Lecture notes on Computer organization

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LECTURE NOTES ON COMPUTER ORGANIZATION AND OPERATING SYSTEMS III B. Tech I semester (JNTUH-R13) Ms. G. GEETHA Assistant Professor Mr. K. CHIRANJEEVI Assistant Professor Mr. CH. SRIKANTH Assistant Professor COMPUTER SCIENCE AND ENGINEERING INSTITUTE OF AERONAUTICAL ENGINEERING DUNDIGAL, HYDERABAD - 500 043 Unit- I Basic Structure of Computers Computer Architecture in general covers three aspects of computer design namely: Computer Hardware, Instruction set Architecture and Computer Organization. Computer hardware consists of electronic circuits, displays, magnetic and optical storage media and communication facilities. Instruction set Architecture is programmer visible machine interface such as instruction set, registers, memory organization and exception handling. Two main approaches are mainly CISC (Complex Instruction Set Computer) and RISC (Reduced Instruction Set Computer) Computer Organization includes the high level aspects of a design, such as memory system, the bus structure and the design of the internal CPU. Computer Types Computer is a fast electronic calculating machine which accepts digital input, processes it according to the internally stored instructions (Programs) and produces the result on the output device. The internal operation of the computer can be as depicted in the figure below: Figure 1: Fetch, Decode and Execute steps in a Computer System The computers can be classified into various categories as given below:  Micro Computer  Laptop Computer  Work Station  Super Computer  Main Frame  Hand Held  Multi core Micro Computer: A personal computer; designed to meet the computer needs of an individual. Provides access to a wide variety of computing applications, such as word processing, photo editing, e-mail, and internet. Laptop Computer: A portable, compact computer that can run on power supply or a battery unit. All components are integrated as one compact unit. It is generally more expensive than a comparable desktop. It is also called a Notebook. Work Station: Powerful desktop computer designed for specialized tasks. Generally used for tasks that requires a lot of processing speed. Can also be an ordinary personal computer attached to a LAN (local area network). Super Computer: A computer that is considered to be fastest in the world. Used to execute tasks that would take lot of time for other computers. For Ex: Modeling weather systems, genome sequence, etc (Refer site: http://www.top500.org/) Main Frame: Large expensive computer capable of simultaneously processing data for hundreds or thousands of users. Used to store, manage, and process large amounts of data that need to be reliable, secure, and centralized. Hand Held: It is also called a PDA (Personal Digital Assistant). A computer that fits into a pocket, runs on batteries, and is used while holding the unit in your hand. Typically used as an appointment book, address book, calculator and notepad. Multi Core: Have Multiple Cores – parallel computing platforms. Many Cores or computing elements in a single chip. Typical Examples: Sony Play station, Core 2 Duo, i3, i7 etc. Functional Units A computer in its simplest form comprises five functional units namely input unit, output unit memory unit, arithmetic & logic unit and control unit. Figure 2 depicts the functional units of a computer system. Figure 2: Basic functional units of a computer Let us discuss about each of them in brief: 1. Input Unit: Computer accepts encoded information through input unit. The standard input device is a keyboard. Whenever a key is pressed, keyboard controller sends the code to CPU/Memory. Examples include Mouse, Joystick, Tracker ball, Light pen, Digitizer, Scanner etc. 2. Memory Unit: Memory unit stores the program instructions (Code), data and results of computations etc. Memory unit is classified as: • Primary /Main Memory • Secondary /Auxiliary Memory Primary memory is a semiconductor memory that provides access at high speed. Run time program instructions and operands are stored in the main memory. Main memory is classified again as ROM and RAM. ROM holds system programs and firmware routines such as BIOS, POST, I/O Drivers that are essential to manage the hardware of a computer. RAM is termed as Read/Write memory or user memory that holds run time program instruction and data. While primary storage is essential, it is volatile in nature and expensive. Additional requirement of memory could be supplied as auxiliary memory at cheaper cost. Secondary memories are non volatile in nature. Arithmetic and logic unit: ALU consist of necessary logic circuits like adder, comparator etc., to perform operations of addition, multiplication, comparison of two numbers etc. Output Unit: Computer after computation returns the computed results, error messages, etc. via output unit. The standard output device is a video monitor, LCD/TFT monitor. Other output devices are printers, plotters etc. Control Unit: Control unit co-ordinates activities of all units by issuing control signals. Control signals issued by control unit govern the data transfers and then appropriate operations take place. Control unit interprets or decides the operation/action to be performed. The operations of a computer can be summarized as follows:  A set of instructions called a program reside in the main memory of computer.  