Basic Computer Organization Even though the size, shape, performance, reliability, and, and cost of computers have been changing over the years, the basic logical structure (based on the stored program concept), as proposed by Von Neumann, has not changed. No matter what shape and size of computer we are talking about, all computer systems perform the following five basic operations for converting raw input data into useful information and presenting it to a user: 1. Inputting: Process of entering data and instructions into a computer system. 2. Storing: Saving data and instructions to make them readily available for initial or additional processing as and when required. 3. Processing: Performing arithmetic operations (add, subtract, multiply, divide, etc.), or logical operations (comparisons like equal to, less than, greater than, etc.) on data to convert them into useful information. 4. Outputting: Process of producing useful information or results for a user, such as printed report or visual display. 5. Controlling: Directing the manner and sequence in which the above operations are performed. The goal of this unit is to familiarize you the units of computer system that perform these operations. It provides an overview of the computer system as computer system architect view them. Internal architecture of computers differs from one model to another. However, basic organization remains the same for all computer systems. Figure 1.1 shows a block diagram of basic computer organization. In this figure, solid lines indicate flow of instructions and data, and dotted lines represent control exercised by control unit. It displays the five major building blocks (functional units) of a digital computer system. These five units correspond to the five operations performed by all computer systems. These five units correspond to five basic operations performed by all computer systems. Functions of each of these units are described below: Input Unit Data and instructions must be entered into a computer system before the computer can perform any computation on the supplied data. The input unit that links a computer with its external environment performs this task. Data and instructions enter a computer through an input unit in a form that depends upon the input devices used. For example, data can be entered through a scanner, another type of input device. However, a computer’s memory is designed to accept input in binary code and hence, all input devices must transform input signals to binary codes. Units called input interfaces accomplished this transformation. Input interfaces match the unique physical or electrical characteristics of input devices to the requirements of a computer system. In short, an input unit performs following functions: 1. I accepts (or reads) instructions and data from outside world. 2. It converts these instructions and data in computer acceptable form. 3. It supplies the converted instructions and data to computer system for further processing. Some of the examples of the input devices are mouse, keyboard, scanner, touch screen, MICR, BCR etc.
Transcript
Basic Computer Organization Even though the size, shape, performance, reliability, and, and cost of computers have been changing over the years, the basic logical structure (based on the stored program concept), as proposed by Von Neumann, has not changed. No matter what shape and size of computer we are talking about, all computer systems perform the following five basic operations for converting raw input data into useful information and presenting it to a user:
1. Inputting: Process of entering data and instructions into a computer system. 2. Storing: Saving data and instructions to make them readily available for initial or
additional processing as and when required. 3. Processing: Performing arithmetic operations (add, subtract, multiply, divide, etc.), or
logical operations (comparisons like equal to, less than, greater than, etc.) on data to convert them into useful information.
4. Outputting: Process of producing useful information or results for a user, such as printed report or visual display.
5. Controlling: Directing the manner and sequence in which the above operations are performed. The goal of this unit is to familiarize you the units of computer system that perform these operations. It provides an overview of the computer system as computer system architect view them. Internal architecture of computers differs from one model to another. However, basic organization remains the same for all computer systems. Figure 1.1 shows a block diagram of basic computer organization. In this figure, solid lines indicate flow of instructions and data, and dotted lines represent control exercised by control unit. It displays the five major building blocks (functional units) of a digital computer system. These five units correspond to the five operations performed by all computer systems. These five units correspond to five basic operations performed by all computer systems. Functions of each of these units are described below: Input Unit Data and instructions must be entered into a computer system before the computer can perform any computation on the supplied data. The input unit that links a computer with its external environment performs this task. Data and instructions enter a computer through an input unit in a form that depends upon the input devices used. For example, data can be entered through a scanner, another type of input device. However, a computer’s memory is designed to accept input in binary code and hence, all input devices must transform input signals to binary codes. Units called input interfaces accomplished this transformation. Input interfaces match the unique physical or electrical characteristics of input devices to the requirements of a computer system. In short, an input unit performs following functions: 1. I accepts (or reads) instructions and data from outside world. 2. It converts these instructions and data in computer acceptable form. 3. It supplies the converted instructions and data to computer system for further processing. Some of the examples of the input devices are mouse, keyboard, scanner, touch screen, MICR, BCR etc.
Output Unit An output unit performs the reverse operation of that of an input unit. It supplies information obtained from data processing to outside world. Hence, it links a computer with its external environment. As computers work with binary code, results produced are also in binary form. Therefore, before supplying the results to outside world, the system must convert them to human acceptable (readable) form. Units called output interfaces accomplish this task. Output interfaces match the unique physical or electrical characteristics of output devices (terminals, printers, etc) to the requirements of an environment. In short, an output unit performs following functions: 1. It accepts results produced by a computer, which are in coded form and hence, we cannot easily understand them. 2. It converts these coded results to human acceptable (readable) form. 3. It supplies the converted results to outside world. Storage Unit Data and instructions entered into a computer system through input units have to be stored inside the computer before actual processing starts. Similarly, results produced by a computer after processing have to be kept somewhere inside the computer system before being passed on to an output unit. Moreover, a computer must also preserve intermediate results for ongoing processing. Storage unit of a computer system caters to all these needs. It provides space for storing data and instructions, intermediate results, and results for output. In short, a storage unit holds (stores): 1. Data and instructions required for processing (received from input devices). 2. Intermediate results for processing.
