Transcript operating system

WELCOME To ESC101N:
Fundamentals of Computing
Instructor: Ajai Jain
[email protected]
• Lecture hours
– Monday, Wednesday, Thursday: 8-9am, L17
– Please come to class in time
• Labs
– 2pm-5pm, 11 lab sessions
– No lab this week
• Tutorial
– Tuesday 8-9 am, Tutorial Block 101-110
– No tutorial this week
Administrivia
• Grading
- Exam: 15+15+30
– Project and/or lab test :20
– Weekly lab sessions: 10
– Two surprise quizzes: 10
• Held in tutorial sessions
• No make up for surprise quizzes (so come to tutorial
regularly)
• There will be a course web page with all
info
• Text book
– Nothing specific: your choice
– Suggestion: “Java Elements: Principles of
Programming in Java” by Bailey and Bailey
– More references are on the webpage
• Visit past course sites: www.iitk.ac.in/esc101
What this course is not about
• This is NOT a course on Java
– You will learn how to solve problems with
computers: especially the ones that you cannot
solve with paper and pencil quickly
– The greater part of the lectures will be devoted
to the concepts involved in developing a
computer algorithm
• Sequence of steps that solve a problem
– Java will be used as a vehicle to demonstrate
the concepts
• Do not expect to become an expert in Java after
taking this course
Anatomy of a computer
• What you see
– A monitor, a keyboard, a mouse, a printer …
– Input/Output devices
– Through these you ask the computer to do
something and the computer tells you the
results
• Need a way to convey your commands to the
computer (it is really a stupid device which cannot
do anything on its own)
– Internally
• A central processing unit and a scratchpad (often
called main memory) accomplish the job
Anatomy of a computer
• Central processing unit does not
understand English, not even Java
– It only understands two symbols: 0 and 1
– These are called bits (short for binary digits)
– You encode your algorithm into a high-level
language called Java
• This is called a program
• This is harder to understand than English, but easier
to understand than a 0-1 encoding
• How do I encode a program in 0-1? This is used
only for storing the program in main memory
Anatomy of a computer
• A system software compiler translates the
program into a binary encoding called an
object program
– This is almost understandable to the central
processing unit (often called a microprocessor)
• Another software called a linker adds
something more to an object program to
convert it to an executable
– This is understandable to the CPU
– But somehow it needs to get started executing
Anatomy of a computer
• An operating system loads the executable
in main memory and hands over the control
to the CPU
– Now the CPU starts executing your program
(essentially the binary executable)
– Once in a while it prints something on the
monitor and you appreciate that
• Notice that it is not doing anything on its own, only
doing whatever you have asked it to do
– At some point the CPU completes the
execution and you have all the results
A simple program
• Let’s write a program in English (almost)
– Want to add five numbers a, b, c, d, e and print
the result on monitor
print (monitor, a+b+c+d+e)
– print is used as a function which takes two
arguments: where to print and what to print
– A binary translation of this could convert each
character i.e. p, r, i, n, t, (, m, … into a binary
string e.g., p is the 16th letter of alphabet, so
represent it as 16 zeros; put a 1 to mark the
end of a character
– Now I can design a CPU which can understand
this translation and execute my program
(caution: this is just an example)
“The Computing Stack”
Problem
ESC101N
Central in CS
Algorithm
HLL=Java
Program (HLLs)
Hardware/ HLL Compiler/Linker
Executable binary
software
interface
Operating System
Microarchitecture
Circuits
Hardware
Transistors
Data Representation
• Recap
• Talking to the computer
– Numbers
• Data types
– Integer and long integer
– Floating-point and double
– Character and String
– Boolean
Recap
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Algorithms
Programs
Compilers: executables (binary: 0 and 1)
Operating systems
Central processing unit (CPU) and memory
Numbers
• How does computer understand numbers?
– Knows only two symbols
– We are mostly familiar with decimal numbers
– General number representation in arbitrary
base
– An algorithm for converting between bases
– Special case: base=2 (binary)
– Is there a decimal to binary converter inside the
computer? Compiler does it
– Negative numbers?
• Two representation conventions: sign-magnitude
representation and 2’s complement representation
2’s complement
• Steps involved in converting decimal to 2’s
complement
– Decide the number of bits (should be at least
2+integer part of log2|N|)
– Write the magnitude in binary and append
zeros at the front to fill up the remaining bits
(this is 2’s complement of the positive number)
– Flip all bits (this is 1’s complement of the
positive number)
– Add 1 to it (this is 2’s complement
representation of the negative number)
2’s complement
• 2’s complement to decimal:
– Write down the polynomial expansion in base 2
– Append a negative sign to the leading
coefficient
• Comparison of 2’s complement and signmagnitude
– Range in sign-magnitude with n bits
– Range in 2’s complement with n bits
– Benefits of 2’s complement in binary arithmetic:
makes logic design simpler
– All computers and calculators use 2’s
complement representation
Variables
• Anything that stores a value
– Symbols made up of characters: A-Z, a-z, 0-9,
_, $, …
– Called identifiers
– Must start with a letter or _ or $
– Examples: dayOfTheWeek, day_of_the_week,
dayoftheweek, _dayoftheweek, myname,
myName, …
– Constants are not variables: 7, 100, 2.5, …
– Variables are useful for holding non-constant
values
Data types
• Integer and Long integer
– int and long
– Called keywords: will learn more keywords
– Keywords cannot be used as identifiers
– Example:
int x;
– x is the variable name which we have declared
as an integer
– x is said to be of type integer
– “int” must be written in small characters
– This is called syntax of a language
– Not following it properly leads to syntax errors
Data types
• Floating-point and double
– Used for representing non-integer numbers
– Examples:
float pi, run_rate, Average_score;
double e, interestRate;
– pi, run_rate, Average_score, e,
interestRate are variable names
– Notice the comma separating the names
Why data types?
• Why can’t I just use a variable in
computation?
– Every variable must have a type
– Why must it be declared to have a type?
– Allocation in the memory
– Every variable should get some space in
thememory; otherwise how can you use it for
computation?
– Help the compiler decide how many bits should
be reserved for a variable
– Observation: compiler must know the data type
to size mapping
More data types
• Character and String
– Non-numeric variables
– You may be surprised: we will see nonnumeric variables in computing
– Examples:
char orange;
String something;
– orange and something are variable names
– Note the syntax
– char is used to store a single symbol e.g.
‘f’, ‘2’, ‘$’, ‘ ‘, …
– String is used to store a sequence of
symbols e.g. “My name is Tintin.”
Even more data types
• Boolean
– Can take two values only: true or false
– true and false are two boolean constants
– You may be surprised to see this type
• Byte
– 8-bit integer
– Example: byte x;
– Range of x?
• Short
– 16-bit integer
– Example: short y;
Constants
• Integer (int)
1, 234, -56, 0, …
• Floating-point and double (float, double)
4.5, 56.789, 3.14, 2.71, 0.693, 4.5e3, 45000e-1
• Character (char)
‘a’, ‘_’, ‘ ‘, ‘A’, ‘m’, ‘$’, …
• String (String)
“Hi there”, “How are you?”, “This is esc101!!”
• Boolean (boolean)
true, false