MEASURING THE MEMORY SIZE OF A COMPUTER.
The size of a computer’s memory is the no. of ‘units of storage’ it contains. The unit of storage can be a Bit, a Byte, or a Word.
A Bit is the smallest unit of storage & can be used to store a 0 or a 1.
A Byte is the amount of storage needed/ required to store 1 character.
A Character is any letter, digit or symbol, which can be obtained by pressing a key on the Keyboard.
Note. 1 Byte can be used to store 1 character.
A Word is a collection of bits. It can also be described as a group of bits or characters considered as an entity and capable of being stored in one storage location.
The no. of bits in a word is called the Word Size. The most common Word sizes are 16, 32 & 64.
On a given computer, a Word is the amount of storage normally needed to store an instruction.
1 Byte A group of 8 bits 1
1 Kilobyte (KB) 1,000 (a thousand) bytes 103 1,024
1 Megabyte (MB) 1,000,000 (a million) bytes 106 1,048,576
1 Gigabyte (GB) 1,000,000,000 (a billion) bytes 109 1,073, 741,824
1 Terabyte (Tera) 1,000,000,000,000 (a trillion) bytes 1012 1,099,511,627,776
A computer memory is made up of many storage cells called Bytes. Each cell (byte) is capable of storing 8 bits (binary digits) and has a unique numeric address.
Generally, the memory size of a computer is usually measured in Bytes. The prefix K is taken to be 1,024 bytes.
For example, when the size of a computer memory is quoted as being, say, 256 Kbytes, this implies that, there are 262,144 memory cells or the computer has (256 x 1,024) = 262,144 bytes of memory.
- How is information stored in a computer?
- (a). Differentiate between a ‘Bit’ and a ‘Byte’.
(b). How many bytes would be required to store the following statement?
COMPUTERS ARE FUN TO USE!
- Define the term ‘Character’ as used in computing.
- (a). What is a Megabyte?
(b). How many Kilobytes and how many characters make a Megabyte?
- The size of a computer memory is quoted as being 256 Kbytes.
- How many bytes can that computer hold in its memory at a particular time, if K is taken to be 1,024 bytes?
- Calculate the precise number of characters that could be stored in the computer. Explain your answer.
- What is a Byte?
- Name 2 standard 8-bit codes used internationally to represent information in computers.
CLASSIFICATION OF COMPUTER MEMORY.
Computer storage is divided into 2:
- Main memory (Primary memory).
- Secondary storage (Backing/ Auxiliary storage).
It can also be described as the Primary storage, Internal Memory, Immediate Access storage, Semi-conductor memories, Core memory, etc.
This is the storage (memory) found within the computer itself.
It is used to hold data, programs & instructions required immediately (or currently being used) by the Processor.
A computer can only obey data and program instructions that are stored in the Main memory.
It is Online (very close) to the central Processor, and therefore, any data within the Main memory is directly accessible to the Processor.
The Primary storage generally consists of the following functional areas:
- Program storage area.
Holds instructions from both System software & Application programs, which enter the central processor from an Input device.
- Working storage area.
Is used to hold the data being processed as well as the intermediate results of such processing.
- Input storage area.
It temporarily holds data that has been read from an Input device. Since Input devices operate at slower speed than the Processor, part of the Input storage area serves as a Buffer. A Buffer helps free the CPU to get on with other work while the slower I/O operations are completing.
- Output storage area.
Holds the finished results of processing unit released to the user.
General features/ characteristics of the Main memory.
- Its operation is wholly electronic, and therefore, very fast, accurate and reliable.
- Data must be transferred to the Main storage before it can be acted on by the Processor.
- It provides direct data access, i.e., data is instantly accessible from the Main memory & the Processor can act directly on the data.
- It is of low/ limited storage capacity.
The Internal memory of the computer is designed in such a way that it reaches a capacity beyond which it cannot extend.
- It is volatile.
For example, the RAM (the section of the Main memory that stores the user programs, application data, instructions and intermediate results during processing) loses its contents immediately when the power is switched off.
The Main memory is therefore, used to store temporary programs and data.
- The speed of the processor depends on the Main memory.
- It is very expensive, due to the technology involved & the elements used in making them.
Question. Identify the advantages and disadvantages of Primary storage.
Functions of the Main memory.
- It stores data awaiting processing.
- It stores instructions waiting to be obeyed.
- It holds the program instructions & the data currently being processed.
- It stores intermediate results of processing awaiting transfer to the output devices, i.e. it stores data awaiting output.
- The size of the Main memory affects the speed, power & capability of the computer.
- All inputs & outputs are transmitted through the Main memory.
Ideally, the Main memory is used to store all data requiring processing in order to achieve maximum processing speed.
Classification of Primary Memory.
The Main memory can be classified into 3 different sections:
- The fast Microprocessor Internal Registers.
- Read-only memory (ROM).
- Random Access memory (RAM).
READ-ONLY MEMORY (ROM).
This is a memory that can only be read, but cannot be written to, i.e., the user can only read the information in it.
ROM provides permanent storage of data, i.e., the contents in ROM cannot be changed at will. This is because the program instructions and the associated data stored in the ROM are developed & installed during the manufacture of the computer hardware by the computer manufacturers & therefore, they cannot be changed during normal computer operations; thus the term “Read only”.
ROM is a Non-volatile memory – its contents are retained (remain intact) when power is switched off. Therefore, it cannot be affected by switching the computer on & off.
ROM forms a small proportion of main storage – it contributes to about 30% of Internal memory.
ROMs are used in situations where the data or instructions must be stored/ held permanently. It is used to store vital data & programs, which need to be held in the Main memory all the time.
For example, they are used to store essential files especially those the computer uses while booting (starting) up.
Common uses of ROMs.
The main functions of the ROM are:
- It stores Firmware (bootstrap programs) –i.e., the essential files the computer uses while booting (starting) up.
- It stores the system data & instructions that are necessary for the normal functioning of the computer system hardware.
For example, it stores the Operating system program, which is necessary for the initial co-ordination of the hardware & the other OS programs.
- It stores Control programs, used for the operation of the computer & peripheral devices.
For example, the BIOS is stored on ROM because the user cannot disrupt the information.
- It stores Translation programs (Code converters), used for converting a user’s program into Machine language.
E.g., TURBO PASCAL, which translates Pascal programs written by users.