Control Unit
Arithmetic Logic Unit
Input Unit
Secondary Storage Output
Unit
Primary Storage
Central Processing Unit (CPU)
Storage Unit
Information (Result)
Program and data
Figure 1.1. Basic organization of a computer system
3. Results for output, before they are released to an output device. Storage units of all computers are comprised of following two types of storage:
1. Primary storage. Primary storage of a computer, also known as its main memory, is used to hold pieces of program instructions and data, intermediate results for processing, and recently produced results of those job(s) on which the computer is currently working. These pieces of information are represented electronically in the main memory chip’s circuitry and while it remains in the main memory, central processing unit can access it directly at a very fast speed. However, primary storage can hold information only while computer on. As soon as the computer is system switches off or resets, the information held in primary storage is erased. Moreover, primary storage normally has limited storage capacity because it is very expensive. Primary storage of modern computer system is made of semiconductor devices.
2. Secondary storage. Secondary storage of a computer, also known as its auxiliary storage, is used to take care of the limitations of primary storage. That is, it supplements the limited storage capacity and the volatile characteristics of primary storage. This is because secondary storage is much cheaper than primary storage and it can retain information even when a computer system switches off or resets. Secondary storage holds the program instructions, data and information for those jobs on which the computer system is currently not working but needs to hold them for processing later. Magnetic disk is the most commonly used secondary storage medium. Arithmetic Logic Unit Arithmetic logic unit (ALU) of a computer system is the place where actual execution of instructions takes place during processing operation. To be more precise, calculations are performed and all comparisons (decisions) are made in the ALU. Data and instructions stored in primary storage before processing are transferred as and when needed to the ALU where actual processing takes place. Intermediate results generated in the ALU are temporarily transferred back to primary storage until needed later. Hence, any data may move from primary storage to ALU and back again to storage many times before processing is over. The type and number of arithmetic and logical operations that a computer can perform is determined by the engineering design for its ALU. However, almost all ALUs are designed to perform the four basic arithmetic operations (add, subtract, multiply, and divide) and logic operations or comparisons such as, less than, equal to, and greater than. Control Unit How does an input device know that it time for it to feed data to storage unit? How does the ALU know what should be done with the data once they are received? Moreover, how it is that only results for output are sent to an output device and not the intermediate results? All this is possible due to the control unit of the computer system. Although, it does not perform any actual processing on data, the control unit acts as a central nervous system for other components of a computer system. It manages and coordinates the entire computer system. It obtains instructions from the program stored in main memory, interprets the instructions and issues signals causing other units of the system execute them. Central Processing Unit Control Unit (CU) and Arithmetic Logic Unit (ALU) of a computer system are together known as the Central Processing Unit (CPU). The CPU is the brain of a computer system. In a human body, the brain takes all major decisions and other parts of the body function as directed by the brain. Similarly, in a computer system, all major calculations and comparisons take place inside the CPU and the CPU is responsible for activating and controlling the operations of other units of the computer system.
Memory CPU contains necessary circuitry for data processing and controlling other components of the computer. However, one thing it does not have built into it is the place to store programs and data needed during data processing. CPU contains several registers for storing data and instructions but they can store only few bytes at a time. They are just sufficient to hold only one or two instructions with corresponding data. If the instructions and data of a program being executed by a CPU were to reside in secondary storage like a disk, and fetched and loaded one by one into CPU registers as the program execution proceeded, this would lead to the CPU being idle most of the time. This is because there is a large speed mismatch between the rate at which CPU can process data and the rate at which data can be transferred from disk to CPU registers. For example, a CPU can process data at a rate of 5 nanoseconds/byte and disk reader can read data at a speed of about 5 microseconds/byte. Hence, within the time in which a disk can supply one byte of data a CPU can process 1000 bytes. This would lead to a very slow overall performance even if a computer used a very fast CPU. To overcome this problem there is a need to have a reasonably large storage space than can hold the instructions and data of the program(s) on which CPU is currently working. The time to fetch and load data from this storage space into CPU registers must also be very small as compared to that time to fetch and load data from this storage space into CPU registers must also be very small as compared to that from disk storage to reduce the speed mismatch problem with CPU speed. Every computer has such a storage space known as primary storage, main memory or simply memory. It is a temporary storage area built into the computer hardware. Instructions and data of a program reside mainly in this area when CPU is executing the program. Physically, this memory consists of some integrated circuit(IC) chips either on the motherboard or on a small circuit board attached to the motherboard of a computer system. This built-in memory allows CPU to store and retrieve data very quickly. The rate of fetching data from this memory is of the order of 50 nanoseconds/byte. Hence the rate of data fetching from main memory is about 100 times faster than that from a high-speed secondary storage like disk. Storage evaluation criteria Any storage unit of a computer system is characterized and evaluated on the following properties:
1. Storage capacity. It is the amount of data that can be stored in the storage unit. A large capacity is desired. As compared to secondary storage units, primary storage has less storage capacity.