The CPU fetches those instructions sequentially one-by-one from the main memory, decodes them and performs the specified operation on associated data operands in ALU.  Processed data and results will be displayed on an output unit.  All activities pertaining to processing and data movement inside the computer machine are governed by control unit.Basic Operational Concepts An Instruction consists of two parts, an Operation code and operand/s as shown below: OPCODE OPERAND/s Let us see a typical instruction ADD LOCA, R0 This instruction is an addition operation. The following are the steps to execute the instruction: Step 1: Fetch the instruction from main memory into the processor Step 2: Fetch the operand at location LOCA from main memory into the processor Step 3: Add the memory operand (i.e. fetched contents of LOCA) to the contents of register R0 Step 4: Store the result (sum) in R0. The same instruction can be realized using two instructions as Load LOCA, R1 Add R1, R0 The steps to execute the instructions can be enumerated as below: Step 1: Fetch the instruction from main memory into the processor Step 2: Fetch the operand at location LOCA from main memory into the processor Register R1 Step 3: Add the content of Register R1 and the contents of register R0 Step 4: Store the result (sum) in R0. Figure 3 below shows how the memory and the processor are connected. As shown in the diagram, in addition to the ALU and the control circuitry, the processor contains a number of registers used for several different purposes. The instruction register holds the instruction that is currently being executed. The program counter keeps track of the execution of the program. It contains the memory address of the next instruction to be fetched and executed. There are n general purpose registers R0 to R which can be used by the programmers during writing n-1 programs. Figure 3: Connections between the processor and the memory The interaction between the processor and the memory and the direction of flow of information is as shown in the diagram below: Figure 4: Interaction between the memory and the ALU BUS STRUCTURES Group of lines that serve as connecting path for several devices is called a bus (one bit per line). Individual parts must communicate over a communication line or path for exchanging data, address and control information as shown in the diagram below. Printer example – processor to printer. A common approach is to use the concept of buffer registers to hold the content during the transfer. Figure 5: Single bus structure SOFTWARE If a user wants to enter and run an application program, he/she needs a System Software. System Software is a collection of programs that are executed as needed to perform functions such as:  Receiving and interpreting user commands  Entering and editing application programs and storing then as files in secondary storage devices  Running standard application programs such as word processors, spread sheets, games etc. Operating system - is key system software component which helps the user to exploit the below underlying hardware with the programs. USER PROGRAM and OS ROUTINE INTERACTION Let‘s assume computer with 1 processor, 1 disk and 1 printer and application program is in machine code on disk. The various tasks are performed in a coordinated fashion, which is called multitasking. t0, t1 …t5 are the instances of time and the interaction during various instances as given below: t0: the OS loads the program from the disk to memory t1: program executes t2: program accesses disk t3: program executes some more t4: program accesses printer t5: program terminates Figure 6 :User program and OS routine sharing of the processor PERFORMANCE The total time required to execute a program is the most important measure of performance for a computer. (t0-t5 of earlier example). Compiler, instruction set and hardware architecture, program all have impact on performance. Basic Performance Equation: The basic performance equation is given by T = (N S) / R where T=execution time, N=number of instructions, S=average cycles per instruction, R=clock rate in cycles per second CACHING Commonly used data are copied to on-processor memory (cache) to reduce access time. Small memories can be made with higher speed than large ones. In a computer, we need both. Figure 7: The processor cache PIPELINING and SUPERSCALR OPERATION Pipelining: Like a production line, instruction execution overlapped so greater parallelism is achieved. Superscalar operation: Execute several instructions simultaneously using multiple ALU‘s. CISC vs RISC Reduced instruction set computer – Large N, small S Complex instruction set computer – Small N, large S COMPILER Translates high level language such as C, C++ and Java to machine instructions. Aims to reduce N×S PERFORMANCE MEASUREMENT Benchmark refers to standard task used to measure how well a processor operates. To