Introduction
Overview of a Computer - Hardware
·
Central
Processing Unit (CPU) controls the flow of instructions and data and performs
the necessary manipulation of data
·
Primary
storage (memory) is used to store information for immediate access by the CPU
Note
that there are many levels of cache
used in primary storage
·
Secondary
storage devices (e.g., the hard drive, CD-ROM drives, tapes, etc.) provide
permanent storage of large amounts of data, but are much slower than primary
storage
·
Input
and Output devices provided interfaces between the computer and the user
Overview
of a Computer - Data Representation
·
The
first computers used vacuum tubes to hold data
·
Vacuum
tubes have two states - ON and OFF
·
An ON
state represents a 1
·
An OFF
state represents a 0
·
Eight
vacuum tubes strung together can represent an 8 digit string of 0s and 1s
·
Put
another way, this string is an 8 digit binary (base 2) number
·
We use
decimal (base 10) numbers in our daily life
·
A
decimal number is a string of digits whose values are drawn from the set
{0,1,2,3,4,5,6,7,8,9}
·
In
general, a number system is simply a way of representing numbers
·
A
number system has a base (the number of digits used in the number system)
·
A binary number has a base of 2 where the
valid digits are 0 or 1
·
E.g.,
1001 binary == 9 decimal (1*8 + 0*4 + 0*2 + 1)
·
An octal number has a base of 8 where the
valid digits are 0 through 7
·
E.g.,
031 octal == 25 decimal (3*8 + 1)
·
A decimal number has a base of 10 where
the valid digits are 0 through 9
·
E.g.,
2000 decimal == 2000 (Y2K bug ;-))
·
A hexadecimal number has a base of 16
where the valid digits are 0 through F, i.e. {0,1,2,3,4,5,6,7,8,9,A,B,C,D,E,F}
·
E.g.,
xABBA hex == 43962 decimal (10*4096 + 11*256 + 11*16 + 10)
If you want to know about basics of conversion then click here
Overview of a Computer - Data
Representation (2)
Powers of 2
2^0
|
1
|
2^11
|
2048 2K
|
2^1
|
2
|
2^12
|
4096 4K
|
2^2
|
4
|
2^13
|
8192 8K
|
2^3
|
8
|
2^14
|
16,384 16K
|
2^4
|
16
|
2^15
|
32,768 32K
|
2^5
|
32
|
2^16
|
65,536 64K
|
2^6
|
64
|
2^17
|
131,072
128K
|
2^7
|
128
|
2^18
|
263,144
256K
|
2^8
|
256
|
2^19
|
524,288
512K
|
2^9
|
512
|
2^20
|
1,048,576 1M
|
2^10
|
1024 1K
|
2^21
|
2,097,152 2M
|
1
KILO = 2^10 = 1024
1
MEG = 2^20 = 1024*1024 = 1,048,576
1
GIGA = 2^30 = 1024*1024*1024 = 1,073,741,824
To evaluate a binary number, say 101101,
simply add up the corresponding powers of 2:
Overview of a Computer - Hexadecimal
Numbers
· A
hexadecimal number is a string of hexadecimal digits
·
Digits
A, B, C, D, E, F represent the numbers 10, 11, 12, 13, 14, and 15
·
Hexadecimal
is popular in the computer field because it can be used to concisely represent
a long string of binary digits
·
Consider
a 16 digit binary number:
1011000111000101
·
Break
this up into groups of 4:
1011
0001 1100 0101
·
Convert
each group of 4 into decimal:
11
1 12 3
·
Then
convert each decimal number into hex:
B1C3
·
And
you now have the number: Baker 1 Charlie 3
Able
= A = 10 = 1010
Baker
= B = 11 = 1011
Charlie = C = 12 = 1100
Dog
= D = 13 = 1101
Easy
= E = 14 = 1110
Fox
= F = 15 = 1111
·
By the
same token, the HEX number 3F2C1596
represents the binary string:
0011 1111 0010 1110 0001 0101 1001 0110
Overview of a Computer -
Primary Storage
·
Primary
storage, also known as main memory
or RAM (random access memory) is used to store information for immediate access
by the Central Processing Unit (CPU)
·
Memory
can be viewed as a series of memory
cells with each cell having its own individual address
·
E.g.,
think of a bank of mailboxes at a post office
·
The
information contained in a memory cell is called the contents of that cell
·
Memory
cells can be used to store data,
such as characters or numbers
·
Internally,
of course, they are all numbers, but we can choose to interpret some numbers as
characters
·
They
can also be used to store program
instructions
·
These
are “special” numbers that are meaningful to a CPU!