2. Access time. It is the time required to locate and retrieved stored data from the storage unit in response to a program instruction. A fast access time is preferred. As compared to secondary storage units, primary storage units have faster access time.
3. Cost per bit of storage. It refers to the cost of a storage unit for a given storage capacity. Obviously, a lower cost is desirable. As compared to secondary storage units, primary storage units have higher cost per bit of storage.
4. Volatile. If the storage unit can retain the data stored in it even when power is turned off or interrupted, it is called non-volatile storage. On the other hand, it the data stored are lost when power is turn off or interrupted, it is called volatile storage. Obviously a non-volatile storage is desirable. In almost all computer systems, primary storage units are volatile and secondary storage units are non-volatile.
5. Random access. If the time taken to access a piece of data from a storage unit is independent of the location of the data in the storage unit, it is called random access storage or random access memory (RAM). Each location of a RAM is as easy to access as any other location and takes the same amount of time. In almost all computer systems primary storage units have random access property and secondary storage units have either pseudo-random access (access time is nearly same for all locations but not exactly same) or sequential access (access time depends on the location of the data) property.
Memory Representation All quantities, physical or otherwise, are measured in units. For example, length is measured in meters and mass in grams. Likewise, for measuring computer memory, a standard unit is required. Digital computers work on only two states: ON (1) and OFF (0). These two values are represented by two different voltages within the circuit. For example, 0 volt represents a false value (0), and +5 volt represents a true value (1). Each of these values (either 0 or 1) is called a binary digit or bit and can be considered a symbol for a piece of information. Although the smallest unit of data that a computer can deal with is a bit, computers generally do not deal with a single bit. Instead, they deal with a group of eight bits, which is referred to as a byte. A byte can have 256 different bit patterns, and thus can represent 256 different symbols. Various units used to measure computer memory are as follows:
1. Bit: It is the smallest unit of data on a machine and a single bit can hold only one of the two values: 0 or 1. Bit is represented by a lower case b.
2. Byte: A unit of eight bits is known as a byte. Hence, a byte is able to contain any binary number between 00000000 and 11111111. It is represented by an upper case B.
3. Kilobyte: In a decimal system, kilo stands for 1000, but in binary system, kilo refers to 1024. Therefore, a kilobyte is equal to 1024 bytes. It is usually represented as KB.
4. Megabyte: It comprises 1024 kilobytes, or 1, 048,576 bytes. However, since this number is hard to remember, a megabyte can be thought of as a million bytes. Megabyte is the standard unit of measurement of RAM and is represented as MB.
5. Gigabyte: It consists of 1024 megabytes (1,073,741,824 bytes). It is the standard unit of measurement of hard disks and is often represented as GB.
6. Terabyte. It refers to 1024 gigabytes. Often represented as TB, terabyte memory is usually associated with super computers only.
Memory Hierarchy The processor is the brain of the computer where all the essential computing takes place. But unlike a human brain, a computer processor has very limited memory. Thus it has to rely on other kind of memories to hold data and instructions and to store results. The memory in a computer system is of three fundamental types: Internal Processor memory: This memory is placed within the CPU (Processor) or is attached to a special fast bus. Internal memory usually includes cache memory and special registers, both of which can be directly accessed by the processor. This memory is used for temporary storage of data and instructions on which the CPU is currently working. Processor memory is the fastest among all the memories but is the most expensive also. Therefore, a very diminutive part of internal processor memory is used in the computer system. It is generally used to compensate for the speed gap between the primary memory and the processor. Primary memory: Random Access Memory (RAM) and read only memory (ROM) fall under this category of the primary memory, also known as main memory. Every computer comes with a small amount of ROM, which contains the boot firmware (called BIOS). This holds enough information to enable the computer to check its hardware and load its operating system into its RAM at the time of
system booting. RAM is the place where the computer temporarily stores it operating system, application programs and current data so that the computer’s processor can reach them quickly and easily. It is a volatile in nature, that is, when the power is switch off, the data in this memory are lost. Unlike RAM, ROM is non volatile. Even when the computer is switched off, the contents of ROM remain available. Secondary memory: Also known as auxiliary memory, secondary memory provides backup storage for instructions (Computer programs) and data. The most commonly used secondary storage devices are magnetic disk and magnetic tapes. These are the least expensive and also have much larger storage capacity then the primary memory. The instructions and data stored on secondary storage devices are permanent in nature. They can only be removed if the user wants it so or if the device is destroyed. Secondary memory can also be used as overflow memory (also known as virtual memory), when the capacity of the main memory is surpassed. Note that unlike processor memory and