Data Storage

Main store (or computer memory) is divided into Read Only Memory (ROM) and Random Access Memory (RAM).

ROM

ROM is memory that cannot be changed by a program or user. ROM retains its memory even after the computer is turned off. For example, ROM stores the instructions for the computer to start up when it is turned on again.

RAM

RAM is a fast temporary type of memory in which programs, applications and data are stored. Here are some examples of what’s stored in RAM:

• the operating system
• applications
• the graphical user interface (GUI)

If a computer loses power, all data stored in its RAM is lost.

Basic Concepts of Storage capacity and file size

Storage capacities and file sizes are measured from lowest to highest in:

• bits
• bytes
• kilobytes
• megabytes
• gigabytes
• terabytes

An operating system abbreviates these measurements, eg 1 megabyte becomes 1MB (megabyte).

Relationship between measurements

The table below outlines the relationship between bits (smallest) and terabytes (largest). Read each row of the table from left to right:

Relationship between measurement

Size	Equal to
8 bits	1 byte
1024 bytes	1 kilobyte
1024 kilobytes	1 megabyte
1024 megabytes	1 gigabyte
1024 gigabytes	1 terabyte

The size of a file and a storage device’s capacity will always be written in its simplest form. For example, an operating system would report a 1 terabyte hard disk’s size as 1TB not as 1024GB, although both are correct.

Example file sizes

The table below lists files commonly found on a computer and their typical file size (compressed):

File	File size
Photo	3MB
Song	5MB
Film	700MB

A file’s size can be influenced by a number of factors but ultimately the more information a file stores, the larger it will be.

________________________________________

The hard disk

The main internal backing store is a computer’s hard disk.

Hard disk

The top side of a hard disk

A hard disk stores:

• the operating system
• software applications or programs
• the majority of your data files

Hard disks spin at very high speeds (around 7,200 RPM – revolutions per minute) within a sealed unit inside the computer. Hard disks store large amounts of data – 200GB to 1TB is common in desktop computers. The data stored on a hard disk is retained until deleted, but it needs to be loaded into main store RAM before it can be used.

Floppy and zip disks

Floppy discs became popular in the 1970s. The most common format was 1.44 MB, capable of holding only very small amounts of data. Computers need a floppy drive to read floppy disks, and many modern computers are no longer supplied with a floppy disc drive because we now work with much larger files.

Now we are capable of storing 16 GB of data on a memory card which is, physically, six times smaller then a floppy disk. It would take roughly 11,111 floppy disks to store 16 GB of data.

In the past, floppy disks were used to:

• transfer small files of data from one machine to another
• backup important small files stored on a hard disk
• store restricted files that you didn’t want other users of your computer seeing

Zip disk

Zip disks are like large floppy disks but can store 250MB or more of data. To read them a computer needs a zip drive. Their use is similar to that of floppy disks.

Newer storage mediums such as DVD-RWs and memory sticks have replaced floppy/zip disks.

_________________________________________

External backing stores: optical discs

There are several different types of optical disc, although they all look pretty much the same.

CD (Compact Disc)

Optical discs that use the same technology as music CDs. They store up to 700MB of data. CDs can be used for multimedia applications such as encyclopaedias and can store pictures, sounds and video clips or anything else that will fit.

There are several formats on the market, such as:

• CD-ROM – read only, the data is written to them before they are sold.
• CD-R – meaning CD-Recordable, the user can write data to the CD once or fill it over time using multi-session (writing to the same disc on separate occasions).
• CD-RW – meaning CD-ReWritable, the CD can be written and re-written to. Unlike multi-session discs, existing data can be overwritten.

DVD (Digital Versatile Disc)

DVDs are the same physical size as CDs but hold much more data – a single sided disc can hold up to 4.7GB. DVDs are commonly used for storing video so you will often see them measured in minutes, eg 4.7GB = 120 minutes.

There are several formats on the market, such as:

• DVD-ROMs – read only, the data is written to them before they are sold.
• DVD-R – meaning DVD-Recordable, the user can write data to the DVD once or fill it over time using multi-session.
• DVD-RW – meaning DVD-ReWritable, the DVD can be written and re-written to. Unlike multi-session discs, existing data can be overwritten.

CD/DVD drives

To read from and write data to CDs and DVDs you will need a suitable drive. Today you can buy CD/DVD drives that are able to:

• read all CD and DVD formats
• write to CD-R, CD-RW, DVD-R and DVD-RW

Data is written to and read from the discs using a laser.