- It stores Special functions (facilities) peculiar to a given machine.
- It stores Character generators for Printers and Video displays.
- It stores ROM Lookup tables.
Types of ROM memories.
- Masked ROM.
- PROM (Programmable Read only memory).
- EPROM (Erasable Programmable Read only memory)
This is a ROM that can only be produced by the manufacturer.
The bit patterns corresponding to the desired contents of this memory must be supplied by the user in a standard format. .
Programmable ROM (PROM).
This is a ROM that can be programmed or “customized” directly by the user using a special PROM programmer to suit the needs of a particular task.
Customizing is the process by which a standard product is adapted for use in a particular situation.
Erasable Programmable ROM (EPROM).
This is a ROM that can be reprogrammed a no. of times.
There are 2 main types of EPROMS:-
- The UV-Erasable Programmable ROM.
- Electrically Erasable Programmable ROM (EEPROM).
It is also called Flash BIOS. This ROM can be rewritten through the use of a special software program, that uses electrical pulses.
Note. This is the way Flash BIOS operate, allowing users to upgrade their BIOS.
RANDOM ACCESS MEMORY (RAM).
It is a type of main memory, which is used by the computer to store data & programs temporarily during the times when they are needed in the Main memory.
The term “Random Access” means that, data in any area of the RAM can be reached or accessed in the same amount of time.
RAM provides “Read and write” facilities, i.e., it allow instructions to be written & read out, and also to be changed at will. Therefore, the computer user/ programmer can control or manipulate the data stored in RAM.
For example, it is this memory that is accessed during installation of programs; deleting, moving & copying of files.
Read refers to the retrieving (recovering) of information from memory, while Write refers to the storing of information in memory.
RAM is a Volatile memory, i.e. the contents of RAM are usually lost (rubbed off) when the power supply or the computer is switched off.
RAM forms the major proportion of Main storage – it contributes about 70% of the Internal memory. It is the memory used in large quantities in Main memory, and every computer must specify its size.
Uses of RAM.
The functions of the RAM are:
- It stores data & instructions awaiting processing.
- It also stores the instructions which are being obeyed or whose parts have been obeyed by the computer.
- Stores the intermediate results – the results of computer working/ calculations, before they are communicated to the users through the Output units.
Note. The RAM are usually stores Application programs (computer user developed instructions for solving specific tasks), such as Word processing or Spreadsheets.
It also stores user data, to be manipulated by the computer using the user input Application programs. Therefore, the RAM of the Internal memory serves the user.
Types of RAM.
- Static RAM (SRAM).
A Static RAM is able to maintain its data as long as power is provided to the memory chips.
It does not need to be re-written periodically. In fact, the only time the data on the memory is refreshed or charged is when an actual “Write” command is executed.
SRAM is very fast & is currently being used in the Main Processor as a small amount of high-speed memory called the Cache memory.
Dynamic RAM (DRAM).
A Dynamic RAM uses capacitors to store information. The information is stored in the Capacitors as a charge. Like any charge, the electrical charges in individual memory capacitors of a DRAM will drive away (leak) causing the data to either be lost or changed within a few milliseconds.
This means that, unlike SRAM, a DRAM must undergo the Refreshing process, i.e., it must be re-written continually in order for it to maintain its data. This is done by placing the memory on a Refresh circuit that re-writes the data several hundred times per second.
Refreshing involves reading the information out of, and then writing it back into the memory, thus restoring a full charge.
DRAM is used widely for most computer memories because it is cheap & small.
Advantages of Static RAM over Dynamic RAM
- SRAM is much faster than DRAM & it able to keep pace with the Main Processor.
- It doesn’t require refresh cycles like DRAM – can retain its data forever.
Disadvantages of Static RAMs.
- They are of low data density, i.e. it stores less data bits.
- SRAM chips are physically large & much more expensive than DRAM chips.
Advantages of Dynamic RAM over Static RAM
- DRAM has a much higher packing density than SRAM, i.e., a DRAM chip is able to store more information than a SRAM chip of the same size.
- DRAM chips are small in size.
- A DRAM can store a lot of information in a very small space, and therefore cheaper (less expensive) that SRAM. This is the main reason why DRAM is the memory used mostly (or in large quantities) as the Main memory in Microcomputers.
Disadvantages of a Dynamic RAM.
- It is much slower than SRAM & is not able to keep pace with the Main Processor.
- It requires refreshing in order to retain the information in its memory cells.
The charge stored in the capacitor of a DRAM leaks & most of the charge cannot be retained for long or may be lost within a few milliseconds. To preserve the information, the charge must be refreshed every 1 or 2 milliseconds by use of a Refresh circuit, which can be incorporated within the CPU.
- Outline THREE characteristics of Main storage.
- (a). What are the functions of the Main memory of the computer?
(b). Why do we say that the Main memory slows down the computer’s processing speed?
- Distinguish between the two types of Primary memory, stating clearly where each one is used.
- (a). What is a RAM? What is it used for?
(b). State 3 important facts about RAM.
(c). What is meant by “Random access?”
(d). Briefly describe the two types of RAM, and state where each one is used.
- (a). What does ROM mean? What is it used for?
(b). Give 3 important facts/characteristics of ROM.
(c). What programs are stored in ROM?
- Define the term “Volatile” in the context of computer memory.
- Identify THREE reasons why Dynamic RAMs are the most widely used memories in microcomputer systems compared to Static RAMs.
- Define and explain the following terms:
- Identify the TWO types of Primary memories found in the Central Processing unit.
- State how RAMs and ROMs are used in the computer system.
- Carefully distinguish between ROM, PROM and EPROM.
- What type of memory is used to store the boot up program (the first program to be executed on switching on a computer)?
- State the main differences between Main memory and Backing storage.
Since the memory chips are too small, they must be combined and put on a medium that can be worked with and added to a system. To achieve this, the designers place the memory chips on a small fiberglass card to create the SIMM (Single Inline Memory Module) or DIMM (Double Inline Memory Module).
These cards are placed in a socket on the motherboard, and then fastened/ bolted in. This design eliminated problems of the past, and made upgrading memory a simple task.
SECONDARY STORAGE (also called Auxiliary or Backing memory).
Secondary memory is used by the computer to hold programs, data files & backup information that is not needed immediately (not currently in use) by the Processor.