main memory, secondary memory is not directly accessible to the processor. Firstly, the data and instruction from the secondary memory have to be shifted to the main memory and then to the processors. Figure 1.2 illustrates the memory hierarchy. The CPU accesses memory according to a distinct hierarchy. When the data come from permanent storage (for example, hard disk), they first go in RAM. The reason behind it is that if the CPU has to access the hard disk constantly to retrieve every piece of required data, it would operate very slowly. When the data are kept in primary memory, the CPU can access them more quickly. Subsequently, the CPU stores required pieces of data and instructions in processor memory (cache and registers) to process the data. INTERNAL PROCESSOR MEMORY Registers As instructions are interpreted and executed by a computer’s CPU, there is movement of information between various units of the computer. In other to handle this process satisfactorily and to speed up the rate of information transfer, a number of special memory units called registers are used. These registers are used to hold information on a temporary basis and are part of the CPU (not main memory). The length of a register equals the number of bits it can store. Hence, a register that can store 8 bits is referred to as an 8-bit register. Most CPUs sold today have 32-bits or 64 bits registers. The length of registers of a computer is sometimes called its word size. The bigger the word size, the faster a computer can process a set of data. With all other parameters being same, A CPU with 32-bit registers
Internal Processor Memory(Registers, Cache
Memory
Primary Memory(RAM, ROM)
Secondary Memory(Magnetic Tape, Magnetic Disk, Floppy Disk, WORM Disk
Smaller capacity, faster access time, and higher cost per bit stored
Figure 1.2. A typical storage hierarchy ladder
can process data twice as large as one with 16-bit registers. Memory Address Register (MAR), Program Counter Register (PC), Accumulator Register (A), and Instruction Register (I) etc. are some of common registers used in most of the computer systems. Cache Memory Use of main memory helps in minimizing disk-processor speed mismatch to a large extent because the rate of data fetching by a computer’s CPU from its main memory is about 100 times faster than that
from a high-speed secondary storage like disk. However, even with the use of main memory, memory-processor speed mismatch becomes bottleneck in the speed with which the CPU can process instructions because there is a 1 to 10 speed mismatch between the processor and memory. That is, the rate at which data can fetched from memory is about 10 times slower than the rate at which CPU can process data. Obviously, the overall performance of a processor can be improved greatly by minimizing the memory-processor speed mismatch. Cache memory (pronounce as “cash” memory) is commonly used for this purpose. It is an extremely fast, small memory between CPU and main memory whose access time is closer to the processing speed of CPU. It acts as a high-speed buffer between CPU and main memory and is used to temporarily store very active data and instructions during processing. Since cache memory is faster than main memory, processing speed is improved by making the data and instructions needed for current processing available in the cache. PRIMARY MEMORY Random Access Memory RAM is like the computer’s scratch pad. It allows the computer to store data for immediate manipulation and to keep track of what is currently being processed. It is the place in a computer where the operating system, application programs and data in current use are kept so that they can be accessed quickly by the computer’s processor. RAM is much faster to read from and write to then the other kinds of storage in a computer, like the hard disk or floppy disk. However, the data in RAM stays there only as long as the computer is running. When the computer is turned off, RAM losses all its contents. When the computer is turned on again, the operating system and other files are once again loaded into RAM. When an application program is started, the computer loads it into RAM and does all the processing there. This allows the computer to run the application faster. Any new information that is created is kept in RAM and since RAM is volatile in nature, one needs to continuously save the new information to the hard disk. Types of RAM RAM is of two types:
Primary Memory
RAM
SRAM DRAM
ROM
Masked ROM PROM EPROM EEPROM
Static RAM (SRAM): the word “static” indicates that the memory retains its content as long as the power is being supplied. However, as soon as the power goes down, the data are lost. This makes SRAM a volatile memory as opposed to ROM. SRAM does not need to be “refreshed” (pulse of current through all the memory cells) periodically. It is very fast but much more expensive than DRAM (Dynamic RAM). SRAM is often used as cache memory due to its high speed.
Dynamic RAM (DRAM): It is named so because it is very unstable. The data continue to move in and out of the memory as long as power is available. Unlike SRAM, DRAM must be continually refreshed in order to maintain the data. This is done by placing the memory on a refresh circuit that rewrites the data several hundred times per second. DRAM is used for most system memory because it is inexpensive and small. The primary difference between SRAM and DRAM is the life of the data they store. Refreshing is not required in SRAM. DRAM must be continuously refreshed after
about every 15 microseconds. SRAM chips are less dense than DRAM chips, that is , total number of cells in SRAM is less than that of DRAM
DRAM chips are more dense than SRAM, that is, total number of cells in DRAM is more than that of SRAM
SRAM is fast, has low latency (the time tag between a request and the action performed).