Magnetic tape

Principally used for backup of important files from the hard disk and for the long term storage and archiving of data.

Advantage

• They are small, robust, portable and provide low cost storage per GB.

Disadvantages

• They are very slow to write to and read from.
• Serial access means all the data on the tape must be read before you can access the data you need.

External hard disks

An external hard disk can store very large amounts of data, eg 1TB, and can be plugged into your computer via a USB or FireWire port to provide extra storage. They’re essentially the same as the hard disk in your computer but due to the casing are usable externally.

Advantage

• High capacity, eg 1TB or more so they can be used to backup data or move large amounts of data between machines.

Disadvantages

• They’re often quite large and therefore inconvenient to carry around.
• They have moving parts so are more likely to break, especially if dropped.

Memory sticks

A memory stick is ‘pen top’ sized USB device and can be used in a similar way to a floppy disk but it is inserted into the USB port – it is then seen by the computer as a removable drive. They typically come in sizes from 512MB to 32GB upwards depending on the price paid.

Advantages

• Memory sticks can hold large quantities of data.
• They are extremely portable, so the user can take them wherever they go.
• They’re durable, because they have no moving parts.

Disadvantage

• Portable storage devices in general are more likely to be lost, stolen or damaged.

Memory Cards

A memory card is a stamp-sized USB device and can be used in a similar way to a floppy disk but it is inserted into either a memory card reader or a USB converter – it is then seen by the computer as a removable drive. They typically come in sizes from 1 GB to 32 GB upwards, depending on the price paid. There are many available memory card formats, but since 2010 the SD card became the more favoured format.

Advantages

• Memory cards can hold large quantities of data.
• They are extremely portable so the user can take them wherever they go.
• Digital accessories such as compact cameras and mobile phones are able to read and write to memory cards allowing the user to transport large collections of photographs, songs or information with them.
• They’re durable because they have no moving parts.

Disadvantage

• Memory cards, specifically the micro SD card, are the smallest storage devices available. This means they are more likely to be lost, stolen or damaged.
• All computers do not come with memory card readers built in. Users will often be required to purchase a card reader or USB converter to view the data on a card.

__________________________________________________________________________________________

Backing up and archiving data

Data needs to be backed up for many reasons:

• a user may delete an important file
• hard disks can fail
• a virus can wipe out data
• a fire may destroy the building where the data was being stored (businesses will often store their backups off-site)

If the data wasn’t backed up then the consequences could be disastrous depending on what data was lost. If a business lost details of all the payments it had yet to receive the business could go bankrupt as they wouldn’t know what was owed to them or by who.

Businesses typically use magnetic tape to backup important data. Your average home user is unlikely to backup their data but those who do will most likely use an external hard disk.

Frequency of backups

Some data is more valuable than other data and some data is changed more frequently than other data. These are the kind of issues that must be taken into account when deciding how often to backup data.

The value of the data should determine how frequently it is backed up.

If the data doesn’t change often then it doesn’t need to be backed up as often, maybe just after each change. If the data changes frequently then it should be backed up frequently (maybe every evening).

Archiving

Some data may not be being used very often but it may still be useful or needed in the future. In this case data can be archived. Archived data is copied to a suitable storage medium (perhaps DVDs or magnetic tape) then it is stored safely and securely. The original data is then deleted from the computer system. This is done to free up storage space for new data.

 

Data Storage:
‘Writing’ data or ‘saving’ data are other ways of saying ‘storing’ data.

‘Reading’ data, ‘retrieving’ data or ‘opening’ a file are ways of saying that we are getting our data back from its storage location.

Main Memory

Main memory (sometimes known as internal memory or primary storage) is another name for RAM (and ROM).

Main memory is usually used to store data temporarily. In the case of RAM, it is volatile (this means that when power is switched off all of the data in the memory disappears).

Main memory is used to store data whilst it is being processed by the CPU. Data can be put into memory, and read back from it, very quickly.

Memory is fast to access, but only holds data temporarily…

Backing Storage

Backing storage (sometimes known as secondary storage) is the name for all other data storage devices in a computer: hard-drive, etc.

Backing storage is usually non-volatile, so it is generally used to store data for a long time.