evaluate the performance of Computers, a non-profit organization known as SPEC-System Performance Evaluation Corporation employs agreed-upon application programs of real world for benchmarks. Accordingly, it gives performance measure for a computer as the time required to execute a given benchmark program. The SPEC rating is computed as follows MULTIPROCESSORS AND MULTICOMPUTERS Multicomputer A computer made up of several computers. The term generally refers to an architecture in which each processor has its own memory rather than multiple processors with a shared memory. Distributed computing deals with hardware and software systems containing more than one processing. Element or storage element, concurrent processes, or multiple programs, running under a loosely or tightly controlled regime. A multicomputer may be considered to be either a loosely coupled NUMA computer or a tightly coupled cluster. Multi computers are commonly used when strong computer power is required in an environment with restricted physical space or electrical power. Common suppliers include Mercury Computer Systems, CSPI, and SKY Computers. Common uses include 3D medical imaging devices and mobile radar. In distributed computing a program is split up into parts that run simultaneously on multiple computers communicating over a network. Distributed computing is a form of parallel computing, but parallel computing is most commonly used to describe program parts running simultaneously on multiple processors in the same computer. Both types of processing require dividing a program into parts that can run simultaneously, but distributed programs often must deal with heterogeneous environments, network links of varying latencies, and unpredictable failures in the network or the computers. Multiprocessor A multiprocessor system is simply a computer that has more than one CPU on its motherboard. If the operating system is built to take advantage of this, it can run different processes (or different threads belonging to the same process) on different CPUs. Multiprocessing is the use of two or more central processing units (CPUs) within a single computer system. The term also refers to the ability of a system to support more than one processor and/or the ability to allocate tasks between them.1 There are many variations on this basic theme, and the definition of multiprocessing can vary with context, mostly as a function of how CPUs are defined (multiple cores on one die, multiple chips in one package, multiple packages in one system unit, etc.). Data Representation Introduction The digital computer is a digital system that performs various computational tasks. The word digital implies that the information in the computer is represented by variables that take a limited number of discrete values. These values are processed internally by components that can maintain a limited number of discrete states. The decimal digits 0, 1, 2, 9, for example, provide 10 discrete values. The first electronic digital computers, developed in the late 1940s, were used primarily for numerical computations. In this case the discrete elements are the digits. From this application the term digital computer has emerged. In practice, digital computers function more reliably only if two states are used. Because of the physical restriction of components, and because human logic tends to be binary (i.e. true-or-false, yes-or-no statements), digital components that are constrained to take discrete values are further constrained to take only two values and are said to be binary. Objectives: After studying this unit, the learner will be able to  Explain various units of a digital computer  Understand different data types  Explain fixed and floating point number representation  Discuss various binary and error detection codes Digital Computers Digital computers use the binary number system, which has two digits: 0 and 1. A binary digit is called a bit. Information is represented in digital computers in groups of bits. By using various coding techniques, groups of bits can be made to represent not only binary numbers but also other discrete symbols, such as decimal digits or letters of the alphabet. By the judicious use of binary arrangements and by using various coding techniques, the groups of bits are used to develop complete sets of instructions for performing various types of computations. In contrast to the common decimal numbers that employ the base 10 system, binary numbers use a base 2 system with two digits: 0 and 1. The decimal equivalent of a binary number can be found by expanding it into a power series with a base of 2. For example, the binary number 1001011 represents a quantity that can be converted to a decimal number by multiplying each bit by the base 2 raised to an integer power as follows: 6 5 4 3 2 1 0 1 x 2 + 0 x 2 + 0 x 2 + 1 x 2 + 0 x 2 + 1 x 2 + 1 x 2 = 75 The seven bits 1001011 represent a binary number whose decimal equivalent is 75. However, this same group of seven bits represents the letter K when used in conjunction with a binary code for the letters of the alphabet. It may also represent a control code for specifying some