DRAM is slow, has high latency (the time tag between a request and the action performed).
It is large, expensive, requires more power to operate and produces a lot of heat
It is smaller, less expensive, requires less power to operate and produce less heat
Read Only Memory Just as a human being needs instructions from the brain to perform actions in a certain event, a computer also needs special instructions every time it is started. This is required because during the start up operation, the main memory of the computer is empty due to its volatile property so there have to be some instructions (special boot programs) stored in a special chip that could enable the computer system to perform start up operations and transfer the control to the operating system. This special chip, where the starts up instructions are stored, is called ROM. It is non volatile in nature, i.e., the contents are not lost when power is switched off. The data and instructions stored in ROM chips are used not only in computer but also in other electronic items like washing machines and microwave ovens. Generally, designers program ROM chips at the time of manufacturing circuits. Burning appropriate electronic fuses to form patters of binary information does the programming. These patterns of binary information are meant for specific configurations, which is why different categories of computers are meant for performing different tasks. For example, a micro program called system boot program contains a series of start-up instructions to check for the hardware, that is, I/O devices, memory and operating system in the memory. These programs deal with low-level machine functions and are alternate for additional hardware requirement. ROM performs the necessary BIOS (Basic input output system) function to start the system and then transfer the control over the operating system. ROM can have data and instructions written into it only one time. Once a ROM chip is programmed, it cannot be reprogrammed or written. If it is erroneous, or the data need to be reorganized, one has to replace it with the new chip. Thus, the programming of ROM chips should be perfect, having all the required data at the time of its manufacturing. Note that in some instances, ROM can be changed using certain tools. The ROM chips consume very little power, are extremely reliable, and in the case of most small electronic devices, contain all the necessary programming to control the device. Types of ROM
Memories in the ROM family are distinguished by the methods used to write data on them and the number of times they can be rewritten. This classification reflects the evolution of ROM devices from “hard-wired” to programmable to erasable-and-programmable. One common feature of all these devices is their ability to retain data and programs even during a power failure. ROMs come in following varieties:
1. Masked ROM: The very first ROMs, known as masked ROMs, were hard-wired devices that contained a pre-programmed set of data or instructions. The contents of such ROMs had to be specified before chip production so the actual data could be used to arrange the transistors inside the chip. 2. Programmable ROM (PROM): Creating a ROM chip from scratch is a time consuming and an expensive process. For this reason, developers created a type of ROM known as programmable read only memory (PROM), which can be programmed. Blank PROM chips can be bought economically and coded by the users with the help of a special device known as PROM-Programmer. However, once a PROM ha s been programmed, its contents can never be changed. As a result, PROM is also known as one-time programmable (OTP) device. Like other ROMs, PROM is also non-volatile. However, it is more fragile than other ROMs as a jolt of static electricity can easily cause the fuses in the PROM to burn out, thus changing the bit pattern from 1 to 0. Nevertheless, blank PROMs are economical and are great for prototyping the data for a ROM before committing to the costly ROM fabrication process. 3. Erasable Programmable ROM (EPROM): An EPROM is programmed in exactly the same manner as a PROM. However, unlike PROM, an EPROM can be erased and reprogrammed repeatedly. It can be erased by simply exposing the device to a strong source of ultraviolet light for a certain amount of time. Note that an EPROM eraser is not selective; it will erase the entire EPROM. Although EPROM is more expensive than PROM, its ability to be reprogrammed makes it more useful. EPROM chips are of two types- one in which the stored information is erased by exposing the chip for some time to ultraviolet radiation and the other one by using high voltage electric pulses. The former is known as Ultra Violet EPROM (UVPROM) and the later is known as electrically EPROM. 4. Electrically Erasable Programmable ROM (EEPROM): This type of ROM can be erased by an electrical charge and then written to by using slightly higher-than-normal voltage. EEPROM can be erased one byte at a time, rather than erasing the entire chip with high voltage electric pulses. Hence, the process of reprogramming is flexible, but slow. Also, changing the contents does not require any additional committed equipment. As these chips can be changed without opening a casing, they are often used to store programmable instructions in devices like printers. EEPROM is also known as flash memory because of the ease with which programs stored in it can be altered. Flash memory is used in many new I/O and storage devices like USB (Universal serial Bus) pen drive and MP3 music player. SECONDARY STORAGE DEVICES
Primary storage of a computer system has following limitations:
1. Limited capacity. It is often necessary to store many millions, sometimes billions, and even trillions, of bytes of data in a computer. Unfortunately, the storage capacity of primary storage of today’s computers is not sufficient to store the large volume of data handled by most data processing centers.