Backing storage devices are slower to access, but can hold data permanently…

The device that actually holds the data is known as the storage medium (‘media’ is the plural).

The device that saves data onto the storage medium, or reads data from it, is known as the storage device.

Sometimes the storage medium is a fixed (permanent) part of the storage device, e.g. the magnetic coated discs built into a hard drive

Sometimes the storage medium is removable from the device, e.g. a CD-ROM can be taken out of a CD drive.

We refer to a collection of data stored in a computer system as a ‘file’. Files are often organised into ‘folders’.

Whenever you click ‘Save’ in an application, burn files to a CD-R, copy music onto your MP3 player, or drag and drop a file onto memory stick, you are using storage devices – devices that can store and retrieve data.

Serial / Sequential Access

A serial (or sequential) access storage device is one that stores files one-by-one in a sequence.

A non-computer serial access device that will be familiar to you is a VHS videotape. Because video is stored on a long piece of tape, when TV shows are recorded onto the tape, they go on one-by-one, in order…

If you want to watch a show that you recorded earlier, you have to rewind / fast-forward through all other shows until you find it.

The shows are only accessible in the same order that you recorded them. This type of one-by-one storage and access is called serial access.

Systems that store things on tape (video, music, computer data, etc.) are always serial access

 

 

Direct / Random Access

A direct (or ‘random’) access storage device is one that stores files so that they can be instantly accessed – there is no need to search through other files to get to the one you want.

An example of a direct access device would be a DVD movie. Unlike the VHS videotape movie, you can jump to any scene on a DVD.

All parts of the DVD are directly accessible. This type of file storage is called direct access.

Some storage media can only store a very limited amount of data, whilst others can store vast amounts…

Data storage capacity is measured in bytes (B).

A thousand bytes is known as a kilobyte (kB)
1,000B = 1kB

A million bytes is known as a megabyte (MB)

1,000,000B = 1MB

A thousand million bytes is called a gigabyte (GB)

1,000,000,000B = 1GB

A million million bytes is called a terabyte (TB)

1,000,000,000,000B = 1TB

Even a very basic storage devices like a floppy disc can storage over a megabyte of data – that’s over 1 million letters or numbers!

And modern hard drives can store a terabyte of data or more – that’s more words than you could type even if you started now, and typed until your old age!

Some storage devices can access data very quickly, whilst others are extremely slow…

* Note: Modern back-up tapes have very fast access speeds, but only to save/read data sequentially (they are serial access devices). Tapes are very slow if you want to read files out of order, since the tape has to be rewound and fast-forwarded.

 

Why Magnetic?

Magnetic storage media and devices store data in the form of tiny magnetised dots. These dots are created, read and erased using magnetic fields created by very tiny electromagnets.

In the case of magnetic tape the dots are arranged along the length of a long plastic strip which has been coated with a magnetisable layer (audio and video tapes use a similar technology).

In the case of magnetic discs (e.g. floppy disc or hard-drive), the dots are arranged in circles on the surface of a plastic, metal or glass disc that has a magnetisable coating.

Hard Drives

Hard-drives have a very large storage capacity (up to 1TB). They can be used to store vast amounts of data. Hard-drives are random access devices and can be used to store all types of films, including huge files such as movies. Data access speeds are very fast.

Data is stored inside a hard-drive on rotating metal or glass discs (called ‘platters’).

Fixed Hard Drive

A hard-drive built into the case of a computer is known as ‘fixed’. Almost every computer has a fixed hard-drive.

Fixed hard-drives act as the main backing storage device for almost all computers since they provide almost instant access to files (random access and high access speeds).

Portable Hard Drive

A portable hard-drive is one that is placed into a small case along with some electronics that allow the hard-drive to be accessed using a USB or similar connection.

Portable hard-drives allow very large amounts of data to be transported from computer to computer.

Many portable music players (such as the iPod classic) contain tiny hard-drives. These miniature devices are just not much bigger than a stamp, but can still store over 100MB of data!

Magnetic Tape

Magnetic tape is a large capacity, serial access medium. Because it is a serial access medium, accessing individual files on a tape is slow.

Tapes are used where large amounts of data need to be stored, but where quick access to individual files is not required. A typical use is for data back-up (lots of data, but rarely only accessed in an emergency)

Tapes are also used and in some batch-processing applications (e.g. to hold the list of data that will be processed).

