Computer Science Engineering : Computer organization & architecture, THE GATE ACADEMY

COMPUTER ORGANIZATION

&

ARCHITECTURE

For

Computer Science &

Information Technology

By

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Syllabus Computer Organization

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Syllabus for

Computer Organization and Architecture

Machine instructions and addressing modes, ALU and data-path, CPU control design, Memory

interface, I/O interface (Interrupt and DMA mode), Instruction pipelining, Cache and main

memory, Secondary storage.

Analysis of GATE Papers

(Computer Organization and Architecture)

Year Percentage of marks Overall Percentage

2013 11.00

9.48%

2012 6.00

2011 7.00

2010 9.00

2009 6.66

2008 16.00

2007 9.33

2006 9.33

2005 14.67

2004 10.00

2003 5.33

Contents Computer Organization

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C O N T E N T S

Chapter Page No.

#1. Introduction 1 – 28

Computer Design 1 – 4

Binary Data Representation 4 – 12

CPU Design 12 – 13

Binary Adder 13 – 19

Arithmetic Logic Unit 19 – 22

Assignment 1 23

Assignment 2 24 – 26

Answer Keys 27

Explanations 27 – 28

#2. Memory 29 – 60

Memory Parameter 29 – 30

Main Memory 31 – 36

Cache Memory 36 – 42

Virtual Memory 42 – 45

Solved Examples 45 – 47



Answer Keys 54


#3. Pipelining and Vector Processing 61 – 82

Parallel Processing 61 – 65

Pipelining 65 – 74

Solved Example 74



Answer Keys 79


#4. Instruction Set and Addressing Mode 83 – 99

Introduction 83

Instruction format 83 – 86

Addressing Modes 86 -87



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CISC and RISC 87 – 88

Solved Example 89



Answer Keys 95


#5. CPU Operation and Design 100 - 110

CPU Operations 100 – 103

Data Path and Control Path 103 – 104

Machine Cycle 104 – 107

Assignment 108 – 109

Answer Keys 110

Explanations 110

#6. I/O Interface (Interrupt and DMA Mode) 111 – 131

Introduction 111

Difference between the Computer and Peripheral Devices 111

I/O Bus and Interface Modules 111 – 112

Modes of Data Transfer 112 – 116

I/O Communication Technique 116 – 118

Direct Momory Access (DMA) 119 – 124



Answer Keys 129


#7. Control Unit Design 132 – 141

Von-Neumann Architecture 132 – 133

Von-Neumann Bottleneck 133

Harvard Architecture 134

Control Unit 134 – 137

Solved Examples 137 – 138

Assignment 139 – 140

Answer Keys 141

Explanations 141



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Module Test 142 – 153

Test Questions 142 – 148

Answer Keys 149


Reference Books 154

Chapter-1 Computer Organization

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CHAPTER 1

Introduction Computer Architecture Computer architecture deals with the structure and behavior of the computer system. It includes the information formats, the instruction set and the hardware units that implement the instructions along with the techniques for addressing memory. Computer Organization Computer organization deals with the way the various hardware components operate and the way they are connected together to form the computer system. It also deals with the units of the computer that receive information from external sources and send computed results to external destinations.

Computer Design Computer design is concerned with the hardware design of the computer. This aspect of computer hardware is sometimes referred to as computer implementation. Figure 1.1 shows the basic building blocks of a typical computer system.

The basic blocks available in any digital computer are, Arithmetic Logic Unit (ALU), Control

Unit, Memory and Input/Output Unit.

Input Unit It is a medium of communication between the user and the computer. With the help of input unit only, it is possible to enter programs and data to the computer.

Examples Keyboard, floppy disk drive, hard disk drive, mouse, Magnetic Ink Character Recognition (MICR), Optical Character Recognition (OCR), paper tape reader, Magnetic tape reader, Scanner etc.

Output Unit It is a medium of communication between the computer and the user. With the help of output unit only, it is possible to get results from the computer. Example: Printers, Video Display Unit (VDU), Floppy disk drive, Hard disk drive, Magnetic tape drive, punched cards, paper tape, plotter, digitizer etc.

MEMORY

INPUT OUTPUT

ALU

CPU

CU

Fig. 1.1 Basic functional units of a computer



Memory The memory unit is responsible for storing the user programs and data. The digital computer memory unit mainly consists of two types of memories: Read Only Memory (ROM) and Read Write Memory (R/WM) or Random Access Memory (RAM).

ROM ROM is used to store permanent programs or system programs. It does not have write capability. Types: PROM, EPROM, EEPROM

RAM It is also called user memory because the user programs or application programs are stored in this memory. The CPU is able to write or read information into or from this type of memory. Types: static, dynamic, scratch pad etc.

Central Processing Unit (CPU) The ALU and Control Unit together are called CPU. It is the heart of any digital computer.

ALU (Arithmetic Logic Unit) The data processing part of CPU is responsible for executing arithmetic and logical instructions on various operand types including fixed point and floating point numbers. Various circuits used to execute data processing instructions are usually combined in a single circuit called an arithmetic logic unit (ALU). The complexity of an ALU is determined by the way in which its arithmetic instructions are realized. Simple ALU’s that perform fixed point addition and subtraction as well as word based logical operations, can be realized by combinational circuits. Much more extensive data processing and control logic necessary to implement is floating point arithmetic in hardware. Example For finding the sum of a series , how will you use an ALU without a multiplier unit but with an adder and barrel shifter? Barrel shifter is a shifter that does shift at an instance. Solution Left shift by 7 means multiplication by 128,6 means multiplication by 64,5 means multiplication by 3 and means multiplication by . Hence the shift operations will be faster for computing . Using the barrel shifter seven times for shifting by 3 .. we find and store left shifted values of in a register and add these. Two Types of ALUs

1. Combinational ALUs The simple ALU combine the functions of ’s complement adder-subtracter with those of a circuit that generates word based logic functions of the form f(x, y). For example AND XOR and NOT. They can thus implement most of a CPU’s fixed point data processing instructions.

