GCSE Computer Science Theory

Secondary Storage

File-Sizing Protocols

Secondary storage sizes are measured in bytes and multiples thereof:

Number of Bytes	Name	Abb.
10³	Kilo	1kB
10⁶	Mega	1MB
10⁹	Giga	1GB
10¹²	Tera	1TB
10¹⁵	Peta	1PB
10¹⁸	Exa	1EB
10²¹	Zetta	1ZB
10²⁴	Yotta	1YB

Basics

Unlike RAM, secondary storage is a more permanent way of storing data.
Secondary storage can be either internal - hard drives - or external - USB memory stick.
Computer programs need to be stored in a way that they can be reloaded when the computer is turned back on.
The user may wish to move programs from the computer
The user may need to back up data against failre fire or thempt

Disk Types

There are three main different types of secondary storage: magnetic storage, optical storage and solid-state storage.

Magnetic Storage

Magnetic storage disks contain iron particles. These particles can be arranged in one of two directions, each represented either 1₂ or 0₂. The drive also contains a head that can reade the direction of the particles and change the direction of the particles using an electromagnetic field. The disk is spun rapidly and the head moves back and forth, reading different parts of the disk.

Advantages	Disadvantages
Very fast access to data	Data can be altered by magnetic fields
Many magnetic discs are comparatively cheap	The discs gradually lose their charge

Optical Storage

Optical storage discs contain dents and flat spots. A laser shone on the disc will not be reflected if it hits a pit; this represents a 0₂. On the other hand, the laser will reflect when it hits a flat spot; this represents a 1₂. The disc, similarly to magnetic storage, is spun rapidly and the laser is guided over different parts of the disc.

Advantages	Disadvantages
Has the potential to last a long time	Require special drives to read/write
Easily transported as they are the same size as CDs	Can suffer from compatibility issues between drives
Allow for random access without regards to the order the data was stored

Solid-State Storage

Solid-state discs contain billions of transistors arranged to make memory cells. The cells can be set to allow current to flow - representing a 1₂ - or not to flow - representing a 0₂. By detecting if current is flowing or not flowing through a particular cell, the 1 or 0 can be read.

It takes about 10-15ms for a hard disk drive to locate and read a sector; an SSD does this in 1ms
Memory cells that are grouped together make a page about 4kB) and pages are then grouped together into blocks (usually 128 pages). You write page by page, but can only erase book by book (slowing down the process slightly)
NAND flash memory can’t overwrite existing data like hard drives, but have to wait until you want to save something new, then it deletes the older data, in a process called garbage collection.
This process causes amplification - the effect of having to read and write more data than is actually needed; as a result, SSDs become slower to use, as they fill up over time.
Write Endurance is also a side-effect of the memory cells; every time you save somthing, the resisitance increases slightly, eventually they sont working.

Advantages	Disadvantages
Faster startup time because they don’t have to ‘spin up’	Considerably more expensive than other secondary storage devices
They have a somewhat longer lifetime than other disc types	Vulnerability to abrupt power loss, magnetic fields and electric charges
Some solid state discs can be waterproof