Removeable Media Magnetic Discs

Floppy Disc

A removable, portable, cheap, low-capacity (1.44MB) storage medium. Floppy discs are random access devices used for transfer small amounts of data between computers, or to back-up small files, etc. Access times are slow.

Almost every PC used to have a floppy disc drive. These are obsolete now, having been replaced by higher capacity technology such as CD-ROMs, DVDs and USB memory sticks.

Zip Disc

A removable and portable storage medium, similar in appearance to a floppy disk, but with a much higher capacity (100MB, 250MB or 750MB).

Zip discs are random access devices which were used for data back-up or moving large files between computers.

Another obsolete storage device, zip discs were a popular replacement for floppy discs for a few years, but they never caught on fully before being superseded by cheaper media like CD-ROMs and CD-Rs

Jaz Disc

A removable and portable storage medium based on hard-drive technology, with a large capacity (1GB or 2GB).

Jaz discs are random access devices which were used for data back-up or moving large files between computers.

Discs were expensive to buy and not very reliable.

Like the Zip disc, this system never really caught on and was superseded by far cheaper and more reliable and cheaper technology.

Why ‘Optical’?

Optical storage devices save data as patterns of dots that can be read using light. A laser beam is the usual light source.

The data on the storage medium is read by bouncing the laser beam off the surface of the medium. If the beam hits a dot it is reflected back differently to how it would be if there were no dot. This difference can be detected, so the data can be read.

Dots can be created using the laser beam (for media that is writable such as CD-Rs). The beam is used in a high-power mode to actually mark the surface of the medium, making a dot. This process is known as ‘burning’ data onto a disc.

This is a magnified view of the dots on the surface of a CD.

The different patterns of dots correspond to the data stored on the disc.

Read-Only Optical Discs

Read-only optical discs have data written onto them when they are manufactured. This data cannot be changed.

CD-ROM

Compact Disc – Read-Only Memory (CD-ROM) discs can hold around 800MB of data. The data cannot be altered (non-volatile), so cannot be accidently deleted. CD-ROMs are random-access devices.

CD-ROMs are used to distribute all sorts of data: software (e.g. office applications or games), music, electronic books (e.g. an encyclopaedia with sound and video.)

DVD-ROM

Digital Versatile Disc – Read-Only Memory (DVD-ROM) discs can hold around 4.7GB of data (a dual-layer DVD can hold twice that). DVD-ROMs are random-access devices.

DVD-ROMs are used in the same way as CD-ROMs (see above) but, since they can hold more data, they are also used to store high-quality video.

High Capacity Optical Discs

Blu-Ray

Blu-Ray disks are a recent replacement for DVDs. A Blu-Ray disc can hold 25 – 50GB of data (a dual-layer Blu-Ray disc can hold twice that). Blu-Ray discs are random-access devices.

Blu-Ray discs are used in the same way as DVD-ROMs (see above) but, since they can hold more data, they are also used to store very high-quality, high-definition (HD) video.

The ‘Blu’ part of Blu-Ray refers to the fact that the laser used to read the disc uses blue light instead of red light. Blue light has a shorter wave-length than red light (used with CDs and DVDs).

Using a blue laser allows more data to be placed closer together on a Blu-Ray disc, than on a DVD or CD, so Blu-Ray has a much higher storage capacity than these older discs.

HD DVD

High-density DVD (HD-DVD) discs can hold around 15GB of data (a dual-layer HD-DVD can hold twice that). HD-DVDs are random-access devices.

HD-DVD discs are used in the same way as DVD-ROMs (see above) but, since they can hold more data, they are also used to store very high-quality, high-definition (HD) video.

The HD-DVD format was launched at the same time as Blu-Ray. For about a year they competed to be the ‘next DVD’. For various reasons, Blu-Ray won the fight, and the HD-DVD format has been abandoned.

Recordable Optical Discs

Recordable optical discs can have data written onto them (‘burnt’) by a computer user using a special disc drive (a disc ‘burner’).

CD-R and DVD-R

CD-Recordable (CD-R) and DVD-recordable (DVD-R) discs can have data burnt onto them, but not erased. You can keep adding data until the disc is full, but you cannot remove any data or re-use a full disc.

CD-RW and DVD-RW

CD-ReWritable (CD-RW) and DVD-ReWritable (DVD-RW) discs, unlike CD-Rs and DVD-Rs, can have data burnt onto them and also erased so that the discs can be re-used.

