds, but it is volatile — it forgets everything when the power goes off. ROM is non-volatile, but it is read-only and has a tiny capacity. So where do your files, your programs and the operating system live when the computer is switched off? In secondary storage: large capacity, non-volatile storage that can be written to as well as read from. The CPU can't use it directly — data is copied into RAM first.
Save Your Work!
Type an essay, then cut the power — first without saving, then again after saving. Watch where the data goes!
Primary vs secondary storage
| Primary (RAM / ROM) | Secondary | |
|---|---|---|
| Accessed directly by the CPU? | Yes | No — loaded into RAM first |
| Read/Write Speed | Fast | Slower |
| Capacity | Smaller | Much larger |
| Cost per byte | Higher | Lower |
| Volatile | ROM no, RAM yes | No (non-volatile) |
What is secondary storage used for?
- Long-term storage of the operating system, programs and all your files — this is its main, everyday job on every computer, not a special case
- Modifiable, semi-permanent storage of your files, kept safe with the power off
Magnetic Storage
Data is stored as tiny regions that are magnetised one way or the other on the surface of a spinning disc (hard disk drive) or along a long strip of tape. A read/write head passes over the surface to read or change the magnetism. Hard disks are used for everyday storage; magnetic tape is used for huge backups and archives.
Magnetised Bits
Each region is of the disk has tiny magnetised grains. In this model, N on the left = 1 and S on the left = 0. Click a region to flip it and build the target byte, bit by bit.
Inside a Hard Disk
The platter spins while the read/write head moves to the right track. Each track is split into sectors. Each sector has the magnetised areas to store/read binary data. Pick a track and a sector — then try knocking the drive.
Advantages
- Large capacity
- Reasonably cheap per byte
- Faster access speeds than optical discs
Disadvantages
- Durability - Moving parts can be damaged
- Slower access speeds than solid state
- Less portable
- Noisy and uses more power
Where is it used?
- Hard disk drive: the main storage in many desktop PCs, servers and external backup drives
Optical Storage
Optical discs are read and written using a laser. A spiral track on the disc has tiny dents called pits between flat areas called lands. Light bounces back differently from a pit than from a land, and the drive turns that pattern of reflections into binary. CD, DVD and Blu-ray are all optical media. R discs can be written once; RW discs can be rewritten.
Laser Reader
This is a side-on slice through a disc track. Step the laser along it. In this simplified model a land reflects the laser to the sensor (1) and a pit scatters it (0). Read all 8 bits — what character is stored?
Typical capacities
Single-layer discs. Even Blu-ray holds far less than a hard disk or SSD.
Advantages
- Cheap per disc
- Thin, light and portable
- Easy to post or hand out
Disadvantages
- Low capacity
- Slow access speeds
- Easily scratched or damaged (durability is an issue)
- Needs an optical drive (many laptops no longer have one)
Where is it used?
- Distributing films, music and software to customers
- Small, cheap backups
Solid State Storage
Solid-state storage keeps data in flash memory chips — millions of tiny cells that either hold an electrical charge or don't. There are no moving parts, and the cells keep their charge with no power, so it is non-volatile. SSDs, USB flash drives and SD cards are all solid state.
Flash Memory Cells
Each cell is a tiny transistor with a “floating gate” inside it. Writing to a cell traps electrons on that gate; erasing drains them off. The more electrons are trapped there, the lower the conductivity. The conductivity indicates whether a 1 or a 0 is stored. This demo treats charged as 1 and discharged as 0, as a simplification).
Click a cell to charge it (1) or discharge it (0). Then cut the power, and knock the drive — compare this with the RAM and hard disk demos.
Advantages
- Fastest access speeds
- No moving parts, so durable and reliable
- Silent, low power use, light and small
Disadvantages
- More expensive per GB
- Smaller capacity for the price than magnetic storage
- Limited number of write cycles — each cell can only be erased and rewritten so many times before it wears out and fails
Where is it used?
- SSD: main storage in laptops and fast PCs
- USB flash drive: carrying files between computers
- SD card: cameras, phones and other small devices
Comparing Storage Types
Exam questions ask you to compare the three types of storage — magnetic, optical and solid state — using these six characteristics, and to pick a suitable type for a given scenario.
Compare the Types
Pick a characteristic. A longer bar is better for that characteristic — except cost, where a longer bar means more expensive per GB.
Typical, relative ratings — real products vary.
Choose the Storage Type
Read the scenario, decide which characteristics matter most, then choose the most suitable type of storage.
Secondary Storage Challenges
A mix of styles — sorting, true/false, which device?, quick-fire questions, ordering and matching definitions.