decision logic in a particular digital computer. In other words, groups of bits in a digital computer are used to represent many different things. This is similar to the concept that the same letters of an alphabet are used to construct different languages, such as English and French. A computer system is sometimes subdivided into two functional entities: hardware and software. The hardware of the computer consists of all the electronic components and electromechanical devices that comprise the physical entity of the device. Computer software consists of the instructions and the data that the computer manipulates to perform various data-processing tasks. A sequence of instructions for the computer is called a program. The data that are manipulated by the program constitute the data base. A computer system is composed of its hardware and the system software available for its use. The system software of a computer consists of a collection of programs whose purpose is to make more effective use of the computer. The programs included in a systems software package are referred to as the operating system. They are distinguished from application programs written by the user for the purpose of solving particular problems. For example, a high-level language program written by a user to solve particular data-processing needs is an application program, but the compiler that translates the high-level language program to machine language is a system program. The customer who buys a computer system would need, in addition to the hardware, any available software needed for effective operation of the computer. The system software is an indispensable part of a total computer system. Its function is to compensate for the differences that exist between user needs and the capability of the hardware. The hardware of the computer is usually divided into three major parts as shown in Fig. 1.1. Random Access Memory (RAM) Central Processing Unit (CPU) Input Input Output Processor Output devices (IOP) devices Fig. 1.1: Block diagram of a digital computer The Central Processing Unit (CPU) contains an Arithmetic and Logic Unit (ALU) for manipulating data, a number of registers for storing data, and control circuits for fetching and executing instructions. The memory of a computer contains storage for instructions and data. It is called a Random Access Memory (RAM) because the CPU can access any location in memory at random and retrieve the binary information within a fixed interval of time. The Input-Output Processor (IOP) contains electronic circuits for communicating and controlling the transfer of information between the computer and the outside world. The input and output devices connected to the computer include keyboards, printers, terminals, magnetic disk drives, and other communication devices. This book provides the basic knowledge necessary to understand the hardware operations of a computer system. The subject is sometimes considered from three different points of view, depending on the interest of the investigator. When dealing with computer hardware it is customary to distinguish between what is referred to as computer organization, computer design, and computer architecture. Computer organization is concerned with the way the hardware components operate and the way they are connected together to form the computer system. The various components are assumed to be in place and the task is to investigate the organizational structure to verify that the computer parts operate as intended. Computer design is concerned with the hardware design of the computer. Once the computer specifications are formulated, it is the task of the designer to develop hardware for the system. Computer design is concerned with the determination of what hardware should be used and how the parts should be connected. This aspect of computer hardware is sometimes referred to as computer implementation. Computer architecture is concerned with the structure and behavior of the computers as seen by the user. It includes the information formats, the instruction set, and techniques for addressing memory. The architectural design of a computer system is concerned with the specifications of the various functional modules, such as processors and memories, and structuring them together into a computer system. Data Types Binary information in digital computers is stored in memory or processor registers. Registers contain either data or control information. Control information is a bit or a group of bits used to specify the sequence of command signals needed for manipulation of the data in other registers. Data are numbers and other binary-coded information that are operated on, to achieve required computational results. The data types found in the registers of digital computers may be classified as being one of the following categories: (1) numbers used in arithmetic computations, (2) letters of the alphabet used in data processing, and (3) other discrete symbols used for specific purposes. All types of data, except binary numbers, are represented in computer registers in binary-coded form. This is because registers are made up of flip-flops