When CD-Rs and DVD-Rs are burnt, the laser makes permanent marks on the silver-coloured metal layer. This is why these discs cannot be erased.

When CD-RWs and DVD-RWs are burnt the laser makes marks on the metal layer, but in a way that can be undone. So these discs can be erased.

DVD-RAM

DVD-Random Access Memory (DVD-RAM) discs are a type of re-writable DVD. They often come in a floppy-disc style case (to protect the disc).

DVD-RAM discs have a similar capacity to a normal DVD, holding 4.7GB of data. DVD-RAM discs are random-access devices.

DVD-RAM discs are used in many camcorders (video recording cameras).

The discs are much higher quality than normal DVD-RWs and can reliably store data for up to 30 years. This means that they are often used for video and data back-up and archiving.

‘Solid-State’?

The term ‘solid-state’ essentially means ‘no moving parts’.

Solid-state storage devices are based on electronic circuits with no moving parts (no reels of tape, no spinning discs, no laser beams, etc.)

Solid-state storage devices store data using a special type of memory called flash memory…

Flash Memory

Flash memory is a type of Electronically-Erasable Programmable Read-Only Memory (EEPROM). Flash memory is non-volatile (like ROM) but the data stored in it can also be erased or changed (like RAM).

Flash memory can be found in many data storage devices…

You might wonder why, since flash memory is non-volatile, normal computers don’t use it instead of RAM. If they did we would have computers that you could turn off, turn back on again and no data would be lost – it would be great!

The reason is speed – saving data to flash memory is very slow compared to saving it to RAM. If a computer were to use flash memory as a replacement for RAM it would run very slowly.

However some portable computers are starting to use flash memory (in the form of solid-state ‘discs’ as a replacement for hard-drives. No moving parts mean less to go wrong and longer battery life.

USB Memory Sticks

Memory sticks (or ‘thumb-drives’) have made many other forms of portable storage almost obsolete (why burn a CD or DVD when you can more easily copy your files onto a memory stick?).

Memory sticks are non-volatile, random-access storage devices.

Each of these small devices has some flash memory connected to a USB interface. Plug it into your computer and it appears as a drive. You can then add files, erase files, etc. You can use it to move any type of file between computers.

Flash memory used to be very expensive, but in recent years it has become much cheaper and you can now buy a 16GB memory stick for just a few dollars.

Memory Cards

Many of our digital devices (cameras, mobile phones, MP3 players, etc.) require compact, non-volatile data storage. Flash memory cards provide this and come in a variety of shapes and sizes.

One of the most common formats used by digital cameras is the SD Card. The cards store the digital images taken by the camera.

Mobile phones contain a Subscriber Identity Module (SIM) card that contains the phone’s number, the phonebook numbers, text messages, etc.

Many phones also have extra memory cards to store music, video, photos, etc. (e.g Tiny Micro-SD cards).

Smart Cards

Many credit cards (e.g. ‘chip-and-pin’ cards), door entry cards, satellite TV cards, etc. have replaced the very limited storage of the magnetic strip (the dark strip on the back of older cards) with flash memory. This is more reliable and has a much larger storage capacity.

Cards with flash memory are called smart cards.

What is a Backup?

A backup simply means making one or more copies of your data.

For example, if you have a folder of photos stored on the hard-drive of your laptop, you might back them up by copying them to a CD-R.

Note: If you move the photos from the hard-drive to a CD-R, you do not have a back-up – you still only have one copy of the photos, but now they are on a CD instead of the hard-drive.

You only have a backup if you have a second copy of your data.

Why Backup Your Data?

If you delete a file by accident, your computer breaks, your laptop is stolen, or your business burns to the ground, having a backup copy means that you have not lost your precious data. You can recover your lost files and continue working.

Most businesses use computers to store very important data (customer records, financial information, designs for products, etc.) If this data is lost, the business could possibly have to close. Backing-up business data is essential.

How Are Backups Created?

Personal backups of the data on your hard-drive can be made by…

• Burning files to a CD-R
• Copying files to an external hard-drive
• Copying the files to another computer on a network

Businesses backup essential data by…

• Making copies of data very regularly
• Using large-capacity media such as magnetic tape
• Keeping old copies of backups, just in case
• Automating the system so that nobody forgets to do it!
• Keeping backup media off-site (in case of fire or theft)

 

__________________________________________________________________________________________