·
The
smallest unit of computer storage is a bit,
as in binary digit
·
Most
computers group bits together to form larger entities
·
E.g., 8 consecutive bits often form a byte and 32 consecutive
bits often form a word
·
A word
on an Intel 286 computer is 16 bits or 2 bytes
·
A word
on an Intel 386, 486, and Pentium computers is 32 bits or 4 bytes
·
The
next generation of Intel computers (e.g. the Merced) will use 64 bit words, i.e. 8 bytes
·
The
DEC Alpha computer is currently using 64 bit words
·
Many
mainframe computers, such as the Unisys A-Series, use 48 bit words
·
Memory
cells in a computer are typically one
byte or one word in size
·
A word
is a unit of information that can be transferred to and from memory
·
A kilobyte of memory, as in 1K, is 1024
bytes of memory
·
A megabyte of memory, as in 1M, is 1024K
of memory, or 2^20 bytes
·
A gigabyte of memory, as in 1G, is 1024M
of memory, or 2^30 bytes
·
Note
that a 32 bit word can represent 2^32 different possible values
Programming Languages - Low Level Languages
·
Computers
only do what they are told to do
·
Except
in Hollywood movies, e.g., 2001, Terminator 2,
the Matrix, etc. ;-)
·
In
order for a computer to perform a task, it must be given a series of specific
instructions in a language it can understand
·
The
fundamental language of any computer is its machine
language
·
This
is typically sequences of zeroes and ones
·
In the
very early days of computers, this
was the only way one could write
programs!!!
·
To relieve
the suffering of these early programmers, a higher level language called assembly language was developed
·
Assembly
language contains mnemonic words and
symbols for the binary machine instructions
·
An assembler maps assembly language
instructions into machine language instructions
·
Assembly
language programming is indeed a significant improvement over machine language
programming
·
However,
it has the following drawbacks:
·
Machine
dependent - each computer architecture has its own unique assembly language
·
Low
level instructions - writing programs is very time consuming, tedious, and
error-prone.
Programming Languages -
High Level Languages
·
A
general trend in computing over the past 4 decades is to elevate programming
from low level to higher level languages
·
I.e.,
high level languages are geared more toward people writing the programs rather
than the computer
·
Assembly
language instructions map directly to machine instructions
·
High
level language instructions must be translated/compiled
into machine instructions
·
High
level languages are more “problem-oriented” than assembly/machine languages
·
E.g,
they require little or no knowledge of the underlying computer architecture
·
Learning
how to write/debug programs in high level languages is much easier and less
error-prone than learning how to write/debug equivalent programs in assembler
·
E.g.,
high level languages required fewer statements to do the same thing as
assembler
·
Programs
written in high level languages can be ported much more easily to different
computer architectures
·
E.g.,
the compiler encapsulates the machine-dependent details of the target assembly
language
·
A
special program called a compiler is
needed to translate a program written in a high level language into assembly
code (which is then transformed into native machine code by an assembler)
·
The
statement written in the high level language are called source code
·
The
compiler/assembler's output is called object
code
Language
Taxonomy
Significant Points about C
Small
number of keywords
Widely
available, particularly on personal computers and workstations
Can be used
very portably
Standard
library
Preprocessor
may be used to isolate machine dependent code
Unlike
Pascal, which has many dialects
Native
language of UNIX (tm) and Windows NT
Terse
Powerful
set of operators
Statements
can be very powerful
Some are bit level operators
Designed to
be implemented efficiently on many machines
Modular --
functions
Parameters
are typically passed `by value’
No nested
functions
Syntax is
complicated
Semantics of
certain features are complex and error-prone.
A
Comparison of Programming Language Philosophy
Pascal
Strict
Parent (a “bondage and discipline” language ;-))
Restricts
programmer for his/her own good
A white,
automatic transmission automobile with lots of safety features (e.g., air bags, controls that limit speed to 55
miles per hour and prohibit leaving the lights on or locking the keys in the
car)
C
A
permissive, easy going parent (a ‘lassize-faire’ language ;-))
Assumes
that the programmer knows what he/she is doing and will assume responsibility
for his/her actions. (Some describe it
as a “gun with which you can shoot yourself in the foot.”)