2. Volatile. Primary storage is volatile and the data stored in it is lost when power is turn off or interrupted. However, computer systems need to store data on permanent basis for several days, several months, or even several years. As a result, additional memory, called auxiliary memory or secondary storage, is used with most computer systems. Secondary storage devices facilitate storing of data and instructions permanently. It has lower cost per bit stored but it generally has an operating speed far slower than of primary storage. It is primarily to stored large volume of data on permanent basis that can be partially transferred to primary storage, whenever required for processing. The data stored on a secondary storage device can be accessed depending upon how it is stored on the device. Primarily, there are two methods of accessing data from the secondary storage devices. Sequential Access: Sequential access means the computer system must search the storage devices from the beginning until the desire data is found. The most common sequential access storage device is magnetic tape where data is stored and processed sequentially. Suppose, a tape contains information regarding employees of an organization. Now to look for employee number 100’s information, the computer will have to start with employee number 1 and then go past 2, 3 and so on until it finally come to 100. The sequential access method is quite simple other methods but searching for data is slow.
Direct Access: Direct access, also known as random access, means that the computer can go directly to the location, where the data that user wants is stored. The common direct access storage devices are magnetic disk and optical disk. In these devices, the data are stored a sequentially numbered 2 and so on. Thus one can access bock 12, then access block 78, then block 2 and so on. The direct access method is ideal for applications like airplane reservation system or computer-based directory assistance operations. In These cases, there is no fixed pattern of requests for data. Based on the access method, secondary storage devices can be classified as shown below: Magnetic Tape
Secondary Storage Devices
Sequencial Access Devices
Magnetic Tape
Direct Access Devices
Magnetic Disk
Floppy Disk Hard Disk
Zip Dsk Disk Pack Winchester Disk
Optical Disk
CD-ROM WORM(CD-R) CD-RW DVD
Memory Storage Devices
Flashb Drive Memory Card
Magnetic tape is the most popular storage medium for large data that are accessed and processed sequentially. Magnetic tape medium is plastic ribbon usually 1/2 inch or 1/4 inch wide and 50 to 2400 feet long. It is coated with a recording material that can be magnetized such as iron oxide or chromium dioxide. Data are recorded on tape in the form of tiny invisible magnetized and non-magnetized spots (representing 1s or 0s) on the coated surface of the tape. Tape ribbon is itself stored in reels or a small cartridge or cassette. Like audio or video tape, magnetic tapes used in computer systems can also be erased and reused indefinitely. Old data on the tape are erased automatically as new data are recorded in the same area. However, the information stored can be read many times without affecting the stored data. Magnetic Disk Magnetic disk ate the most popular direct-access secondary storage device. They are also the most popular on-line secondary storage device. A magnetic disk is a thin circular plate/platter made of metal or plastic and coated on both sides with a recording material that can be magnetized such as iron oxide. Data are recorded on a disk in the form of tiny invisible magnetized and non-magnetized spots (representing 1s or 0s) on the coated surfaces of the disk. A standard binary coat, usually 8-bit EBCDIC, is used for recording data. The disk itself is stored in a specially designed protective envelope, or several of them are stacked together in a sealed contamination free container. Like magnetic tapes, magnetic disks can also be erased and reused indefinitely. Old data on a disk are erased automatically by recording new data in the same area. However, stored data can be read many a times without affecting the data. Storage Organization The surface of a disk is divided into a number of invisible concentric circles called tracks. The tracks are numbered consecutively from outermost starting from zero. The number of tracks varies from low-capacity disks to high-capacity disks. In addition to the concentric circles, a disk’s surface is also divided into invisible pie shape segments called sectors. To access a piece of data (a record) stored on a disk, the record’s disk address must be specified. A disk address represents the physical location of a record on the disk and it comprises of sector number, tract number, and surface number (when double-sided disks are used). Often multiple disks are stacked and used together to create large capacity disk-storage systems. In this case, a set of magnetic disks is fixed to a central shaft, one below the other, to form a disk pack. The disk pack is sealed and mounted on a disk drive consisting of a motor to rotate the disk pack about an axis. The disk drive also has an access arms assembled having separate read/write heads for each surface of the disk pack on which data can be recorded or read. Normally, the upper surface of the topmost side and lower surface of the bottommost disk are not used in a disk pack because these surfaces may be scratched easily. However, this drawback has been eliminated in modern disk drives using miniaturization and precision components. Access Time Disk access time the interval between the time a computer makes a request for transfer of data from a disk system to primary storage and the time this operation is completed. To access data stored on a disk address of the desired data is specified in terms of surface/head number, track/cylinder number, and sector number. Information is always written from the beginning of a sector and can read only from the track beginning. Hence, disk access time depends on the following three parameters: 1. Seek time. The time required to position the read/write head over the desired track/cylinder is called seek time. Seek time varies depending on the position of the access arms assembly when a read/write command is received.