and flip-flops are two-state devices that can store only 1‘s and 0‘s. The binary number system is the most natural system to be used in a digital computer. But sometimes it is convenient to employ different number systems, especially the decimal number system, since it is used by people to perform arithmetic computations. Number Systems A number system of base, or radix, r is a system that uses distinct symbols for r digits. Numbers are represented by a string of digit symbols. To determine the quantity that the number represents, it is necessary to multiply each digit by an integer power of r and then form the sum of all weighted digits. For example, the decimal number system in everyday use employs the radix 10 system. The 10 symbols are 0, 1, 2, 3, 4, 5, 6, 7, 8, and 9. The string of digits 724.5 is interpreted to represent the quantity. 2 1 0 -1 7 x 10 + 2 x 10 + 4 x 10 + 5 x 10 that is, 7 hundreds, plus 2 tens, plus 4 units, plus 5 tenths. Every decimal number can be similarly interpreted to find the quantity it represents. To distinguish between different radix numbers, the digits will be enclosed in parentheses and the radix of the number inserted as a subscript. For example, to show the equality between decimal and binary forty-five we will write (101101) = (45) . Besides the decimal and binary number 2 10 systems, the octal (radix 8) and hexadecimal (radix 16) are important in digital computer work. The eight symbols of the octal system are 0, 1, 2, 3, 4, 5, 6, and 7. The 16 symbols of the hexadecimal system are 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D, E, and F. The last six symbols are, unfortunately, identical to the letters of the alphabet and can cause confusion at times. However, this is the convention that has been adopted. When used to represent hexadecimal digits, the symbols A, B, C, D, E, F correspond to the decimal numbers 10, 11, 12, 13, 14, 15, respectively. A number in radix r can be converted into the familiar decimal system by forming the sum of the weighted digits. For example, octal 736.4 is converted to decimal as follows: 2 1 0 -1 (736.4) = 7 x 8 + 3 x 8 + 6 x 8 + 4 x 8 8 = 7 x 64 + 3 x 8 + 6 x 1 + 4 / 8 = (478.5) 10 The equivalent decimal number of hexadecimal F3 is obtained from the following calculation: (F3) = F x 16 + 3 = 15 x 16 + 3 = (243) . 16 10 Integer = 41 Fraction = 0.6875 41 0.6875 20 1 x 2 10 0 1.3750 5 0 x 2 2 1 0.7500 1 0 x 2 0 1 1.5000 X 2 1.0000 (41) = (101001) (0.6875) = (0.1011) 10 2 10 2 (41.6875) = (101001.1011) 10 2 Fig. 1.2: Conversion of decimal 41.6875 into binary The conversion of decimal 41.6875 into binary is done by first separating the number into its integer part 41 and fraction part 0.6875. The integer part is converted by dividing 41 by r = 2 to give an integer quotient 20 and a remainder of 1. The quotient is again divided by 2 to give a new quotient and remainder. This process is repeated until the integer quotient becomes 0. The coefficients of the binary number are obtained from the remainders with the first remainder giving the low-order bit of the converted binary number.The fraction part is converted by multiplying it by r = 2 to give an integer and a fraction. The new fraction (without the integer) is multiplied again by 2 to give a new integer and a new fraction. This process is repeated until the fraction part becomes zero or until the number of digits obtained gives the required accuracy. Octal and Hexadecimal Numbers The conversion from and to binary, octal, and hexadecimal representation plays an 3 4 important part in digital computers. Since 2 = 8 and 2 = 16, each octal digit corresponds to three binary digits and each hexadecimal digit corresponds to four binary digits. The conversion from binary to octal is easily accomplished by partitioning the binary number into groups of three bits each starting from the least significant bit (LSB) position. The corresponding octal digit is then assigned to each group of bits and the string of digits so obtained gives the octal equivalent of the binary number. Consider, for example, a 16-bit register. Physically, one may think of the register as composed of 16 binary storage cells, with each cell capable of holding either a 1 or a 0. Suppose that the bit configuration stored in the register is as shown in Fig.1.3. Since a binary number consists of a string of 1‘s and 0‘s, the 16 16- bit register can be used to store any binary number from 0 to 2 – 1. For the particular example shown, the binary number stored in the register is the equivalent of decimal 44899. Starting from the low-order bit, we partition the register into groups of three bits each (the sixteenth bit remains in a group by itself). Each group of three bits is assigned its octal equivalent and placed on the top of the register. The string of octal digits so obtained represents the octal equivalent of the binary number. Octal 1 2 7 5 4 3 Binary 1 0 1 0 1 1 1 1 0 1 1 0 0 0 1 1 Hexadecimal A F 6 3 Fig. 1.3: Binary, octal and hexadecimal conversion. Conversion from binary to hexadecimal is similar except that the bits are divided into groups of four. The corresponding hexadecimal digit for each group