1.1 GENERATION OF
COMPUTERS
The first electronic computer was designed and built at the
University of Pennsylvania based on
vacuum tube technology. Vacuum tubes were used to perform
logic operations and to store data.
Generations of computers has been divided into five according
to the development of technologies
used to fabricate the processors, memories and I/O units.
I Generation : 1945 – 55
II Generation : 1955 – 65
III Generation : 1965 – 75
IV Generation : 1975 – 89
V Generation : 1989 to present
First Generation
(ENIAC - Electronic Numerical Integrator And Calculator
EDSAC – Electronic Delay Storage Automatic Calculator
EDVAC – Electronic Discrete Variable Automatic Computer
UNIVAC – Universal Automatic Computer
IBM 701)
ƒ Vacuum tubes were used – basic arithmetic operations
took few milliseconds
ƒ Bulky
ƒ Consume more power with limited performance
ƒ High cost
ƒ Uses assembly language – to prepare programs. These
were translated into machine level
language for execution.
ƒ Mercury delay line memories and Electrostatic
memories were used
ƒ Fixed point arithmetic was used
ƒ 100 to 1000 fold increase in speed relative to the
earlier mechanical and relay based
electromechanical technology
ƒ Punched cards and paper tape were invented to feed
programs and data and to get results.
ƒ Magnetic tape / magnetic drum were used as secondary
memory
ƒ Mainly used for scientific computations.
Second Generation (Manufacturers – IBM 7030, Digital Data
Corporation’s PDP 1/5/8
Honeywell 400)
ƒ Transistors were used in place of vacuum tubes.
(invented at AT&T Bell lab in 1947)
ƒ Small in size
ƒ Lesser power consumption and better performance
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ƒ Lower cost
ƒ Magnetic ferrite core memories were used as main
memory which is a random-access
nonvolatile memory
ƒ Magnetic tapes and magnetic disks were used as
secondary memory
ƒ Hardware for floating point arithmetic operations was
developed.
ƒ Index registers were introduced which increased
flexibility of programming.
ƒ High level languages such as FORTRAN, COBOL etc were
used - Compilers were
developed to translate the high-level program into
corresponding assembly language
program which was then translated into machine language.
ƒ Separate input-output processors were developed that
could operate in parallel with CPU.
ƒ Punched cards continued during this period also.
ƒ 1000 fold increase in speed.
ƒ Increasingly used in business, industry and
commercial organizations for preparation of
payroll, inventory control, marketing, production planning,
research, scientific &
engineering analysis and design etc.
Third Generation (System 360 Mainframe from IBM, PDP-8 Mini
Computer from Digital
Equipment Corporation)
ƒ ICs were used
ƒ Small Scale Integration and Medium Scale Integration
technology were implemented in
CPU, I/O processors etc.
ƒ Smaller & better performance
ƒ Comparatively lesser cost
ƒ Faster processors
ƒ In the beginning magnetic core memories were used.
Later they were replaced by
semiconductor memories (RAM & ROM)
ƒ Introduced microprogramming
ƒ Microprogramming, parallel processing (pipelining,
multiprocessor system etc),
multiprogramming, multi-user system (time shared system) etc
were introduced.
ƒ Operating system software were introduced (efficient
sharing of a computer system by
several user programs)
ƒ Cache and virtual memories were introduced (Cache
memory makes the main memory
appear faster than it really is. Virtual memory makes it
appear larger)
ƒ High level languages were standardized by ANSI eg.
ANSI FORTRAN, ANSI COBOL etc
ƒ Database management, multi-user application, online
systems like closed loop process
control, airline reservation, interactive query systems,
automatic industrial control etc
emerged during this period.
Fourth Generation (Intel’s 8088,80286,80386,80486 ..,
Motorola’s 68000, 68030, 68040,
Apple II, CRAY I/2/X/MP etc)
ƒ Microprocessors were introduced as CPU– Complete
processors and large section of main
memory could be implemented in a single chip
ƒ Tens of thousands of transistors can be placed in a
single chip (VLSI design implemented)
ƒ CRT screen, laser & ink jet printers, scanners
etc were developed.