2. Latency. Once the heads are positioned on the desired track, the head on the specified surface is activated. Since disk is continuously rotating, this head should wait for the desired data to come under it. This rotational waiting time, i.e., the time required to spin the desired sector under the head is called latency. Latency, also known as rotational delay time, is also a variable and depends on the distance of the desired sector from the initial position of the head on the specified track. It also depends on the rotational speed of the disk that normally varies from 300 rpm to 7200 rpm. 3. Transfer Rate. Transfer rate refers to the rate at which data is read from or written to a disk. Once the read/write head is positioned over the addressed sector, the desired data is read/write at a speed determined by the rotational speed of the disk. The data transfer rate of a disk system depends on the density of stored data and rotational speed of the disk. Since data transfer time is negligible (due to high transfer rate) as compared to seek time and latency, average access time for a disk system is its average seek time plus latency, average access time for disk system is its average seek time plus its average latency. Types of Magnetic Disks All magnetic disks are round platters. They come in different sizes, use different types of packaging, and are made of rigid metal or flexible plastics. Based on these differences, there are many types of magnetic disks available today. However all of them can be classified broadly in two types- floppy disks and hard disks. Floppy disks are individually packaged in protective envelopes or plastic cases whereas hard disks are packaged individually or in multiples in cartridge or contamination-free containers. Depending on the types of packaging, hard disks are further classified into Zip/Bernoulli disks, disk packs, and Winchester disks. Floppy Disks A floppy disk is a flat, circular piece of flexible plastic coated with magnetic oxide. It is encased in a square plastic or vinyl jacket cover. The jacket gives handling protection to the disk surface. Moreover, it has a special liner that provides a wiping action to remove dust particles, as they are harmful to disk surface and read/write head. Floppy disks are so called because they are made of flexible plastic plates that can bend. They are also known as floppies or diskettes. They were introduced by IBM in 1972 and are now produced in various sizes and capacities by many manufacturers. 3 1/2 is the most commonly used floppy disk. It consists of the most commonly used floppy disk. All 3 1/2 inch floppy disks are of double-sided type that record data on both disk surfaces. However they come in three different capacities –double density, high density, and very high density. Hard Disks Hard disks are the primary on-line secondary storage device for most computer systems today. They are made of rigid metal platters and come in many sizes ranging from 1 to 14-inch diameter. Types of Hard Disks Depending on how they are packaged, hard disks are normally categorized into three types: 1. Zip/Bernoulli Disk. It consists of a single hard disk platter encased in a cartridge. The disk is commonly of 3 1/2 inch size having storage capacity of about 100 MB. The storage capacity slightly varies depending on the formatting style used by a computer system with which it is used. Its disk drive, called zip drive, may be of portable or fixed type. The fixed type is part of a computer system permanently connected to it. The portable type can be disconnected and taken away. A zip disk can be easily loaded/unloaded into a zip drive just as we insert/remove a floppy disk in a floppy disk drive or a video cassette in a VCR. 2. Disk Pack. It consists of multiple hard disk platters mounted on a single central shaft. All the disks revolve together at the same speed. Its disk drive has a separate read/write head for each usable disk surface. Its disk drive is of interchangeable type and allows loading/unloading of differently disk packs as and when they are to be used. When not in use, a disk pack is stored off-line in a plastic case. This gives virtually unlimited storage capacity to disk packs.
3. Winchester Disk. A Winchester disk consists of multiple hard disk platters mounted on a single central shaft. However, unlike a disk drive, a Winchester disk drive is of fixed type. That is, its hard disk platters and disk drive are sealed together in a contamination-free container and cannot be separated from each other. Hence, Winchester disks have limited capacity. However, for the same number of disk platters of the same size, Winchester disks can have larger storage capacity than disk packs. Optical Disk As compared to magnetic tape and magnetic disk, optical disk is a relatively new secondary storage medium. During the last years, it has proved to be a promising random access medium for high capacity secondary storage because it can store extremely large amount of data in a limited space. An optical-disk storage system consists of a rotating disk coated with a thin metal or some other material that is highly reflective. It uses laser beam technology for recording/reading of data on disk surface. Optical disks are also known as laser disks or optical disks because they use laser beam technology for data read/write. Storage Organization Unlike magnetic disks having several concentric tracks, an optical disk has one long track starting at the outer and spiraling inward to the center. This spiral track is ideal for reading large blocks of sequential data, such as audio or video. But it causes slower random access of data than in case of concentric tracks used by magnetic disks. This is because in case of concentric tracks organization, sectors can be located faster as they are always found on a given track at a fixed distance from the center. The spiral track of an optical disk is split up into equal-length sectors regardless of the position of a sector from the center. An optical disk drive uses beam technology for recording/reading of data on an optical disk surface. An optical disk drive contains all the mechanical, electrical, and electronic components for holding an optical disk and for reading/writing of information on it. Optical disk drives are slower (have larger access time) than magnetic disk