However, contents in a secondary storage media can be quickly transferred into the computer’s Main memory for processing when required.
It is also used by the computer to supplement the computer’s main (internal) memory in case of mass storage purposes.
This storage is provided by less expensive devices such as:
- Magnetic disks (Hard disks &Floppy diskettes).
- Winchester disks.
- Magnetic tapes.
- Cassette tapes.
- Punched cards.
- Zip disks.
- Optical disks, which include CD-ROMs & WORM (Write once Read Many) disks, and
- Digital Video Disks (DVDs), which can be connected to the computer.
Most of these storage media are magnetic based, i.e., they use the principles of magnetism to store data and instructions in form of binary.
The data is stored permanently in Disk drives. The disk drives can either be fixed inside the computer, as in the case of Hard disks, or inserted anytime you want to read or write in them.
What is a Disk drive?
- A Disk drive is a computer device for reading or writing data from or into a storage media.
- A Disk drive is a hardware on which files can be stored.
- A Disk drive is a unit that houses a disk.
- Hard disk drive (HDD or drive C:).
- Floppy disk drive (FDD or drive A:).
- CD-ROM drive.
- DVD-ROM drive.
- Tape drive.
- Zip drive.
A Disk drive can be used as an Input device, Output device or Secondary storage device.
Characteristics of Secondary storage devices.
- They provide slow access of information – they process data very slowly compared to primary storage.
Modern secondary storage devices normally operate in milliseconds. It can take between 25 – 50 milliseconds to locate information in a disk drive.
- They have high data storage capacity.
Disks & Tapes can store large amounts of data and instructions; however, the amount of storage is limited by the no. of disk packs or tapes you buy.
- The devices are cheap.
- They are non-volatile. Secondary storage units store data permanently.
- Used for mass storage of data & program files not currently being operated on, but which will be transferred to the main storage when required.
Question. Identify the advantages of secondary storage devices.
THE NEED FOR SECONDARY STORAGE IN COMPUTERS.
- The amount of storage needed on a typical Microcomputer system might be greater than the storage space available in the Main memory. This requires the use of backing storage devices, which can be used to store large quantities of information.
- Whatever is in memory is lost when the computer is switched off. Thus, there is a need to store programs & data in secondary storage devices from which it can be retrieved when needed.
- Primary storage is expensive, thus the need for secondary storage devices which are cheaper.
FUNCTIONS OF SECONDARY STORAGE DEVICES.
- Used to store backup data & instructions that is not needed immediately (or not currently in use) by the CPU. This helps in creating space for another data to be stored in the memory.
- Used for transportation & distribution of data & software, i.e., for transferring files from one machine to another.
- Used to back up files (keep copies of data & programs) for safe-keeping.
Whatever is in memory is lost (or can be corrupted) when the computer or the power supply is switched off. Disks can therefore be used to store programs & data, which can be retrieved when needed.
- Used to install new software.
- (a).What is meant by ‘Secondary Storage’?
(b). Explain 3 reasons why it is necessary for a computer system to have secondary storage
(c). Outline 3 basic characteristics of Secondary storage devices.
- (a). State THREE differences between Primary storage and Secondary storage.
(b). Give TWO reasons for using secondary storage devices instead of using Primary
- (a). Give two examples of secondary storage devices.
(b). State 4 functions of secondary storage devices in computer systems.
- Explain the salient features of computer secondary storage.
- Why do we need secondary storage on a computer system?
- (a). State any four advantages of secondary storage over main memory.
(b). State one primary storage device and one secondary device.
A Magnetic disk is a round platter made of a plastic or a metal & coated with a magnetic material, which is used for storage of information.
Magnetic disks can be used:
- Store backup data that is not being used currently.
- To install new software.
- To transfer/ transport files from one machine to another.
- To back up small amounts of files for safekeeping.
Magnetic Disk storage – A storage device or system consisting of magnetically coated disks, on the surface of which information is stored in the form of magnetic spots arranged in a manner to represent binary data. The data are arranged in circular tracks around the disks and are accessible to reading and writing heads on an arm which can be moved mechanically to the desired disk and then to the desired track on that disk.
Data recorded on a magnetic disk can be read/ retrieved in 2 ways: –
- Sequentially or Serially – whereby data from a given track are read or written sequentially as the disk rotates.
Suppose there are 5 records stored on track 0, and 5 records on track 1.
In Sequential access/ retrieval, the records from track 0 are read, followed by the records from track 1, and so on until all records have been retrieved.
In Direct data access/ Retrieval, the records are accessed directly, in any order, moving the Read/write heads to the track that contains the data required.
For example, suppose you want to read record 99, followed by record 20, then followed by record 43. If the records are located on tracks 19, 3 & 8 respectively, then the read/write head will move to track 19, then back to track 3 & then to track 8.
In order to read record 99, there is no need to access records 1 to 98; the head can proceed directly to where record 99 is located.
The storage capacities of disks are commonly expressed in terms of the no. of bytes of data they can hold.
A Magnetic disk can be of 2 forms: –
- Floppy diskettes (Soft disks).
- Hard disks.
A Floppy disk is a disk that can be inserted in & removed from a disk drive.
The 3.5-inch disk is inserted via a slot in front of the System unit/ cabinet.
Floppy disk units are single-drive units able to hold a single disk. The disk unit is incorporated physically into the body of PCs. Such disk drives are called Internal disk drives.
A Floppy disk is made from a thin, flexible plastic circular material. The plastic material is coated with a magnetic substance (usually Iron Oxide), which enables data to be recorded on the disk.
The plastic disk is protected in a rigid smoothly lined plastic envelope; that safeguards the recording surface against external influences, e.g., touch and dust accumulation onto the recording surface.
Floppy diskettes can bend easily.
The diskettes are relatively cheap and conveniently handled. However, they are only suitable as storage media in Microcomputer systems because, they store relatively low volumes of data and have a short life.
The storage capacity is influenced by the no. of sides of the plastic base coated with magnetic material & the storage density, measured in bytes. The common storage capacities are 360, 720, & 1,440 bytes.
In single-sided disks, data can only be recorded on one side, whereas in double-sided disks, data can be stored on both sides.
The disks come in different densities. The Low density (single-sided) disks, which hold 720KB of data & High-density (double-sided) disks, which usually hold 1.44MB of data.
Uses of Floppy diskettes.