of four bits is written as shown below the register of Fig.1.3. The string of hexadecimal digits so obtained represents the hexadecimal equivalent of the binary number. The corresponding octal digit for each group of three bits is easily remembered after studying the first eight entries listed in Table 1.1. The correspondence between a hexadecimal digit and its equivalent 4-bit code can be found in the first 16 entries Table 1.1 lists a few octal numbers and their representation in registers in binary- coded form. The binary code is obtained by the procedure explained above. Each octal digit is assigned a 3-bit code as specified by the entries of the first eight digits in the table. Similarly, Table 1.2 lists a few hexadecimal numbers and their representation in registers in binary-coded form. Here the binary code is obtained by assigning to each hexadecimal digit the 4-bit code listed in the first 16 entries of the table. Comparing the binary-coded octal and hexadecimal numbers with their binary number equivalent we find that the bit combination in all three representations is exactly the same. For example, decimal 99, when converted to binary, becomes 1100011. The binary-coded octal equivalent of decimal 99 is 143 (001 100 011) and the binary-coded hexadecimal of decimal 99 is 63 (0110 0011). If we neglect the leading zeros in these two binary representations, we find that their bit combination is identical. This should be so because of the straight forward conversion that exists between binary numbers and octal or hexadecimal. The point of all this is that a string of 1s and 0s stored in a register could represent a binary number, but this same string of bits may be interpreted as holding an octal number in binary-coded form (if we divide the bits into groups of three) or as holding a hexadecimal number in binary-coded form (if we divide the bits into groups of four). The registers in a digital computer contain many bits. Specifying the content of registers by their binary values will require a long string of binary digits. It is more convenient to specify content of registers by their octal or hexadecimal equivalent. The number of digits is reduced by one-third in the octal designation and by one-fourth in the hexadecimal designation. For example, the binary number 1111 1111 1111 has 12 digits. It can be expressed in octal as 7777 (four digits) or in hexadecimal as FFF (three digits). Computer manuals invariably choose either the octal or the hexadecimal designation for specifying contents of registers. Decimal Representation The binary number system is the most natural system for a computer, but people are accustomed to the decimal system. One way to solve this conflict is to convert all input decimal numbers into binary numbers, let the computer perform all arithmetic operations in binary and then convert the binary results back to decimal for the human user to understand. However, it is also possible for the computer to perform arithmetic operations directly with decimal numbers provided they are placed in registers in a coded form. Decimal numbers enter the computer usually as binary- coded alphanumeric characters. These codes, introduced later, may contain from six to eight bits for each decimal digit. When decimal numbers are used for internal arithmetic computations, they are converted into a binary code with four bits per n digit.A binary code is a group of n bits that assume up to 2 distinct combination of 1s and 0s with each combination representing one element of the set that is being coded. For example, a set of four elements can be coded by a 2-bit code with each element assigned one of the following bit combinations; 00, 01, 10, or 11. A set of eight elements requires a 3-bit code; a set of 16 elements requires a 4-bit code, and so on. A binary code will have some unassigned bit combinations if the number of elements in the set is not a multiple power of 2. The 10 decimal digits from 0 to 9 form such a set. A binary code that distinguishes among 10 elements must contain at least four bits, but six combinations will remain unassigned. Numerous different codes can be obtained by arranging four bits in 10 distinct combinations. The bit assignment most commonly used for the decimal digits is the straight binary assignment listed in the first 10 entries of Table 1.3. This particular code is called Binary Coded Decimal (BCD). Other decimal codes are sometimes used and a few of them are given in the section 1.7. It is very important to understand the difference between the conversion of decimal numbers into binary and the binary coding of decimal numbers. For example, when converted to a binary number, the decimal number 99 is represented by the string of bits 1100011, but when represented in BCD, it becomes 1001 1001. The only difference between a decimal number represented by the familiar digit symbols 0, 1, 2, …, 9 and the BCD symbols 0001, 0010, …, 1001 is in the symbols used to represent the digits – the number itself is exactly the same. A few decimal numbers and their representation in BCD are listed in Table 1.3.

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