ƒ Semiconductor memory chips were used as the main
memory.
ƒ Secondary memory was composed of hard disks – Floppy
disks & magnetic tapes were used
for backup memory
ƒ Parallelism, pipelining cache memory and virtual
memory were applied in a better way
ƒ LAN and WANS
were developed (where desktop work stations interconnected)
ƒ Introduced C language and Unix OS
ƒ Introduced Graphical User Interface Computer Organization
1. Introduction
3
ƒ Less power consumption
ƒ High performance, lower cost and very compact
ƒ Much increase in the speed of operation
Fifth Generation (IBM notebooks, Pentium PCs-Pentium
1/2/3/4/Dual core/Quad core.. SUN
work stations, Origin 2000, PARAM 10000, IBM SP/2)
ƒ Generation number beyond IV, have been used
occasionally to describe some current
computer system that have a dominant organizational or
application driven feature.
ƒ Computers
based on artificial intelligence are available
ƒ Computers use extensive parallel processing, multiple
pipelines, multiple processors etc
ƒ Massive parallel machines and extensively distributed
system connected by communication
networks fall in this category.
ƒ Introduced ULSI (Ultra Large Scale Integration)
technology – Intel’s Pentium 4
microprocessor contains 55 million transistors millions of
components on a single IC chip.
ƒ Superscalar processors, Vector processors, SIMD
processors, 32 bit micro controllers and
embedded processors, Digital Signal Processors (DSP) etc have
been developed.
ƒ Memory chips up to 1 GB, hard disk drives up to 180
GB and optical disks up to 27 GB are
available (still the capacity is increasing)
ƒ Object oriented language like JAVA suitable for
internet programming has been developed.
ƒ Portable note book computers introduced
ƒ Storage technology advanced – large main memory and
disk storage available
ƒ Introduced World Wide Web. (and other existing
applications like e-mail, e Commerce,
Virtual libraries/Classrooms, multimedia applications etc.)
ƒ New operating systems developed – Windows 95/98/XP/…,
LINUX, etc.
ƒ Got hot pluggable features – which enable a failed
component to be replaced with a new
one without the need to shutdown the system, allowing the
uptime of the system to be very
high.
ƒ The recent development in the application of internet
is the Grid technology which is still in
its upcoming stage.
ƒ Quantum mechanism and nanotechnology will radically
change the phase of computers.
1.2 TYPES OF COMPUTERS
1. Super Computers
2. Main Frame Computers
3. Mini Computers
4. Micro Computers
1. Super Computers
E.g.:- CRAY Research :- CRAY-1 & CRAY-2, Fujitsu (VP2000),
Hitachi (S820), NEC (SX20), PARAM 10000 by C-DAC, Anupam by
BARC, PACE Series by
DRDO
ƒ Most powerful Computer system - needs a large
room
ƒ Minimum world length is 64 bits
ƒ CPU speed: 100 MIPS
ƒ Equivalent to 4000 computers
ƒ High cost: 4 – 5 millions
ƒ Able to handle large amount of data
ƒ High power consumption
ƒ High precision Computer Organization
1. Introduction
ƒ Large and fast memory (Primary and Secondary)
ƒ Uses multiprocessing and parallel processing
ƒ Supports multiprogramming
Applications
ƒ In petroleum industry - to analyze volumes of seismic
data which are gathered during oil
seeking explorations to identify areas where there is
possibility of getting petroleum
products inside the earth
ƒ In Aerospace industry - to simulate airflow around an
aircraft at different speeds and
altitude. This helps in producing an effective aerodynamic
design for superior performance
ƒ In Automobile industry – to do crash simulation of
the design of an automobile before it is
released for manufacturing – for better automobile design
ƒ In structural mechanics – to solve complex structural
engineering problems to ensure safety,
reliability and cost effectiveness. Eg. Designer of a large
bridge has to ensure that the bridge
must be proper in various atmospheric conditions and
pressures from wind, velocity etc and
under load conditions.