Floppy diskettes are used: –
- To distribute software on microcomputers.
- To collect or input data for subsequent transfer and input to another system.
- As backup media for small hard disks.
Common types of Floppy drives.
The diskettes come in different sizes.
- 3.5-inch drives, which accept the small 3.5” disks.
- 5¼-inch drives, which accept the big 5¼” disks.
- 8-inch drives.
The 3.5” & 5.25” diskettes are used in Microcomputers, while the 8” diskette is normally used in Minicomputers & Mainframe computers not for storage but as a data collection/capture medium.
The diskettes can only be read by drives that are designed to read/write onto them, i.e., a low-density 3.5-inch drive will only read and write the low-density diskettes.
However, most high-density drives are able to read and write both low-density & high-density diskettes.
Physical Appearance of a 3.5-inch Floppy disk.
- Permanent label.
- It is incorporated on the diskette when it is bought.
- It has on it an arrow indicating the direction of inserting the diskette into its drive, information about the diskette, such as the no. of tracks per inch, the version of the diskette, e.g., Single-sided Single density (1S1D) & the trade name of the diskette.
- Temporary label.
This is the label, which is attached onto the diskette by its user to specify, e.g. the name of the owner, name of programs maintained on the diskette, etc.
- Security tags.
Are used to safeguard the contents of the diskette. When the hole is covered by the shutter (a plastic band), the diskette cannot be written to or its contents altered.
- Spindle hole.
It is the hole used by the disk drive to hold/ clump the diskette over its turntable for the drive motor to spin the diskette past the Read/Write heads so that the reading or writing operations can be performed on the recording surface(s) of the diskette.
- Read/Write region (Read/write head slot)
This is where the diskette’s recording surface is exposed. It is used to give the heads of the disk drive access to the disk.
Comparison between 5¼-inch and 3½-inch diskettes.
|5.25-inch Floppy B:||3.5-inch Floppy A:|
DS – Double-Sided. HD – High-Density.
- Both types are either of low density or High density.
- The 5.25 inch diskettes are flexible, while the 3.5 inch are rigid.
- 5.25 inch diskettes have a max. of 15 sectors, while 3.5 inch diskettes have a max. of 18 sectors.
- The 3.5-inch disks store more data, and are better protected. They have now replaced the 5.25-inch diskettes, which are only used on existing 8088 PCs.
The disk surface is divided into concentric circles called Tracks. The Tracks are further sub-divided into Sectors, which are used for data storage. Data or information is recorded on the Tracks & sectors. Typically, each sector is 512 bytes.
The tracks are described as concentric, because they allow the moving around one track from a given start point and end up at the starting position.
Hard disk is made of metal & is usually rigid/ firm.
Hard disk is not removable like the floppy disk, but it is fixed inside the computer. However, it works on the same basic principles as the floppy disk.
A hard disk is made up of one or more platters (disk plates), arranged one on top of the other to form a disk pack. The platters are made from a metallic material, usually an Aluminum alloy or Glass in order to make them light.
The glass platters have enough Ceramic within it to resist cracking & also they can better resist the heat produced during operation.
Each platter is coated on both sides with a magnetic material, usually Iron Oxide, which enables data to be recorded on the platter. This is why many platters are brownish orange in colour.
The mixture (of the magnetically sensitive substance) is poured on the platter, then spinned to evenly distribute the film over the entire platter.
The disk pack plates are held on a rotational Spindle, which is used by the drive motor to rotate the plate surfaces past the Read/write heads in order for the read & write operations to be performed on the recording surfaces.
Data is written on & read from the disk using Read/write heads in the disk drive, under the influence of the computer’s command signals.
The heads are attached to a device or an access arm called the Head Actuator, which is used to move the read/write heads across the platters to the destination track.
There is usually 1 Read/write head on each side of a platter & all the heads are attached to a single actuator shaft so that the heads move in unison. Each head has springs to force it into the platter it reads.
When off, the heads float between the surfaces of the platters, which are held in a vacuum that enables it to spin/ rotate around very quickly.
When the drive is running, the platters rotate causing air pressure that lifts the heads slightly off the platter surface. The disk rotates & the heads can move in & out over the surface to record or read data on the various tracks.
- The Read/Write heads do not touch the disk plate’s recording surface. They fly over to avoid the R/W head’s ‘crash’, which may result in the wearing away of the magnetic coating over the recording surfaces that may cause loss of the recording property of the magnetism.
- The distance between the head & the platter is very small such that the drive must be assembled & repaired in a very clean room because one dust particle can throw the whole drive off.
DATA STORAGE IN A HARD DISK.
The surface of each disk is divided into a no. of concentric circles called Tracks, each track being divided into Sectors.
The storage capacity of a hard disk is much higher than that of a floppy disk, & is therefore able to store much more data than a floppy disk of the same size because of technical differences.
The storage capacity of the hard disk is determined by the no. of recording surfaces, no. of tracks per surface & the recording density.
The computer identifies the record sought for by using its track no., or cylinder no. & the sector no. for its direct retrieval.
HARD DISK ACCESS MECHANISMS.
In order for a drive to read or write to a disc, it must be spinning at a constant speed. Floppy disk drives only begin rotating whey they are required to read or write data. However, Hard disks spin continuously, often at 3,000 revolutions per second.
The Read/write heads are capable of crossing the disk surface from one track to another very fast, making it possible to locate a data file or even a particular record/item within a file on the disc very quickly.
Terms used to define Access Times in Hard disks.
The Hard disk is a Direct Access Storage Media (DAS/m). Its Access Time is obtained in same manner as that for the diskette. However, the Access Time is influenced by:
- The arrangement of the Read/Write heads.
- The rotational speed, which is faster than that of the diskette.
If the disk pack is removable from the unit, the disk drive or unit is referred to as an Exchangeable Disk Unit (EDS). If the disk pack is permanently held in a unit, the disk drive or unit is referred to as a Fixed Disk Unit (FDU).
Disk unit – is the device in which the disk pack is placed.
Features of a Fixed Disk unit.
- The unit houses a no. of non-removable disks.
- It has a motor that rotates the drive at a high contact rate.
- In a Fixed-Head drive, there isusually 1 read/write head for each track on a given surface.
