1.3 Storage

Secondary Storage

Although RAM can store the data and program files used when a computer system is actually running, it is volatile, meaning it loses its contents without a power source. A secondary storage device is needed in a computer system to store data and program files when the power supply is turned off.

This picture shows a 2015 model of a 500gb hard drive with the cover taken off. You can see the metal coated disks which spin at 7200 revolutions per minute.

The large magnet on the bottom right of the picture is used to move the disk head across the disk, in a similar way that a needle moves across a vinyl record.

Larger capacity hard drives use more than one platter which makes them capable of transferring data at a faster rate.

Accessing data on a secondary storage device is considerably slower than accessing data that is stored in primary storage (RAM)

Secondary storage is used to store programs and data that the computer is not currently using. It is a long term storage and the data will not be lost when the power is turned off.

Types of Secondary Storage

Magnetic

Traditionally, secondary storage used devices which could store data using magnetic signals stored on magnetic surfaces.

A magnetic cassette tape is a good example of this. A long, thin plastic tape coated with a magnetic oxide could store data by using a strong electro magnet. If a pulse of electricity was sent to the magnet, then a small part of the tape will be magnetised one way, and this corresponds to a binary 1. If the current was reversed, then a binary 0 would be stored. As this technology developed, the tape was replaced with a solid disk of glass or metal, which had a more durable coating but used exactly the same method to store data. Recently these "Hard" drives have become physically smaller and faster but can store very large amounts of data. They have also become much cheaper.

Tape drives are still used occasionally as a back up device.

Magnetic storage is often used in desktop PCs, network storage, backup systems and very large document files.

Some disadvantages of magnetic storage is that they have a limited life and will eventually fail, they can be easily damaged, they are physically large and they have complex moving parts.

Optical

Magnetic storage devices are limited in terms of the speed that they can save and retrieve data because the have to spin a disk at very high speeds and physically move a magnet across the surface. If we use a strong laser beam to melt a silver coated plastic disk, then a binary 1 can be represented by a small hole, and a binary 0 can be represented by no hole. The same laser, but as a lower power setting can then be directed at the disk and will be reflected by the reflective coating, or will not be reflected because there is a hole. The reflected beam can be detected using a light sensor. this whole process is much faster.

The main optical storage formats that we use today are CDs, DVDs and BluRay which all work in a similar way but have different properties which enable more data to be stored.

Optical storage is commonly used for storing music, video and games. CDs can store 700mb, DVD can store 4.7-9.4gb and BluRay can store 25-128gb.

Some disadvantages of optical storage is that the disks can be easily scratched and degrade over time, they have a limited capacity and there are sometimes compatibility issues between different formats.

Solid State

As the need for faster and higher density storage devices is increasing all the time, new technologies have become available. Solid State memory devices use a special form of RAM style chip to store the data. Unlike volatile RAM chips, the solid state storage device can retain its data even when the power is turned off. Because there are no moving parts at all, they can operate at extremely high speeds. At the moment, they are the most expensive storage type, but as with all computing devices, they will get physically smaller, faster, cheaper whilst their performance gets faster and capacity increases.

As well as the solid state drive shown in the picture, we also class memory cards (SD cards etc), USB pen drives and even mobile phone SIM cards as solid state devices. Solid state technology is also used in mobile phones and tablet computers.

Currently, solid state devices are more expensive than magnetic and optical devices.