ƒ Meteorological centers use super computers for
weather forecasting
ƒ In Biomedical research – atomic nuclear and plasma
analysis – to study the structure of
viruses such as that causing AIDS
ƒ For weapons research and development, sending rockets
to space etc
2. Main Frame Computers
E.g.:- IBM 3000 series, Burroughs B7900, Univac 1180, DEC
ƒ Able to process large amount of data at very high
speed
ƒ Supports multi-user facility
ƒ Number of processors varies from one to six.
ƒ Cost: 3500 to many million dollars
ƒ Kept in air conditioned room to keep them cool
ƒ Supports many I/O and auxiliary storage devices
ƒ Supports network of terminals
SYSTEM ROOM
(Entry restricted to system Administrators & maintenance
staff)
User Terminals
Printer
Plotter
Magnetic
Tape Drives
Magnetic Tape Library
Console
Host Processor
Front-end Processor
Back-end Processor
Magnetic
Tape Drives
USERS ROOM
(Entry restricted to authorized persons)
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Applications
ƒ Used to process large amount of data at very high
speed such as in the case of Banks/
Insurance Companies/ Hospitals/ Railways…which need online
processing of large number
of transactions and requires massive data storage and
processing capabilities
ƒ Used as controlling nodes in WANs (Wide Area
Networks)
ƒ Used to mange large centralized databases
3. Mini Computers
E.g.:- Digital Equipments PDP 11/45 and VAX 11)
ƒ Perform better than micros
ƒ Large in size and costlier than micros
ƒ Designed to support more than one user at a time
ƒ Posses large storage capacities and operates at
higher speed
ƒ Support faster peripheral devices like high speed
printers
ƒ Can also communicate with main frames
Applications
ƒ These computers are used when the volume of
processing is large for e.g. Data processing
for a medium sized organization
ƒ Used to control and monitor production processes
ƒ To analyze results of experiments in laboratories
ƒ Used as servers in LANs (Local Area Networks)
4. Micro Computers
E.g.:- IBM PC, PS/2 and Apple Macintosh
ƒ A microcomputer uses a microprocessor as its central
Processing Unit. Microcomputers are
tiny computers that can vary in size from a single chip to
the size of a desktop model
ƒ They are designed to be used by only one person at a
time
ƒ Small to medium data storage capacities 500MB – 2GB
ƒ The common examples of microcomputers are chips used
in washing machines, TVs, Cars
and Note book/Personal computers.
Applications
Used in the field of desktop publishing, accounting,
statistical analysis, graphic designing,
investment analysis, project management, teaching,
entertainment etc
ƒ The different models of microcomputers are given
below:-
a) Personal computers:- The name PC was given by the IBM for
its microcomputers. PCs are
used for word processing, spreadsheet calculations, database
management etc.
b) Note book or Lap Top:- Very small in terms of size – can
be folded and carried around –
Monitor is made up of LCD and the keyboard and system units
are contained in a single
box. Got all the facilities of a personal computer (HDD, CDD,
Sound card, N/W card,
Modem etc) and a special connection to connect to the desktop
PC which can be used to
transfer data.
c) Palm Top:- Smaller model of the microcomputer- size is
similar to that of a calculator –
pocket size- It has a processor and memory and a special
connection to connect to the
desktop PC which can be used to transfer data.
d) Wrist PC:- Smallest type of microcomputer – can be worn on
our wrist like a watch- It has
a processor and memory and a wireless modem
Computer
Organization
1. Introduction
Cost Speed Applications
Superfast Weapon
design
Weather forecasting
Super Computers
6
Aircraft design
fast Biomedical applications
Scientific
calculations Main frame computers
Data Processing for
large business
Teaching systems in
Universities
medium
Large multi-user
systems
Manufacturing
processes Mini Computers
Hospital
Administration
Teaching systems in
Colleges
slow Office automation
Small business
systems Micro Computers
Control applications
Teaching systems in
schools
FUNCTIONAL UNITS OF A COMPUTER
Computer is a device that operates upon information or data.
