Storage Main Memory Secondary Memory Different speeds Magnetic Media Optical Media Access Modes

A Computer System As we already know a computer system is made up of different components such as; 1. the CPU, 2. input devices, 3. output devices, 4. storage devices and others.

What is Storage? Storage devices are used to store data used by the computer. Two main types of storage devices are; 1. Primary Storage/Memory: used to store essential data or data that is currently being used by the computer 2. Secondary Storage/Memory: are used to store data permanently

Primary/Main Memory

ROM – Read Only Memory ROM is a primary memory It is a non-volatile computer chip; this means that the data saved in ROM is not lost once the computer is turned off The programs stored on a ROM cannot be changed or deleted since it is read only

Other types of ROM 1. PROM (programmable read only memory) – a blank chip which can be programmed only once using a special device called a programmer. Once it’s programmed its contents cannot be modified or erased. 2. EPROM (erasable programmable read only memory) –can be programmed multiple times. Its contents can be erased by using UV (ultraviolet) light. Exposure to the UV light will erase all contents. 3. EEPROM (electrically erasable programmable read-only memory) – similar to EPROM but its contents can be electrically erased and re-written without having to remove it from the computer. 4. Flash Memory – a modern type of EEPROM, it can be erased and re-written to electrically. It is much faster than EEPROM. It is ideal for electronic devices such as mobile phones and digital cameras.

BIOS – Basic Input Output System In a PC, the ROM is found on the motherboard The ROM stores the programs required to start up the computer, this program is called the BIOS (basic input output system) The main task of the BIOS is to load and start the operating system The BIOS identifies system devices such as the hard-disk, CD, video card, mouse, keyboard and other hardware devices. BIOS can also be used to modify some computer settings, for instance configuring hardware, setting the time, select the boot devices, set passwords and others.

RAM – Random Access Memory RAM (random access memory) is another type of primary memory. Unlike ROM, it is a volatile computer chip; which means once the computer is turned off all the data is lost RAM is a read and write memory hence data stored on the RAM can be modified

Where is RAM found? In a PC, the RAM (also called DRAM – dynamic RAM) is also plugged in the motherboard. The size of the RAM is measured in GB (gigabytes). Modern PCs or Laptops have around 3GB of RAM. RAM in a computer system is used for fast processing of data.

Example of why is RAM needed? Charlie buys a PC game from a shop; this game is stored on a DVD. Before James is able to use it, he has to install the game on the hard-disk. The first step is to insert the DVD into the DVD Drive and follow the on-screen wizard to install the game. This process will copy the data from the DVD to the hard-disk for permanent storage.

… The game cannot be run directly from the DVD for two reasons: 1. DVD is slower than the hard-disk, therefore this will affect the performance of the game 2. Charlie cannot save progress to DVD since it’s read only Once the game is copied on to the hard-disk, Charlie opens the game by double-clicking on its icon. Now, when the game is being executed, the files related to the game are copied to the RAM. This happens because the RAM is much faster than the HD, so the game will benefit from this performance!

Cache Memory/SRAM Although RAM is very fast, sometimes the CPU might require data to be processed even faster. This is when the cache memory is used; this is just another type of RAM which is also called SRAM – static RAM. This is not plugged in the motherboard but it is found inside the CPU. This is faster than DRAM but it is limited in size; CPUs have around 2MB, because it’s very expensive.

Diagram CPU Control Unit, Arithmetic Logic Unit, Cache Memory Main Memory RAM ROM Fast AccessFaster Access

Magnetic Storage Magnetic storage uses magnetic charges to store data. The presence of a magnetic spot represents a 1, whilst the absence represents a 0. 10

Floppy Disk When the floppy disk was first invented, it was consider to be large and could store only a few kilobytes of data. With improvements, the floppy disk evolved to store 1.44MB of data. At the time, the main advantage of the floppy disk was its portability. The floppy disk was a very convenient device which could store a number of files and can be transferred from one computer to another very easily

Disadvantages 1. Too small – the floppy disk can only store 1.44MB which when compared to other devices such as the CD (700MB) or the DVD (4.7GB) is very little. 2. Relatively expensive – once again compared to much larger devices, the floppy disk is more expensive. 3. Reliability – data stored on a floppy disk can be corrupted easily due to temperature, condensation, magnetism, dust and scratches. 4. Slow – data read and written to a floppy takes a long time to be accessed.