drives. Types of Optical Disk All optical disk are round platters. They come in different sizes and capacities. Commonly used types of optical disks are CD-ROM, WORM (CD-R), CD-RW, and DVD disks. They are described below. CD-ROM CD-ROM stands for Compact Disk-Read-Only Memory. It is a spin-off audio CD technology and works much like audio CDs used in music systems. In fact, if your computer has sound card and speakers, you can play audio CDs with your computer. CD-ROM disk is a shiny, silver color metal disk usually of 51/4- inch (12cm) diameter. It is made of polycarbonate plastic and thin layer of pure aluminum is applied to make the surface reflective. It has storage capacity of about 650 Megabytes or 700 Megabytes in newer ones. It is so called because of its large storage capacity on a compact-size disk and because it is read-only storage medium. That is, these disks come pre-recorded and information stored on them cannot be altered. WORM Disk/CD-Recordable (CD-R) Disk WORM stands for write once, read many. A WORD disk allows users to create their own CD-ROM disks by using a CD-recordable (CD-R) drive attached to a computer as a regular peripheral device. WORM disks look like standard CD-ROM disks, are purchased blank, and later encoded using a CD-R drive. The information recorded on a WORM disk by a CD-R drive can be read by any ordinary CD-ROM drive. As the name implies, data can be written only once on a WORM disk but can be read many times. That is, like a CD-ROM disk, once data has been written on the surface of WORM disk it becomes permanent and can be read but never be altered. However, all the data to be recorded on a WORM disk can be written on its surface in multiple sessions. The sessions after the first one are always additive and cannot alter the etched/burned information of earlier sessions. The information
added in a session can be hidden in a subsequent session by creating the File Allocation Table (FAT) at a new location but etching on the surface cannot be removed. Such disk is called Multi-Session Disk. Laser beam technology is used for data recording/reading. CD Read/Write (CD-RW) Disk A CD Read/Write (CD-RW) disk is very similar to a WORM disk with the exception that you can erase the previous content and write on it multiple times. Such disks are metallic alloy layers. Laser beam changes the chemical property during writing (or burn process) changing reflectivity at desired places. The land-pit difference on CD-RW is not significant and hence CD drives have to be compatible to read such disks. A CD-RW disk usually has a lifetime of 100 or more erase-write cycle. A disk written once can be erased by changing the chemical property again and then it can be written on to afresh. CD-RW drives have such erase capability. CD-RW disks are little expensive than CD-R disks but are a great cost saver because they can be reused many times due to their erase capability. Digital Video (or Versatile) Disk (DVD) DVD was designed primarily to store and distribute movies. However, it is fast becoming mainstream optical disk as prices are reducing and needs for large capacity storage is increasing. It is similar to CD ROM in principal but is denser in recording the data. There are two variants of DVD- single-layer disk and double-layer disk. Single-layer disk has storage capacity of 4.7GB, whereas double-layer disk has storage capacity of 8.5GB. Like CD-ROM, DVD also has many types- DVD-R/W, DVD+R/RW, DVD-Video, and DVD-Audio. DVD-Video is now the most dominant movies storage format used. DVD has enough space to store movie and support multi-lingual subtitles, multilingual audio, multiple camera angles etc. It also supports region marking to protect against piracy and use of DVD from one region to another. It also supports content Protection for Prerecorded Media (CPPM) security technique to safeguard against copying, etc. Other Memory Storage Devices Flash Drive (Pen Drive) Flash drive is a compact device of the size of a pen, comes in various shape and stylish designs (such as pen shape, wallet shape etc), an may have different added features (such as with a camera, with a built-in MP3/WMA/FM Radio play back for music on the go, etc.). It enables easy transport of the data from one computer to another. It is a plug-and-play device that simply plugs into a USB (Universal Serial Bus) port of a computer. The computer detects it automatically as removable drive. Now, one can read, write, copy, delete and move data from the computers’ hard disks to the flash drive or from the flash drive to the hard disk drive. One can even run applications, view videos, or play mp3 files from it directly. Once done, it can be simply plugged out of the USB port of the computer and kept into the pocket for being carried anywhere. A flash drive does not require any battery, cable, or software and is compatible with most computers. All these features make it ideal external data storage for mobile people to carry or transfer from one computer to another. As the name implies, it is based on the flash memory storage technology discussed above. Recall that flash memory is none-volatile, electrically erasable programmable read only memory (EEPROM) chips. It is a highly durable solid-state storage having data retention capability of 10 years. Available storage capacities are 8MB, 16MB, 64MB, 128 MB, 256MB, 512MB, 1GB, 2GB, 4GB and 8 GB. The main body of a flash drive usually has a write protected, a read/write LED (Light emitting diode) indicator, and a strap hole. Some manufacturers also provide software to be used with the drive. Memory Card (SD/MMC)
Similar to flash drive, flash memory based cards are available as removable storage device in different types of electronic equipments. Some of the most popular ones are secure Digital (SD) and multimedia card (MMC). Some other manufacturers have their own proprietary cards based on flash memory. Storage capacity of these cards ranges from 8 MB to 2GB. In addition to computers, these cards used in various types of digital devices such as digital camera and cell phones. This also facilitates easy transfer of data from these devices to a computer for storage in d computer’s hard disk or for further processing by the computer. Each of these cards has its own interface and specific design features for use with certain types of devices.