A bright
red ’65 Corvette with a big block
engine, manual transmission, optional seat belt, and with fuzzy dice hanging
from the rear view mirror.
C++
A less
permissive, yet open minded parent (e.g.,
‘Thomas Huxtable’ ;-))
Assumes
that the programmer generally knows what he/she is doing, but provides more
checking by default. (“With C++ it’s
harder to shoot yourself in the foot, but when you do, you’ll blow off both of
your legs” – Bjarne Stroustrup)
A bright
red 2000 Corvette with a 6 speed manual
transmission, air bag, and heads up
display, many on board computers
Example C Program
/*
File: hello.c
Description: Prints a greeting
to stdout.
*/
#include <stdio.h>
int main (void) /*
execution starts in main */
{
printf ("Hello
world.\n");
return 0;
}
|
All
C programs must have a function in it called main
Execution
starts in function main
C
is case sensitive!
Comments
start with /* and end with */. Comments
may span over many lines.
C
is a “free format” language.
The #include
<stdio.h> statement instructs the
C compiler to insert the entire contents of file stdio.h in its place and
compile the resulting file
Review – Lecture - 1
Introduction to C
Computer Program:
A
set of instructions given to the computer to perform a particular task is
called Computer program software.
Programming Language:
·
Provides
a way of communication between user and computer.
·
Computer
program is written in a programming language.
·
There
are many programming language. Like C, C++, etc.
·
Each
language has its own set of rules to write computer programs.
Computer language is used
to compile the software. A software is compiled by computer language and is a
program on the computer.
Types
of Programming Language:
·
High level programming
languages
·
Low level programming
languages
ü High level programming
languages are more structured, are closer to spoken language and are more
intuitive than low level languages.
ü Higher level languages
are also easier to read and can typically sometimes work on many different
computer operating systems.
ü Some examples of higher
level languages are Java, Visual Basic, COBOL, BASIC, C++, and Pascal to name
only a few.
ü Lower level languages are
typically assembly languages that are machine specific.
Computers run
instructions at the binary level, interpreting zeros and ones as instructions.
Rather than writing programming code at this level, we've developed languages
that compile into the zeros and ones that computers understand. As these
languages become more robust, they get further and further way from zeros and
ones, becoming higher level languages.
Characteristics of High
level programming languages
ü Easy to Learn
ü Machine Independent
ü Shorter Programes
ü Well defined Syntax and
Standard
ü Easy to detect Errors
Machine Language:
Machine language is the actual bits used to control the
processor in the computer, usually viewed as a sequence of hexadecimal numbers
(typically bytes). The processor reads these bits in from program memory, and
the bits represent "instructions" as to what to do next. Thus machine
language provides a way of entering instructions into a computer (whether
through switches, punched tape, or a binary file).
Assembly Languge:
Assembly language is a more human readable view of
machine language. Instead of representing the machine language as numbers, the
instructions and registers are given names (typically abbreviated words, or
mnemonics, eg ld means "load"). Unlike a high level language, assembler
is very close to the machine language. The main abstractions (apart from the
mnemonics) are the use of labels instead of fixed memory addresses, and
comments.
An assembly language program (ie a text file) is
translated to machine language by an assembler. A disassembler
performs the reverse function (although the comments and the names of labels
will have been discarded in the assembler process).
Machine language faster
than assembly language even than assembly language depend upon machine language
A programming language that is once removed from a computer's machine language.
Machine languages consist entirely of numbers and are almost impossible for
humans to read and write. Assembly languages have the same structure and set of
commands as machine languages, but they enable a programmer to use names
instead of numbers.
Each type of CPU has its own machine language and assembly language, so an
assembly language program written for one type of CPU won't run on another. In
the early days of programming, all programs were written in assembly language.
Now, most programs are written in a high-level language such as FORTRAN or C.
Programmers still use assembly language when speed is essential or when they
need to perform an operation that isn't possible in a high-level language.
Program Source Code and Object Code:
"Source" and "object" are not well-defined classes of code. They are actually relative terms. Given a device for transforming programs from one form to another, source code is what goes into the device, and object code (or "target" code) is what comes out. The target code of one device is frequently the source code of another. For example, both early C++ compilers and the Kyoto Common Lisp compiler produced C code as their output. C++ (or Common Lisp) was the source language; C was the target language. This output was then run through a C compiler to produce symbolic assembler code as output. This then had to be run through yet another program, the "assembler", to produce binary machine code that could be directly executed by a processor.
