A-level Computing 2009/AQA/Computer Components, The Stored Program Concept and the Internet/Hardware Devices/Secondary storage devices

This section will be looking at the various forms of secondary storage devices (media). For each device you should be familiar with the following details:
 * Seek time - The average time taken from requesting data to starting to read the requested data
 * Capacity - The amount of data it is possible to store on a medium
 * Write type - Whether it is read only, write only, or readable and writable
 * Cost - How much it costs per megabyte
 * Access type - Whether it uses Random Access or Serial Access

Pay special attention to devices with an orange background, you need to be able to describe exactly how they work!

Magnetic media
Magnetic media stores data by assigning a magnetic charge to metal. This metal is then processed by a read head, which converts the charges into ones and zeros. Historically, magnetic media has been very popular for storing programs, data, and making backups. It looks set to continue in this role for some time. However, solid state technology is starting to be used more and more, storing programs and data on new devices such as mobile phones and cameras.

Hard disk
Hard disks are usually found inside computers to store programs and data. They are increasingly cheap and more and more companies are using them to back things up. Hard disks can vary in physical size with some disks getting as small as your thumb. More closely packed platters, greater density of data on each platter to allow for more tracks and cylinders and the ability to write smaller magnetics spots have all been developments in the design of hard disks to increase their storage capacity. The capacity of a commercial disk is currently up to about 4 terabytes allowing users to read and write to them. They are constructed from several key components: For the exam you must be able to explain how a hard disk works: Writing data is very similar:
 * Platter - Metallic disks where One or both sides of the platter are magnetized, allowing data to be stored. The platter spins thousands of times a second around the spindle and spins continuously when in operation.  There may be several platters, with data stored across them. The disk is divided into tracks and sectors with data represented by magnetising spots on the disk.
 * Head - The head reads magnetic data from the platter. For a drive with several platters there may be two heads per platter allowing data to be read from top and bottom of each
 * Actuator Arm - used to move the read heads in and out of the disk, so that data can be read and written to particular locations and you can access data in a Random fashion, you don't need to read your way through the entire disk to fetch a particular bit of information, you can jump right there. Seek time is very low.
 * Power connector - provides electricity to spin the platters, move the read head and run the electronics
 * IDE connector - allows for data transfer from and to the platters
 * Jumper block - used to get the disk working in specific ways such as RAID
 * 1) The platters spin around the spindle
 * 2) data is requested to be read from a particular area of a platter
 * 3) the actuator arm moves the read head to that track
 * 4) Once the data sector that is required has spun around and under the read head, data is read
 * 5) Read data is sent from the IDE connector to main memory
 * 1) The platters spin around the spindle
 * 2) data is sent to the hard disk using the IDE connector
 * 3) the actuator arm moves the write head to the track that will be written to
 * 4) Once the data sector that is required has spun around and under the write head, data is written to the platter

Pros

Cons

Magnetic Tape drive
Increasingly obsolete, the tape has been a medium to deliver software and back up data since the early days of computing. Nowadays they are used mostly for corporate backing up and archiving of data. Tapes are sequential data stores, meaning that if you had information stored at the end of the tape you would have to wind your way through the entirety of the tape before you could read it. There is no random access like with a hard disk! Tapes can be several terabytes in size and reading and writing can be very fast as long as you read or write continuous sections of the tape at once.

Pros

Cons

Optical media
Optical media works by creating a disc with a pitted metallic surface. There are several different types of disk out there ranging from 650 MB to 128 GB, with the pits and lands getting closer together for higher volume disks. The principle behind how each of them works is the same.

CD-ROM
A CD-ROM is a metal disc embedded into a plastic protective housing. Each disc has to be 'mastered'; this is the process of creating the CD and placing the data on it. CDs are WORM (Write Once, Read Many) media; this refers to the fact that once they have been mastered, there is no way to change the data on them.

Reading from a CD-ROM
 * 1) A single track runs in a spiral pattern from the centre of the disc to the outside, this track is made of pits and lands to represent the ones and zeroes of binary data
 * 2) A low-powered laser is shone on the metallic surface and the reflection is captured in a photodiode sensor, the lands reflect differently to the pits, meaning it can tell the difference between a 1 and a 0
 * 3) The disc spins and the laser follows the track
 * 4) The binary data (the 1s and 0s) are put together and the CD-ROM has been read

Pros

Cons

CD-R
The CD-R is made of a reflective metal disk with a layer of (usually green, opaque) dye on top.

Writing to a CD-R
 * 1) A single track runs in a spiral pattern from the centre of the disc to the outside.
 * 2) A high-powered laser is shone onto the CD-R, changing the transparency (permanently) of the dye above. The transparent and opaque parts represent binary 1s and 0s
 * 3) The disc spins and the laser follows the track, putting the binary data onto the CD-R in a spiral track
 * 4) The data has been written

Reading from a CD-R
 * 1) A single track runs in a spiral pattern from the centre of the disc to the outside, this track is made of pits and lands to represent the ones and zeroes of binary data
 * 2) A low-powered laser is shone on the surface and the reflection is captured in a photodiode sensor. The opaque dye will reflect differently to the transparent dye (which would just reflect the metal underneath it), meaning it can tell the difference between a 1 and 0
 * 3) The disc spins and the laser follows the track
 * 4) The binary data (the 1s and 0s) are put together and the CD-R has been read

Pros

Cons

CD-RW
The CD-RW is made of a reflective metal disk with a layer of a special ('phase change') metal on top.

Writing to a CD-RW
 * 1) A single track runs in a spiral pattern from the centre of the disc to the outside.
 * 2) A high-powered laser is shone onto the CD-RW. Depending on whether this is very high powered or heats at a slightly lower temperature, the top layer of metal cools differently. These will result in different amounts of reflectivity, which represent the 1s and 0s.
 * 3) The disc spins and the laser follows the track, putting the binary data onto the CD in a spiral track
 * 4) The data has been written

Reading from a CD-RW
 * 1) A single track runs in a spiral pattern from the centre of the disc to the outside, this track is made of pits and lands to represent the ones and zeroes of binary data
 * 2) A low-powered laser is shone on the surface and the reflection is captured in a photodiode sensor. The different ways the metal has cooled reflect different amounts, meaning it can tell the difference between a 1 and 0
 * 3) The disc spins and the laser follows the track
 * 4) The binary data (the 1s and 0s) are put together and the CD-RW has been read

Pros

Cons

USB (memory stick) Flash Drive
USB Flash drives are solid state, that means that there are no moving parts. This is very useful for seek times as we don't have to wait for mechanical movement, meaning seek time is very low and it allows for fast Random Access Memory. Flash drives can be set to read only mode, but they will always allow for reading and writing. The size of flash drives is not as great as a Hard Disk and they are generally much more expensive per megabyte


 * 1) put drive into USB socket
 * 2) USB driver loads, providing the computer with code on how to read and write from the USB
 * 3) The USB is read, giving information on the file and folder structure (File Allocation Table) to the Computer
 * 4) [Reading] The user chooses to open a file, the Computer sends the address wanted to the USB port
 * 5) [Reading] The USB returns the data at the location requested
 * 6) [Writing] The computer sends data to the USB port where it is placed into empty space on the drive
 * 7) [Writing] The computer then requests a new version of the file and folder structure

Pros

Cons

Memory cards
Work in much the same way as a Flash drive and can often be converted into Flash Drives. They have different connectors and are generally smaller than USB Flash drives allowing for them to be used in cameras, mobile phones and game consoles.