For example, if there are 200 tracks per recording surface, then there will be 200 R/W heads serving each surface, such that, when accessing data, there is no head movement in reading data from one track followed by data from another track. This means that during the Read and Write operations, the R/W heads doesn’t have to move in order to locate the right track because, each track is already located, hence the seek time is zero. This implies that the access time for the disk pack of a fixed head drive is reduced.
- Fixed head drives are more expensive than moving head drives.
Moving Head Drive.
- The recording surface of each disk plate is supplied by only 1 Read/Write, regardless of the no. of tracks the surface contains. Therefore, during the read and write operations, the R/W head servicing the surface must move in order to locate the right track containing the contents requested.
Suppose the R/W head is positioned over track 20 & the data required is on track 20. Then this data can be read as the disk rotates past the head.
Suppose the data required is on track 64. Then the access arm must first move the R/W head from track 20 to track 64. Once the head is positioned over track 64, the data is then read.
- After the head is positioned over the desired track, it has to wait for the right sector. The time taken for the disk to rotate from its present position to the position on the track at which the data starts is called Rotational delay (latency) & is measured in Milliseconds.
The faster the hard drive spins, the shorter the rotational latency time.
- The time taken to read & transmit the data to the computer is called the Transmission Time.
For a moving head drive, the time taken to access data (i.e., Access Time) usually ranges between 25 – 100 Milliseconds for a hard disk system & 100 – 600 milliseconds for a floppy disk system.
Advantages of Magnetic Disks.
Magnetic disks (Floppy disks & Hard disks) are the most commonly used medium for online secondary storage in microcomputer systems because of the following reasons: –
- They are cheap (Low cost).
Although disk drives are expensive, the use of removable disk packs enables storage capacity to be increased very cheaply.
E.g., to improve the storage capability of a floppy disk system, you simply need to buy additional disks at low cost.
The cost of Hard disks has decreased making them to be widely used on microcomputers.
- Have relatively fast access times for data stored anywhere on the disk.
For hard-drives, the data Transfer rate between memory & disk is 300,000 – 2 million characters per second, while that of floppy disks is between 30,000 – 150,000 cps.
- Have high storage capacities.
Hard disks can store tens of millions to hundreds of millions of characters while floppy disks can store between 100KB – 2 MB of data.
- They are re-usable – the disk space can be re-used by simply recording new data over old data. Also, the data stored in a magnetic disk can be easily corrected or updated.
- They are Non-volatile – information is stored permanently.
Disadvantages of Magnetic Disks.
- Data stored on magnetic disk is not human-readable, i.e., to verify the accuracy of data stored on the disk, a computer run has to be made, which reads the contents on the disk.
- A disk is susceptible (prone) to dust, stroke & magnetic fields; which can distort (deform/disfigure) data on the disk causing disk-reading errors.
- Require enough skills to manage the disks effectively.
Differences between Hard disks & Floppy diskettes.
|Floppy diskette.||Hard disk.|
|Can be inserted in & removed from a disk drive. It can also be transferred between computers.Made of a flexible plastic material & can bend easily.Consists of a single platter/ disk. Cheaper.Floppy disk drives only begin rotating when they are required to read or write data.Have a spindle hole. Low storage capacities compared to hard disks, e.g. a Floppy disks store between 100KB – 2MB of characters.Slower accesstimes, e.g. the data transfer rate between memory & the disk is between 30,000 – 150,000 characters per second.||It is not removable like the floppy disk, but permanently housed in a disc unit inside the computer.Made of a metal & is usually rigid/ firm. Made of more than 1 platter arranged one on top of the other to form a disk pack.Relatively expensive than floppy disks.Hard disk drives spin continuously, i.e., they start rotating when a computer is switched on.Have a rotating spindle that holds the disk plates together in a disk pack & is used to rotate the disk pack when reading or writing onto the disk.Hard disks have a much higher storage capacity than floppy disks, e.g. can store between 10 – 80 million characters.Have faster access times for data stored in it than a floppy disk, e.g. the data transfer rate between memory & the disk is between 300,000 – 2 million cps.|
Zip drives act as either external or internal devices.
- The Zip disk is found in a hard plastic case, and like the diskette, it uses a magnetic material for double-sided recording & reading.
- Zip drives are larger & their read/write heads can operate more efficiently than those on a regular floppy disk drive.
- Zip disks are usually portable.
- Each disk can hold up to 100 MB.
These are the storage media mostly used in Mini and Mainframe computers.
A Magnetic Tape reel is made of a plastic ribbon/ band coated on one side with a magnetic material that enables data & instructions to be recorded/ stored on the tape.
Magnetic Tape – a tape with a magnetic surface on which data can be stored by selective polarization of portions of the surface.
The reels of the Tape are stored in a protective case, which safeguards the recording surface of the reel from environmental destructions, e.g., touch, dust, direct sunlight radiations, etc.
Usually, a plastic ring (the Permit ring) is affixed on the Permit Ring Groove, which is on the case, before the tape is mounted in its deck. The Permit ring is used to protect/ safeguard the contents of the tape.
If the permit ring is affixed, the tape surface can be written to & read from, hence it is possible to alter the contents of the tape. If the permit ring is not affixed onto its groove, the tape surface can be read but cannot be written to; hence the tape user cannot alter the tape contents.
The width of the tape is divided into Tracks, while the length is divided into vertical columns called Frames. Each frame is made up of 7 storage unit areas (bit positions). These frames are used to store individual characters across the tape width.
The recording surface of a tape has 7 or 9 tracks running along its length. Each recording position on a track can be magnetized to represent a ‘1’, while that which is not magnetized represents a 0. Thus for a 9-track tape, each frame contains 9 bits & is used to represent 1 character.
A tape is usually ½ (0.5) inch wide & 2,400-feet long. The characters are recorded across the tracks on the tape.
Advantages of Magnetic Tapes as secondary storage medium.
- Tapes have a high storage capacity (or high data recording density), i.e. they can store lot of information in a small space.
Usually 1-inch of the tape can store between 1,600 – 6,400 characters. This also means that, a tape can allow a complete hard disk to be backed-up without the need to changemedia during the process.
- Tapes are cheaper compared to other removable storage media.
- Have high data transfer rates.
Reels of a magnetic tape have a transfer rate of approx. 10,000 – 1 million cps, whereas incassette tapes, the transfer rate is about 1,000 cps.
- Tapes are re-usable. When information on a tape is no longer required, it can simply be ‘written over’ with new information.