It is an electronic device which
accepts input data, stores the data, does arithmetic and
logic operation and outputs the information
in desired format.
Even though the size, shape, performance, reliability and
cost of computers have been changing
over the years, the basic logical structure proposed by Von
Neumann has not change. The internal
architecture of computers differs from one system model to
another. A block diagram of the basic
computer organization specifying different functional units
is shown below. Here the solid lines
indicate the flow of instruction and data and the dotted
lines represent the control exercised by the
control unit.
Control
Unit
Information
(Result)
Primary
Storage
Secondary
Storage
Output Unit
Program
& Data
Input Unit
ALU
Central Processing
Unit Computer Organization
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7
INPUT UNIT
Input unit accepts coded information from human operators
through electromechanical
devices such as the keyboard or from other computers over
digital communication lines. The
information received is either stored in the memory for later
reference or immediately used by the
Arithmetic and Logic circuitry to perform the desired
operation. Finally the result is sent back to
the outside through the output unit.
The keyboard is wired so that whenever a key is pressed, the
corresponding letter or digit is
automatically translated into its corresponding code and sent
directly to either the memory or the
processor.
Other kinds of input devices: Joy stick, track ball, mouse
(pointing devices), scanner etc.
MEMORY UNIT
The memory unit stores program and data. There are two
classes of memory devices :-
Primary memory and Secondary memory.
Primary memory (Main memory)
ƒ Contains a large number of semiconductor cells each
capable of storing one bit of
information
ƒ These cells are processed in group of fixed size
called words containing ‘n’ bits. The main
memory is organized such that the contents of one word can be
stored or retrieved in one
basic operation.
ƒ For accessing data, a distinct address is associated
with each word location.
ƒ Data and programs must be in the primary memory for
execution.
ƒ Number of bits in each word is called the word length
and it may vary from 16 to 64 bits.
ƒ Fast memory
ƒ Expensive
ƒ Time required to access one word is called Memory
Access Time - 10nS to 100nS. This
time is fixed and independent of the location.
E g. Random Access Memory (RAM)
Secondary storage
ƒ They are used when large amount of data have to be
stored (also when frequent access is not
necessary)
E.g. Hard Disk, Compact Disk, Floppy Disk, Magnetic Tapes
etc.
PROCESSOR UNIT
ƒ The heart of the computer system is the Processor
unit.
ƒ It consists of Arithmetic and Logic Unit and Control
Unit.
Arithmetic and Logic Unit (ALU)
ƒ Most computer operations (Arithmetical and logical)
are executed in ALU of the processor.
ƒ For example: Suppose two numbers (operands) located
in the main memory are to be added.
These operands are brought into arithmetic unit – actual
addition is carried. The result is
then stored in the memory or retained in the processor itself
for immediate use.
ƒ Note that all operands may not reside in the main
memory. Processor contains a number of
high speed storage elements called Registers, which may be
used for temporary storage of
frequently used operands. Each register can store one word of
data.
ƒ Access times to registers are 5 to 10 times faster
than access time to memory.
Control Unit Computer Organization
1. Introduction
ƒ The operations of all the units are coordinated by
the control unity (act as the nerve centre
that sends control signal to other units)
ƒ Timing signal that governs the I/O transfers are
generated by the Control Unit.
ƒ Synchronization signals are also generated by the
Control Unit
ƒ By selecting, interpreting and executing the program
instructions the program instructions
the control unit is able to maintain order and direct the
operation of the entire system.
The control unit and ALU’s are usually many times faster than
other devices connected to a
computer system. This enabled a single processor to control a
number of external devices such as
video terminals, magnetic taped, disk memories, sensors,
displays and mechanical controllers
which are much slower than the processor.
