esc101-lec4

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ESc101: Fundamentals of Computing

2011-12-Monsoon Semester

Please switch off your mobile phones.

Lec-04 Dheeraj Sanghi, CSE Dept., IIT KanpurESc101, 2011-12-Monsoon

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Announcements

• Section E11 will have a lab on Fridays.

• Section E15 will have a lab on Wednesdays.

• Monday lab and grading policy is on moodle.

• Tuesday/Wednesday labs will be on moodle today.

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Recap

• Algorithm – Converting binary to decimal– Flow chart is on moodle

• Started with basic model of a computer


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Recap: Model of a Computer

• From the perspective of programming, the main components are:– CPU: Central Processing Unit

• ALU: Arithmetic and Logic Unit

• Control Unit

– RAM: Random Access Memory

• Additional devices of interest are:– Storage (e.g., Disks)

– Input (e.g., keyboard, mouse)

– Output (e.g., monitor, printer)

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Recap: Memory

• Bit: Having a value of 0 or 1

• Byte: Having 8 bits

• Kilobyte (KB): 1024 bytes

• Megabyte (MB): 1024 KB

• Gigabyte (GB): 1024 MB

• Is generally byte addressable– 32 bit addresses would mean a maximum memory of 4 GB.

• Any byte can be read at any time – random access

• Programs and Data are stored in memory.


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Recap: ALU

• Can execute arithmetic and logical operations– Add, Subtract, Multiply, Divide (for integers)

– Right shift, Left shift

– Comparison of two bytes or two numbers

– What operations needs to be done next on what inputs is controlled by the program

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Recap: Control Unit

• Responsible to bring the next instruction of the program.

• It then decodes the instruction and tells ALU what to execute next

• It is also responsible to bring in operands from memory to ALU.

• Similarly, it stores the results of the operation back to memory.


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Recap: Binary Format

• The basic memory unit in computer is a bit– Computer can only understand binary: 0 and 1.

– Computer can only operate on binary input and produce binary output.

– For example, if number 17 has to be stored in a byte, it will be stored as 00010001.

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Program

• The Control Unit can only understand a sequence of binary bits.

• So the program has to be in binary format, also known as “machine language.”

• Program is a sequence of instructions in machine language.

• The execution happens as follows:

– Control unit fetches an instruction

– If operands are in memory locations, fetch them as well.

– Ask ALU to execute the instruction on the operands

– The result needs to be stored in the memory location.

– Next instruction is fetched, unless the previous instruction asked the Control Unit to fetch a difference instruction.


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A Small Program

0000010010011001 – read memory location 0010000010010011010 – read memory location 0100000011011001000 – add two numbers read

0000010011010001 – store the result in memory location 001

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Assembly Language

• Extremely difficult to write program in machine language.

• Even more difficult to understand machine language.

• Hence the designers of CPU also design a higher level language called, Assembly Language.

• This has mnemonics for both operations and the operands.– Hence somewhat easier to follow


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Example Program

• MOV NUM1, R1

• MOV NUM2, R2

• ADD R1, R2

• MOV R1, NUM1

• Move contents of memory location NUM1 to location R1

• Move contents of memory location NUM2 to location R2

• Add contents of R1 and R2 and store the result in R1

• Move the contents of location R1 to Memory location NUM1

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Assembler

• Assembler translates the assembly language program to a machine language program.


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Need for a Higher Level Language

• Very difficult to write large programs in assembly language.

• New programming languages have been designed to write programs easier.

• These include: C, Pascal, Fortran, Cobol, C++, Java, Lisp, Python, …

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Compiler

• Compiler is a program that translates a higher level language to machine language

• There are many C Compilers available.– We will be using ‘gcc’.


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Input/Output

• To solve problems, we need to provide input to the program, and the program needs to give output.

• The program should be able to read from keyboard, understand mouse clicks, etc.

• The program should be able to write on the monitor or display unit.

• The programs should be stored on permanent storage, e.g., disks.

• All this is done with the help of Operating System.

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Operating System

• Manages all resources of the computer.– Allocates space on disk for files.

– Allocates memory for your programs

– Allows multiple programs to be running in “parallel.”

– Reads what you type on keyboard.

– Maintains access control• Files of one user cannot be read by another user, unless

allowed.

• Examples: Linux, Windows, Solaris, Android, …


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Our First ‘C’ Program

/* This program computes Body-Mass Index */

#include <stdio.h> /* To include I/O Header files */main () /* Main function heading */{

int mass; /* To store weight of the body in KGs */float height; /* To store height in meters */float bmi; /* To store body-mass-index */

printf (“Body Weight in KG = ?”); /* Prompt user for Weight */scanf (“%d”, &mass); /* Read Weight in KG */printf (“Height in Meters = ?”); /* Prompt user for height */scanf (“%f”, &height); /* Read Height in meters */

bmi = mass / (height * height); /* Compute BMI */

printf (“BMI value is: %f\n”, bmi); /* Print the answer */}

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Points to Note

• #include

• Comments

• main ()

• int

• float

• printf

– %f, \n

• scanf

– %d, %f, &

• Variable names

• Arithmetic expressions

• Assignment

• Types

• Integer versus Floating point operations


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Any Questions?

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