- The domestic cassettes can also be used as storage media in small home computers where the speed of retrieval is not a necessity and the volume of the capacity of the cassette is enough. This is because domestic cassettes operate on the same principles as the magnetic tapes.
Disadvantages of storing records on Magnetic Tapes.
- Data stored on a tape must be read/ accessed sequentially, one record after another.
E.g., if you need to update the 100th record, all the previous 99 records must be read, pass under the Read/Write head (or at least skipped over) to reach the record the user is searching. Hence, slow data, instruction and information retrieval.
This means that, if you need to process records in a different order, let say, record 100 followed by record 5, followed by record 50, the processing would be slowed down a lot because the tape would have to move back & forth. However, if the records have to be processed in sequence, i.e. from the first to the last, it would be fast.
- Data stored on magnetic tape (& disk) is not human-readable.
E.g., if you wish to verify the accuracy of data stored on the tape, a computer run would have to be made, in order to read the contents on the tape and print it.
- Tapes have short life spans (average of about 2 yrs).
- A tape is susceptible (prone) to dust, stroke & magnetic fields; which can distort (deform) data on the tape causing tape-reading errors.
- Tapes do not fully use their recording surface.
An inch of tape may hold 1,600 – 6,400 characters & the IRG may be 0.5-inch. This means that, almost a ⅓ of the unused space on the tape is wasted.
MAGNETIC TAPE CARTRIDGE & CASSETTE TAPES.
They operate on the same principle as ½-inch reel-to-reel tape.
The domestic Cassette tapes are very similar to magnetic tapes. The only difference is that Magnetic tapes are wider & longer than the domestic cassettes.
Many cartridges are designed to overcome the bother of loading and unloading tapes.
A tape cartridge gives greater protection against dust & dirt and then makes the tape trouble-free.
Tape cartridges provide an effective way to copy the contents of disks to guard against data loss.
On cassettes tapes, characters are stored serially down the length of the tape, one at a time. This slows down the processing speed of the information stored on the tape.
Advantages of Cassette Tapes.
- They are very cheap & convenient, making them to be a widely used form of secondary storage in many home computers.
- It can store hundreds of thousands of bytes of data.
- Can be re-used.
- An ordinary cassette player can be used to record & play back the data on the tapes. Therefore, no expensive Input/Output device needs to be bought.
COMPARISON BETWEEN MAGNETIC DISKS & MAGNETIC TAPES.
- Both are coated with magnetic materials.
- Have high data storage capacities, i.e. can store hundreds of thousands of bytes of data.
- Have high data transfer rates.
- Hold data permanently, i.e. are Non-volatile.
- Cheap and convenient; hence, the reason why they are mostly used for secondary storage in PCs.
- Require drives in order to read or write data from or into a disk or tape.
- Data stored a magnetic tape & disk is not human readable, i.e., to verify the accuracy of data stored on the tape or disk, a computer run would have to be performed.
- Both are adversely affected by dust, stroke & magnetic fields, which can distort data stored in them causing data reading errors.
- Tapes or disks do not fully use their recording surfaces. The Inter-Block Gaps in tapes occupy a large space; while in disks, a space has to be left for purposes such as copying & moving of files, defragmentation of the disk, etc.
- Both tapes & disks are re-usable. When information on a tape or disk is no longer required, it can simply be “written over” with new data.
- Data in them can be read sequentially, e.g., when playing music or watching a movie on a disk.
|Magnetic Tape||Magnetic Disk|
|Consist of a strip of plastic, i.e., reels of tape.Only 1 side of the tape is coated with a magnetic material for recording data.1 track of the tape is not used for data storage, but for parity check, i.e., to ensure that data recorded & transmitted is accurate. Requires tape drives to write information to & read data from the tape.Data is stored on a tape in form of records that are organized in blocks.Have Inter-Block/Record Gaps (blank spaces) separating two successive blocks or records. The records on a tape are read sequentially, i.e. one record after another in the order they occur on the tape.Data is recorded across the tracks on the tape or serially down the length of the tape.||Consist of round platters made of plastic or metal.Both sides of the platters can be coated with a magnetic material for recording data.The whole disk surface can be used for recording data. However, in hard disks, the top most surface of the 1st plate & bottom most surface of the last plate are not used for recording data as they can easily be scratched.Require disk drives to read or write data in the disks.Data is stored on the disks in files, folders or directories.Have no blank spaces between the tracks on the recording surfaces. However, they use a recording method known as Tunnel Erasure; which is used to keep each track of data separate from the others. Data recorded on a disk can be read sequentially or directly. Data is recorded on concentric circles on the disks called tracks.|
PUNCHED CARDS & PUNCHED PAPER TAPES.
These are paper media, which were used as storage media by the early computers.
They been replaced by the magnetic media, due to the following reasons:
- They are bulky.
- Provide slow input.
- They are non-reusable.
- They can be destroyed due to dust.
- Costly to produce – the punching & verification are tedious and expensive.
Optical disks use Lasers to read or write data. When writing, a laser beam is used to align a permanent data pattern on the disk surface. When reading, the data contents are sensed by the pattern of light reflected from the beam by the data on the disk surface.
There are 2 types of Optical disks:
- CD-ROMs (Compact disc Read-Only Memory)
- WORM (Write Once Read Many) discs.
CD-ROMs (Compact Disc Read Only Memory).
What is CD-ROM (Data CD)?
A CD-ROM (also known as a data CD) is a compact disc used to store computer data.
- CDs (Compact discs) were originally developed for the music industry. They use small disks identical to the ones that hold music to hold computer information.
- They have higher storage capacities than traditional Magnetic disks.
The current CDs can hold about 650 MB of data compared to the 3.5” floppy diskette, which can only hold 1.44 MB of data.
This storage capability enables programmers and other data distributors to write more sophisticated programs for computer users, because they are no longer limited by data storage space.
The CD-ROM Technology
A CD is made by having information burnt into the Polymer material using a laser. The indentations appear as fine circular tracks in the CD.
Data is written on the CD in a continuous spiral running from the center of the CD to its outside rim. All the bits in a file are written one after the other from beginning to end, then the next file is written, and so on.
A logical format (or file system) structures the raw bits on the CD in a virtual tree of directories and files, which makes it easier for both humans and computers to use the information.