Write Protect Tab The floppy disk has a write-protect tab, situated in the upper left corner. When locked the contents of the floppy cannot be modified or deleted. The tab must be moved in order to be able to write content to the disk. Write Protect Tab

Floppy Disk Structure A floppy disk stores data on a magnetic disc which is protected by fabric, which is then enclosed in a plastic cover. It has a metal flap which protects the magnetic disk from being scratched. Plastic Cover Fabric Magnetic Disk

Formatting the Floppy Disk Before it can be used for the first time, a floppy disk has to be formatted. When formatting two things occur; 1. all data stored on the floppy is erased 2. a filing system (FAT) is created. In the case of the floppy disk its surface is divided into tracks and sectors Track Sector Track-Sector

Hard Disk The hard disk was first invented by IBM in the late 1950s, at that time is was big and very expensive and had limited storage Nowadays, the hard disk is an essential part of the computer system It stores all the applications, operating system and user’s files. A hard disk works on the same way a floppy disk works but it is more reliable

Hard Disk Structure It’s made up of a number of platters stacked on top of each in a metal case and all air is removed. Each platter has a read/write head attached to an arm which moves in and out to reach data stored on these platters. The arms are then attached to a motor which allows movement. When the hard-disk is switched on the platters will start to rotate, the head moves to the required location Once switched on the hard-disk platters will constantly rotate so that whenever the computer requires data, it can be accessed very fast

Diagram The heads will never touch the platter, since if this happens the hard disk will crash and data would be lost.

Formatting the Hard Disk When a hard disk is formatted (just like the floppy disk) tracks and sectors are created. However, since the hard disk has more than one surface, the tracks on top of each other form a track. Track Cylinder

Hard Disk Characteristics Capacity240GB The capacity of the hard disk. The larger it is, the more files it can store. Average Seek Time <9ms This is the time the hard disk takes to rotate the platters to the correct location and move the heads at the correct position. Rotational Speed7200RPMIn a minute the platters can rotate 7200 times. Data Transfer Rate130MB/sIn one second the hard-disk can read 130MB.

Magnetic Tape A magnetic tape reel system stores its data on meters of magnetic tape. The surface of the magnetic tape is split into 9 tracks; 8 of them to store the data and an additional one for parity checking/error correction. A character can be written on the tape by using magnetic spots to represent binary patterns (so 9 bits). A group of 9 bits is called a frame

Diagram

Storage Method Data on the tape is stored in data blocks. The blocks are then separated by an inter- block gap. In order to access a particular block all the previous blocks have to be accessed (serial access)

Tape Streamer A tape streamer is a smaller version of the magnetic tape reel. This can be used with a normal PC; it’s also a serial device so it’s mostly ideal for backups.

Data Access Methods 1. Serial Access 2. Direct Access

Serial Access In serial data access the items are organized one after each other, In order to access a particular item all the previous items have to be accessed. In this example in order to access Item 6 all the previous items (from 1 to 5) must be processed. Item 1Item 2Item 3Item 4Item 5Item 6Item 7Item 8Item 9...

Why Serial Access? Serial access is useful when all the items stored are required, so they must be loaded. In other cases it would be very slow since unwanted items must still be accessed This type of access is mostly used for backing up storage

Direct Access In case of direct access the required files can be accessed immediately without having to go through the other files. This can be done because the device stores the location of all the data stored Modern storage devices such as disks and electronic media are all direct access

Optical Storage Optical devices use laser beams to store data. Data is stored on these devices by using laser beams and their reflective properties.

CD – Compact Disk The CD stores 700MB of data It became very popular due to its portability and also its ability to store various media such as 1. videos, 2. text files 3. images 4. and sound

CD Structure A CD is made up of different layers; A. A polycarbonate plastic which is one of th e thickest layers where the bumps are created, B. A sheet of aluminum on top of the plastic which reflects light C. A lacquer layer to protect the aluminum D. Art work E. A laser beam used to read the data

Diagram

How is data Stored? Data is stored in pits and lands which are used to reflect ones and zeros; If there is a pit the laser beam is not reflected back, whilst if there is a land the laser beam is reflected. This change of light reflection allows data to be stored as binary.

Diagram Since there is a land the laser is reflected and received by the laser picks- up data LaserPick Up In this case since there is a pit, the laser is not reflected back so no data is picked up. LaserPick Up

How it works Although the CD is slower than the hard- disk it has a very good speed. The CD starts to spin as soon as data is required from it. So it will only spin when data is being read from it (or written to it). Data is stored spirally on a CD starting from the center moving outward

The different CD types There are different types of CDs, 1. CD-ROM: CD read only memory which has data stored on it and it cannot be modified or deleted. 2. CD-R: CD recordable which can be used to write data on it but then it cannot be deleted. If the CD is not full more data can be added. 3. CD-RW: CD rewritable allows the user to write data and erase it.

DVD - DVD - Digital Video Disc The DVD is an improvement over the CD It allows more data to be stored on it; on average 4.2GB. This is achieved by reducing the size of the lands and pits.

The different DVD types There are different types of DVDs, 1. DVD-ROM: DVD read only memory which has data stored on it and it cannot be modified or deleted. 2. DVD-R: DVD recordable which can be used to write data on it but then it cannot be deleted. If the DVD is not full more data can be added. 3. DVD-RW: DVD rewritable allows the user to write data and erase it.