Programs typically go through a series of transformations from higher level to lower level languages. The assembler language code that a C compiler produces as "object" code is source code for the assembler. Today, the GNU C compiler (known as gcc) will deliver assembler language code instead of binary if the user requests it by specifying the -S switch.
Language Translator:
All
computer programs run on machine code that is executed directly on computer.
Types of Language Translator:
Compiler -- reads human-readable source code, produces
machine-executable binary code. Examples are C, COBOL, Java, etc. Easiest for
humans to program, but does not always produce the most efficient executables.
Interpreter -- Reads human-readable code, line at a time, and produces and
executes machine instructions "on the fly". Example is good old
BASIC. Good for testing, but is VERY slow.
Assembler -- Converts machine-manipulation coding directly into binary machine
instructions. Produces the most efficient executables, but is the most
difficult (for humans) to work with.
What is the difference
between assemblers interpreters and compilers?
Assemblers use the basic building blocks of the command processor code to write
programs and is the language closest to the binary on which all computers
operate, although it is difficult to use it does work well for things like
networking and communication protocols.
Interpreters are just
what they say, they translate the code in real time as you operate the program,
then process it, and are therefore the slowest.
Compilers translate the
code into a format the computer understands prior to the execution (or
distribution) of the code and is therefore the easiest to use as it combines
the better attributes of both programming methods into one easy to use package.
Assemblers convert Assembly code to machine
code. Interpreters convert high level code to real-time machine code and store
it in the memory for direct execution. Compilers convert high level code to
real-time machine code or some intermediate code and store it in a file for
later execution.
Compiler convert whole of the sentence once into the machine langauge but the
interpreter convert one by one. A compiler translates code from a source
language (e.g. C, C++, Java) to a target language, which can then be executed
by a (virtual or physical) machine. An interpreter reads code in an interpreted
language (e.g. PHP, Perl, JavaScript) and directly executes the contained
instructions.
C-Language
C is one of the most
widely used programming languages of all time and there are very few computer architectures for which a C compiler does not exist. C has
greatly influenced many other popular programming languages, most notably C++, which began as an
extension to C.
Advantages of C-Language:
ü Easy to Understand,
Learn, Use
ü Middle-Level Language
ü Basis for C++
Basic Strutruture of C Programe:
Every
C program consists of one or more functions. A function is nothing but a group
or sequence of C statements that are executed together. Each C program function
performs a specific task. The ‘main()’ function is the most important
function and must be present in every C program. The execution of a C
program begins in the main() function.
The
basic structure of C program consists of the following main pasrts.
ü Preprocessor Directives
ü The main() function
ü
C
statements
Preprocessor directives:
The instructions that are given to compiler before
the beginning of the source code are called preprocessor directivities. It is
also known is compiler directivities. The preprocessor directivities begins
with a # sign followed by word “include” or “define”.
Header files in C-Language
ü At the beginning
of program headers files are included.
ü The headers files
are the part of the C compiler.
ü It contains the
information of standard library functions.
ü The functions are
called in the main body of the program to perform different tasks.
ü In C language,
there are several header files. Each header file contains declarations of one
type of functions. i.e. “math.h” contains the declarations of mathematical
functions.
Each header file has an extension “.h”. Include # is used to add the header
file into program. The name of header file is written between angle brackets
“< >”
The syntax of header file
# include
<name of header file>
For examples;
“#include <stdio.h>”. “stdio.h” is header file. The word “stdio”
stands for standard input and output.
“# include <conio.h>, is use its functions
“clrscr()”. The “clrscr()” is used to clear the clear screen of monitor.
The main() Function
ü C-programs
consist of one or more functions.
ü Every C program
must have the main () functions.
ü The main ()
functions comes after the preprocessor directives.
ü That point at
which execution of program is started.
The program is executed within an operating system.
At the end of program execution, the main () function returns a value to the
OS. The OS uses this value to determine whether the program is executed or not.
- int main() can be used
to return integer value to the OS.
- void main() means that
the program will return any value to the OS after its execution.
- main(void) indicates
that program takes no parameters when program is executed from command
line.
The syntax of main () function is:
Void main (void)
{
C-program statements, each end with semicolon (;)
}
Statements of C-program is written between Curly
brackets i.e. {}
For example:
# include <stdio.h>
#include<conio.h>
main ()
{
clrscr();
printf (“AOA”);
}
In the above program two statements are written in
the body of main () functions. Each statements end with semicolon (;).