ISO 9660 is a worldwide standard specifying the logical format for files and directories on a CD-ROM.
To use the CD-ROM discs, a CD-ROM drive is needed.
The devices in a CD-ROM drive are only able to read back pre-recorded sound or data by using a laser and detecting the pattern of light reflected from its beam.
The current CD-ROM drives use Multi-session & Multi-speed technologies due to the intense requirements of most multimedia applications. The Multi-session technology allows data to be written to a CD again & again until it reaches its maximum capacity (650MB).
The Multi-speed technology increases the rotational speed of the disk, thus increasing the data transfer rate of a CD-ROM drive.
Uses of CD-ROMS (CDs).
Typical uses for CD-ROMs include:
- Archiving data.
- Backing up a hard drive or other media.
- Creating a test copy of a CD before having it factory-duplicated.
- Disseminating information to field offices.
- CDs are the primary methods of installing software.
Most software companies distribute application software in CD-ROMs.
- CDs are used in Multimedia (the integration of text, motion video, graphics, & sound). Programmers pack Multimedia in the CD-ROMs enabling consumers to enjoy the work of multimedia inventions.
- To provide reference works, catalogues, directories, encyclopedias, software front descriptions, graphical images and sound.
Types of Compact Discs:
There are 2 types of Compact Discs (CD-ROMs): –
- CD-Recordable (CD-R) discs.
- CD-ReWritable (CD-RW) discs.
A CD-R holds data permanently, i.e., once written, it cannot be erased or overwritten.
CD-R can be used to store or backup a lot of data (about 650MB), thus making it suitable for businesses that need to record/ store a lot of information.
Advantages of CD-R
- Good for permanent data storage.
- Less expensive per disc than CD-RW disc (CD-R discs are cheaper compared to CD-RW).
- Readable on virtually all CD-ROM and CD-R drives.
- Use when you do not need to erase the data.
- CD-R drives can write faster than most CD-RW drives.
Disadvantages of CD-R.
- The disc can only be written once, (i.e. once something has been recorded on a CD-R, it can’t be erased or written over again).
- The future of CD-R drives seems to be in doubt since CD-RW drives can accomplish the same thing as CD-R drives.
Note. In a CD-R drive, there are 2 speeds listed.
The faster of the two speeds is the speed at which the drive reads information from a disc, while the slower one is the speed at which the drive records information onto a CD-R.
It is possible to record data on a CD-RW more than once. With CD-RW drive you can also erase the contents of a CD and re-write new information to it.
In addition, CD-RW drives can play audio CDs, use the regular CD-ROMs & read CD-R discs. This makes CD-RW drives very desirable.
Most CD-RW drives can also record to CD-R discs, making it possible to use CD-R discs with a CD-RW drive.
Advantages of CD-RW.
- Used when you need to erase the data and re-write new information (e.g., updating files). Data written to a CD-RW is not permanent, i.e. it can be overwritten or erased.
- Used to make a practice CD or to test the contents of a CD before making a permanent one.
- More cost effective for near line data storage requirements than CD-R.
Disadvantages of CD-RW.
- CD-RW drives & media are expensive/more costly than CD-R drives.
- CD-RW are slower than CD-R & CD-ROM drives.
It takes about 1hr to format CD-RW discs & about ½ an hour to copy 250 MB of data to the disc, while CD-R discs take a few minutes to format and have the same write time.
- Data can be read and written to CD-RW discs only by CD-RW drives.
- CD-RW drives are currently facing stiff competition from the DVD-Recordable (DVD-R) because the DVD-R can store more information than CD-RW.
Note. Both CD-RW & CD-R can be read by standard CD-ROM drives.
WORM (Write Once Read Many) discs.
A WORM disc looks like a CD. Also, data is read from the disk in a similar way to that on a CD.
The WORM disc surface has 40,000 Tracks, 25 Sectors and a total storage capacity of 1GB.
Data is written into the disc by burning a permanent pattern into the surface of the disc by means of a high precision laser beam.
The WORM discs are exchangeable.
WORM discs are non-erasable & are less prone to data loss compared to Magnetic disks.
Access speed of a WORM disc is slower than that of Magnetic disks.
To use the WORM discs, a WORM disk unit/drive is needed. The drive is similar to magnetic disk unit.
An example of a WORM drive is the CD-R, which uses the same size of disks as CDs and once written using the CD-R drive, it can be read in a CD-ROM disk drive as well as in a CD-R drive.
Uses of WORM disks.
- WORM drives store large amounts of data.
- They are used to put data online for reference purposes.
Advantages of Optical discs as secondary storage media.
- Have very high storage capacities. This enables them to be used for multimedia applications.
- Have relatively high access speeds.
- Are Non-volatile, i.e. information kept in them is permanent. Therefore, they are more secure against alteration.
- Are cost effective (cheap) especially if used for large storage volumes.
- They are robust – they resist temperature, electromagnetic fields, and not affected by water or dust.
- Have very high data transfer rates.
Modern CD-ROM drives have data transfer rates of between 150 – 4,800 KB/second.
- Some Optical discs allow data to be written to them a no. of times, e.g., CD-RW.
Reasons why Optical discs (CD-ROMs) are not mostly used in microcomputer systems as secondary storage media.
- CD-ROMs require special writers to write to the disk. The CD Writers are usually expensive, thus limiting the utility/ usefulness of CD-ROMs as computer storage devices.
- Are expensive (not economical) especially if used for low storage volumes.
- Require CD-ROM drives to use the discs, which are not installed on most desktop computers.
- Most CD-ROMs are produced by the manufacturer or can only be written once. Therefore, they are not reliable especially for businesses that may need to re-programme the CDs to suit their needs.
- Are slow to prepare, i.e., it takes time to format & also to copy data into the discs, e.g. CD-RW disks take about 1hr to format & about ½ an hr to copy 250 MB of data to the disc.
- The Access speed of an Optical disk is low.
DVD (Digital Video Discs).
A DVD looks like a CD-ROM. However, a DVD can store much more information.
A single-sided DVD can hold 4.7 GB of information (a 133 minutes of full-motion video), while a double-sided DVD can hold 17GB of information (the equivalent of 8 hrs of studio quality video); enabling most movies to be stored on a single disc.
This amount of storage gives software programmers flexibility when it comes to designing programs. They are able to store all the high-quality graphic images, digital sound & tools they want in a single DVD.
