Secondary Storage

What are secondary storage devices?

Devices that store data even when the computer is off

Why do we need secondary storage devices?

Main memory is volatile and loses its contents when it loses power. We need somewhere to store the data even after the computer loses power.

What are the parts of a hard disk drive?

Metallic platters coated with magnetic iron particles and read-write heads

How is data stored in a hard disk drive?

One bit is represented by the polarity of one group of magnetic particles

How is data read from and written to a hard disk drive?

Read-write heads move over each platter and write data using a magnet to change the polarity or use a magnetic reader to sense the polarity and read data

What are the advantages of hard disks?

-Low price per gigabyte

-More durable compared to optical disks

-Faster than optical disks

What are the disadvantages of hard disks?

Moving parts so is very vulnerable to breaking

Where are hard disks used?

-Personal computers

-Backups

-Transporting large amounts of data

What does CD stand for?

Compact Disc

What does DVD stand for?

Digital Versatile Disc

What is the maximum capacity of a CD?

700MB

What is the maximum capacity of a single layer DVD?

4.7GB

What is the maximum capacity of a single layer Blu-ray?

25GB

What are the aspects of CD-R, DVD-R and BD-R?

Write to only once, read any amount of times

How are CD-R, DVD-R and BD-R written to?

A high power laser 'burns' the surface of the disk and changes the chemical properties therefore making some parts less reflective

What are the aspects of CD-ROM, DVD-ROM and BD-ROM?

Read only

How are CD-ROM, DVD-ROM and BD-ROM written to?

Data is permanently written to during manufacture.

Physical machinery imprints pits into the disk.

The pits are less reflective than the unpitted areas.

What are the aspects of CD-RW, DVD-RW and BD-RE?

Read and write to as many times as you like

How are CD-RW, DVD-RW and BD-RE written to?

A laser heats the surface and changes the properties therefore making it less reflective.

It can be cooled to reverse the state.

How are optical disks read from?

A low power laser moves over the surface.

Some areas reflect more light than others.

A photodiode records the reflection pattern and converts it into digital form.

What are the advantages of optical disks?

Cheap and very compact

What are the disadvantages of optical disks?

Can be damaged from scratching and sunlight

Where are optical disks used?

Distributing software such as games, movies, music etc.

What technology do solid state drives use?

NAND flash memory

What are the parts of a solid state drive?

NAND memory chips, a controller and a SATA interface

How do solid state drives store data?

Floating gate transistors trap negative charge in the form of electrons. If there are electrons, the bit is 0. If not, the bit is 1.

How are floating gate transistors arranged?

Grids of transistors are called blocks.

The rows of each block are called pages.

NAND chips are made up of many blocks.

One block is the smallest amount of data that can be read or written to at once.

How is data overwritten on a solid state drive?

Cannot be overwritten, each block must be erased before being written to again. To prevent wear, a separate block is designated and the data from the original block is mirrored onto the new block. The original block's contents are then erased.

What does a solid state drive controller do?

Reads from and writes to the blocks using a buffer and is responsible for organising the data in the blocks and pages

What are the advantages of solid state drives?

-Very fast and durable as no moving parts

-Low latency

-Low power consumption

What are the disadvantages of solid state drives?

Very expensive

Where are solid state drives used?

-Personal computers

-Laptops

-Backup

-Transporting data between computers