OUTPUT UNIT
ƒ Counter part of input unit
ƒ Output devices accept binary data from the computer -
decodes it into original form and
supplies this result to the outside world.
E.g. Printer, Video terminals (provides both input &
output functions), graphic displays etc
Basic Operational Concepts:-
ƒ Activity in a computer is governed by instructions
ƒ To perform a given task, a set of instructions called
program must be there in the main
memory
ƒ Individual instructions are brought from the memory
into the processor which executes the
specific operation.
ƒ Data to be used as operands are also stored in the
memory.
E.g. Add LOCA, R0
This instruction adds the operand at the memory location LOCA
to the operand in the Processor R0
and places the sum into the register R0. Here the original
contents of LOCA are preserved whereas
those of R0 are overwritten.
Steps:-
1. Instruction is fetched from the main memory into the
processor Memory access
2. Operand at LOCA is fetched operation
3. Add the contents to the contents of R0 ALU operation
4. Finally store the result in R0
Note: Data transfer between the main memory and the processor
are started by sending the
address of the memory location to be accessed to the memory
unit and issuing the appropriate
control signal by the control unit.
INTERNAL ORGANIZATION OF PROCESSOR
Processor contains a number of registers used for temporary
storage of data other than ALU and
Control circuitry
Instruction Register (IR) – holds the instruction that is
currently being executed – its output is
available to the control circuits which generate the timing
signals that control the various
processing elements involved in executing the instruction.
8 Computer Organization
1. Introduction
Program Counter (PC) – It contains the address of the
instruction currently being executed. During
the execution of an instruction, the contents of the program
counter are updated to hold the address
of the next instruction to be executed. i.e. PC points to the next instruction that
is to be fetched
from the memory.
n General Purpose Registers (R0 to Rn-1) – Facilitates
communication with the main memory.
Access to data in these registers is much faster than to data
stored in memory locations because the
registers are inside the processor. Most modern computers
have 8 to 32 general purpose registers.
Memory Address Register (MAR) – holds the address of the
location to or from which data are to
be transferred
Memory Data Register (MDR) – contains the data to be written
into or read out of the address
location.
9
MAR
PC
IR
Ro
R1
.
.
Rn-1
MDR
ALU
CONTROL
n general purpose registers
MAIN MEMORY
Fig: Processor
Steps involved during operation:-
1. Program is stored in the main memory
2. PC is set to point to the first instruction of the program
3. Contents of the PC are transferred to the MAR and a Read
Control signal sent to the
memory
4. After the access time, the addressed word (in this case
the first instruction) is read out of the
memory and is loaded into the MDR
5. Contents of the MDR are transferred to the IR. Now the
instruction is ready to be decoded
and executed.
6. If the instruction involves an operation to be performed
by the ALU, the required operands
are to be fetched from the memory (or CPU registers). This is
done by sending its address to
the MAR and initiating a Read cycle. Computer
Organization
1. Introduction
10
7. Operands are read from the memory into the MDR and are
transferred from MDR to the
ALU.
8. ALU will perform the desired operation.
9. If the result is to be stored in the memory, then it is
sent to the MDR.
10. The address of the location where the result is to be
stored is sent to the MAR and a Write
cycle is initiated.
11. At some point during the execution of the current
instruction, the contents of the PC are
incremented so that the PC now points to the next instruction
to be executed.
12. As soon as the execution of the current instruction is
completed, a new instruction fetch
may be started.
NOTE:- In addition to transferring data between the memory
and the processor, the computer
accepts data from input devices and sends data to output
devices. For example, a sensing device in
a computer controlled industrial process may detect a
dangerous condition. Here the device raises
an interrupt signal. An interrupt is a request from an I/O
device for service by the processor. Now
the processor provides the requested service by executing an
appropriate interrupt-service routine.
The internal state of the processor at such moments (like the
contents of the PC, the general
registers, and some control information) are saved in memory
locations. When the interrupt-service
routine is completed, the state of the processor is restored
so that the normal program may be
continued
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