Currently, DVDs are used primarily for movies. The DVD videos offer superior pictures & sound, the ability to play audio CDs in a DVD player, and pictures that are sharp & clear than VHS videotapes.
To use a DVD, your computer should have a DVD-ROM. To read the DVD-ROM, a DVD-ROM drive is required.
In order to enjoy all the functionality of a DVD, the computer must have a Motion Picture Expert Graph (MPEG) decoder card or MPEG software. This enables the user to view full screen video or video clips from a DVD video disc.
DVD drives are reasonably/ fairly priced.
DVD drives can read all other ROM formats – audio CDs, CD-ROMs, CD-R, CD-RW & DVD-ROM discs.
The current DVDs are not recordable, i.e. they don’t have the ability to record information.
HANDLING PRECAUTIONS FOR MAGNETIC MEDIA.
- Should be stored in optimum temperature ranges, e.g. 10oC – 52oC for diskettes.
- Should be protected in their cases when not in use in order to safeguard their recording surfaces against environmental influences such as dust, touch, direct sunlight, radiations, etc.
- When loading/ mounting the media into its reading/writing unit, care should be taken to avoid brushing the recording surfaces against the mechanical components of the drive.
- Should never be brought near moving or magnetic bodies. Such bodies might cause the demagnetization of the recording surfaces (i.e., remove the magnetic property from the surfaces) making recording in terms of magnetism impossible.
- Put on the power before mounting the media and off after removing the media from the drive. This is because the fluctuation in power might also cause de-magnetization.
Physical storage considerations.
Recording density – The no. of useful storage cells per unit of length or area.
- The no. of characters per inch on a magnetic tape or punched card.
- The no. of bits in a single linear track measured per unit of length of the recording medium.
Volume – A term used for any individual physical storage medium that can be written to or read from. E.g., a fixed hard disk, floppy disk, CD-ROM, a disk cartridge or tape cartridge.
Formatting – Before a diskette can be used, it must be formatted. This prepares the disk so that the drive can use it.
Initialization – Before a disk is recorded, it has to be initialized, i.e., writing zeros to every byte on every track. This eliminates all trace of any existing data.
Fragmentation – When data is written on a newly formatted disk, it is usually written to unused contagious sectors. If data is erased, then the deleted sectors may leave spaces among used sectors. Overtime, after many inserts and deletes, these free sectors may be scattered across the disk. In such a phenomenon, the disk is said to be fragmented.
- (a). What is a Disk?
(b). Explain in detail the uses of a disk in a computer system.
(c). State the two basic types of magnetic disks.
(d). Give THREE differences between the two types of magnetic disks identified in 1(c).
(e). Give 3 reasons why magnetic disks have become the most commonly used medium for
online secondary storage in microcomputer systems.
- (a). What is a Disk drive?
(b). State FOUR drives of a computer.
- (a). What is a hard disk and what is it meant for?
(b). What are the components of a hard disk.
- The disk pack of a hard disk has 6 disk plates.
(a). Calculate the number of surfaces that can be used for recording data. Explain your
(b). Apart from the number of recording surfaces, identify TWO other features that can be
used to determine the storage capacity of the hard disk.
- (a).What is a Floppy disk?
(b). How many types of floppy disks are there in terms of size? Name them and state their
(c). Draw a well-labeled diagram of a 3.5-inch floppy disk showing its parts.
(d). How does a Floppy disk differ from a Hard disk?
(e). State FOUR precautions that should be taken when handling diskettes.
- Distinguish the following:
- Magnetic tape unit and Magnetic tape.
- Optical disk drive and Optical disk.
- What is meant by:
- An even parity check?
- An odd parity check?
- Explain why the use of blocks of records can enable data to be stored more efficiently on magnetic tapes.
- Define ‘Transfer time’ and ‘Transfer rate’ with regard to tapes.
- Give 3 advantages of using magnetic tapes as secondary storage devices in microcomputer systems.
- Give 5 similarities & 5 differences between magnetic tapes and Magnetic disks.
- Explain the meaning of Serial Access and Direct Access. Give examples of backing storage devices that uses each of these methods of access.
- List 3 advantages of each of the following types of secondary storage media.
- Magnetic Tape.
- Magnetic Disk.
- Optical Disk.
- (a). Name the two types of Optical disks.
(b). Clearly differentiate between the TWO types of Compact Disks (CD-ROMs).
(c). List 3 reasons why Optical discs (CD-ROMs) are not mostly used in microcomputer
systems as storage devices.
- (a). Explain the term “Backing store”. Give examples.
(b). State 3 reasons why do most computers require backing store?
(c). Name two different types of backing storage media and compare the accessibility of data
from each of these types.
(d). Draw a diagram to show the construction of ONE backing storage device.
- A floppy disk drive is an auxiliary storage drive:
- With which type of computer would you normally associate this device?
- Why does this type of computer commonly have disk drives as well as Main memory?
- If one character is stored in an 8-bit byte, and a floppy disk is said to store 360 KB. Calculate the precise number of characters that could be stored in the disk.
Explain your answer.
- Compare the two types of diskettes used in terms of size, capacity and whether they are low or high density
- Define the following terms as used in Magnetic disks:
- Access time.
- Seek time.
- Rotational delay.
- Data transfer time.
- (a). List THREE examples of Optical storage devices.
(b). Give THREE reasons why Optical disks are better storage devices compared to floppy
- Write short notes on the following:
- Hard disks.
- Magnetic tapes.
- Optical disks.
- (a). Explain the term “Access time” and how it can be calculated.
(b). Draw a labeled diagram of Magnetic tape deck/unit.
- Explain the following terms with regard to magnetic tape systems:
- Load-Point marker.
- Inter-Block Gap.
- Header label.
- Recording density.
- What is the importance of Inter-Block Gaps in a magnetic tape?
- Explain the precautions of handling magnetic media.
- “In future computer diskettes will be obsolete as every computer will rely on Optical disks”. State whether or not you agree with this claim and give TWO clear reasons to support your answer.
- Assuming that a computer DVD has a storage space of 4GB and a normal movie (video) file takes 700MB of storage space. How many movies can be stored in a single DVD storage device? (Give your answer to the nearest whole number).
- How many optical disks of 720MB storage capacity are needed to store 20GB storage of hard disk data? (Give your answer to the nearest whole number).
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