Writing
your first C program
Learning
any programming language becomes easy with a hands-on approach, so let’s get
right to it. The following is a simple C program that prints a message ‘Hello,
world’ on the screen.
/*
This
program prints a message
*/
1.#include<stdio.h>
2.main()
3.{
4. printf(“Hello,
world”);
5.}
|
Type
this program in any text editor and then compile and run it using a C-compiler.
However, your task will become much easier if you are using an IDE such as
Turbo C (available freely from http://community.borland.com/downloads/). Download and install Turbo C on your
machine and follow the steps given below in order to type and run the program
given above:
1. Go to the directory where you have
installed Turbo C.
2. Type TC at the DOS command prompt.
3. In the edit window that opens, type the
mentioned program above.
4. Save the program as hello.c by pressing F2
or Alt + ‘S’.
5. Press Alt + ‘C’ or Alt + F9 to compile the
program.
6. Press Alt + ‘R’ or Ctrl + F9 to execute the
program.
7. Press Alt + F5 to see the output.
If
you are using a Linux machine, you have to follow the steps mentioned below:
- Go
to the Linux command prompt (# or $).
- Type
vi.
- The
vi editor will open.
- Type
the program in the editor.
- Press
‘Esc + Shift + ‘:’.
- Type
‘w’ + ‘q’ followed by file name ‘hello.c’.
- At
the command prompt type ‘gcc hello.c’.
- Type
‘./a.out’ to run the program.
Note:
C is a case-sensitive language. It makes
a distinction between letters written in lower and upper case. For example, ‘A’
and ‘a’ would be taken to mean different things. Also, remember that all C
language keywords should be typed in lower case.
Understanding
the program
In
the program you saw above, the information enclosed between ‘/* */’ is called a
‘comment’ and may appear anywhere in a C program. Comments are optional and are
used to increase the readability of the program.
The
‘#include’ in the first line of the program is called a preprocessor
directive. A preprocessor is a program that processes the C program
before the compiler. All the lines in the C program beginning with a hash (#)
sign are processed by the preprocessor.
‘stdio.h’
refers to a file supplied along with the C compiler. It contains ordinary C
statements. These statements give information about many other functions that
perform input-output roles.
Thus,
the statement ‘#include<stdio.h>’ effectively inserts the file ‘stdio.h’
into the file hello.c making functions contained in the ‘stdio.h’ file
available to the programmer. For example, one of the statements in the file
‘stdio.h’ provides the information that a function printf() exists, and
can accept a string (a set of characters enclosed within the double quotes).
The
next statement is the main() function. As you already know, this is the
place where the execution of the C program begins. Without this function, your
C program cannot execute.
Next
comes the opening brace ‘{’, which indicates the beginning of the function. The
closing brace ‘}’ indicates the end of the function.
The
statement printf() enclosed within the braces‘{}’ informs the compiler
to print (on the screen) the message enclosed between the pair of double
quotes. In this case, ‘Hello, world’ is printed. As mentioned earlier,
the statement printf() is a built-in function shipped along with the C
compiler. Many other built-in functions are available that perform specific
tasks. The power of C lies in these functions.
Be cautious about errors!
Errors/bugs
are very common while developing a program. Errors in the program are called
bugs. The process of finding and removing bugs in a program is called
debugging. If you don't detect them and correct them, they cause a program to
produce wrong results. There are three types of errors — syntax, logical, and
run-time errors. Let us look at them:
- Syntax
errors: These errors occur because of wrongly typed statements, which are
not according to the syntax or grammatical rules of the language. For
example, in C, if you don’t place a semi-colon after the statement (as
shown below), it results in a syntax error.
printf(“Hello,world”)
– error in syntax (semicolon missing)
printf("Hello,world");
- This statement is syntactically correct.
- Logical
errors: These errors occur because of logically incorrect instructions in
the program. Let us assume that in a 1000 line program, if there should be
an instruction, which multiplies two numbers and is wrongly written to
perform addition. This logically incorrect instruction may produce wrong
results. Detecting such errors are difficult.
- Runtime
errors: These errors occur during the execution of the programs though the
program is free from syntax and logical errors. Some of the most common
reasons for these errors are
- when you instruct your computer to divide a number by zero.
- when you instruct your computer to find logarithm of a negative
number.
- when you instruct your computer to find the square root of a
negative integer.
Unless
you run the program, there is no chance of detecting such errors.
Try
it out
1. Write a C
program to print your name on the screen.