2. Sequential ALU’s Both multiplication and division can be implemented by combinational logic. It is generally impractical to merge these operations with addition and subtraction into a single, combinational ALU.



The combinational multipliers and dividers are costly in terms of hardware. They are also much slower than addition and subtraction. An n-bit combinational multiplier or divider is typically composed of n or more levels of add-subtract logic making multiplication and division at least ‘n’ times slower than addition (or) subtraction, where addition and subtraction each take one clock cycle, while multiplication and division are multicycle operations.

Example How many 16-bit ALU slices can be used for designing a 64-bit ALU? Solution Four slices will be needed in parallel if ALU 64-bits operations are to take nearly the same time as 16-bit slice, and four slices will be needed in series if ALU 64- bits operations are to take nearly four times of 16-bit slice. Interconnection of basic of blocks In order to form an operational system individual components or parts of computer that we have discussed need to be connected in some organized way for this purpose ‘bus’ is introduced which is defined as a group of lines that serves as a connecting path for several devices. These lines carry data, address and control signals. Figure 1.2 shows a exemplanary interconnection using single bus structure

Since, this bus can be used only for single transfer at a time Multiple buses are introduced so as to achieve more concurrency in operations so that two or more transfers can be carried out at the same time. Hereby, increasing performance but at an increased cost. Control Unit The control unit (CU) is the heart of the CPU. Every instruction CPU supports has to be decided by the CU and executed appropriately. Every instruction consists of a sequence of micro instructions that carries out that instruction the CU controls the elements inside the CPU the interface to the external data rate. Following are the basic tasks of the control unit 1. For each instruction the CU causes the CPU to go through a sequence of control steps: 2. In each control step the CU issues a set of signal which caused the corresponding

operation to be executed.

Output

Memory

CPU

Fig1.2. Single-bus Structure

Input



3. The signals to be generated by CU depends upon the actual step to be executed, the condition and states of flag register of the processor, the actual instruction to be executed and any external signals received on the system bus (i.e., interrupt signals)

Binary data representation In digital computer, integers are represented as binary numbers of fixed length. A binary number of length x is an ordered sequence (x x x . . . x x ) Of binary digits where each digit x (also known as a bit) Can assume one of the values 0 or 1 The above sequence of x digits represents the integer value

X x x

. . . . x x ∑x .

Since operands and results in an arithmetic unit are started in resisters of a fixed length, there is a finite number of distances that can be represented with in an arithmetic unit. Let x and x denote the smallest and largest representable value. We say that (x x ) is the range of the number A sequence of n digits in a register does not necessarily have to represent an integer. We can get such a sequence to represent a mixed number that has a fractional part as well as an integral part. This is done by portioning the n digits into two series; k digits in integral part and in digits in the fractional part, satisfying k +m = n

(x x . . . . . x x ⏟

x x . . . . . x ⏟

)

The above sequence of n digits represents the value

X x x

. . . . . x x x

. . . . . x ∑ x

The radix point is now stored in the register but is understood to be in a fixed position between k most significant digits and the m least significant digits. For this reason we call such representation as fixed-point representation

IR

Common signals internal to the CPU

Common signals internal on system bus

Signals from system bus

Control Unit (CU)

Clock

Flags



Representation of negative numbers For fixed point numbers, we have to determine the way negative numbers are represented. Two different terms are commonly used:

1. Sign and magnitude representation Which is also called the signed-magnitude method

2. Complement representation Which comprises two alternatives

Signed magnitude In this representation the first digit is the sign digit and the remaining ( ) digits represent magnitude. In binary case sign bit is normally selected to be o for +ve numbers and 1 for +ve ones. Let the ( ) digits representing the magnitude be partitioned into ( ) and m digits in the integral and fractional part, respectively. The largest representable number is then,

( )

Thus the range of +ve numbers is [ ] and the range of –ve number is ( ).

We therefore have two representations of zero, one +ve and –ve. A major disadvantage of the signed-magnitude representation is that the operation to be performed may depend on the signed of the operands. This is avoided in the complement representation methods Complement representation There are two alternatives 1. Radio complement (also called ’s complement) 2. Diminished radix complement (also called ’s complement) In both complement method, a +ve number is represented in the same way as in the signal

magnitude method, where a negative number – is represented by ( ), where R is a constant. Such a representation satisfies the basis identify ( ) One of the major advantages of a complement method is that no decisions have to be made before executing an addition or substation ( ) ( )

If the –ve result – ( ) is already in the same complement form. However if , the correct result should be , while ( ) ( ). The addition term R must be discarded, and the value of R should be selected to simplify or completely eliminate this correction step. In case of ’s complement the value of R is and for complement the value of R is . The range of numbers in the two’s complement for is

This range is slightly a symmetric as then is one more –ve number than are +ve numbers, on the hand these is a unique representation for 0. The range of representable number in the one’s complement system is symmetric

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Computer Science Engineering : Computer organization & architecture, THE GATE ACADEMY