Computer System Architecture INTRODUCTION

Computer System
Architecture
SHAHID SULTAN

Introduction
 Computer architecture
 It focuses on the structure and behavior of the computer system
 It refers to the logical aspects of system implementation as seen by the programmer.
 It includes many elements such as
• instruction sets and formats, data types, addressing modes, number and type of
registers,..
• It helps us to answer the question: How do I design a computer?

 Computer organization
 It encompasses all physical aspects of computer systems.
 How components are connected together?
 How components interact with/talk to each other?
 It addresses issues such as
 Control signals, signaling methods
 Memory types, …
 It helps us to answer the question: How does a computer work?

Generations of Computer
 The computer has evolved from a large-sized simple calculating machine
to a smaller but much more powerful machine.
 The evolution of computer to the current state is defined in terms of the
generations of computer.
 Each generation of computer is designed based on a new technological
development, resulting in better, cheaper and smaller computers that are
more powerful, faster and efficient than their predecessors.
 Currently, there are five generations of computer.

First Generation Computers
(1940-1956)
 The first computers used vacuum tubes(a sealed glass tube containing a
near-vacuum which allows the free passage of electric current.) for circuitry
and magnetic drums for memory.
 They were often enormous and taking up entire room.
 First generation computers relied on machine language.
 They were very expensive to operate and in addition to using a great deal
of electricity, generated a lot of heat, which was often the cause of
malfunctions(defect or breakdown).
 The UNIVAC and ENIAC computers are examples of first-generation
computing devices.

Advantages :
 It was only electronic device
 First device to hold memory
Disadvantages :
 Too bulky i.e large in size
 Vacuum tubes burn frequently
 They were producing heat
 Maintenance problems

Second Generation Computers
(1956-1963)
 Transistors replaced vacuum tubes and ushered in the second generation
of computers.
 Second-generation computers moved from cryptic binary machine
language to symbolic.
 High-level programming languages were also being developed at this
time, such as early versions of COBOL and FORTRAN.
 These were also the first computers that stored their instructions in their
memory.
 e.g PDP-8, IBM 1401, CDC 1604

Advantages :
 Size reduced considerably
 The very fast
 Very much reliable
Disadvantages :
 They over heated quickly
 Maintenance problems

Third Generation Computers
(1964-1971)
 The development of the integrated circuit was the hallmark of the third
generation of computers.
 Transistors were miniaturized and placed on siliconchips, called
semiconductors.
 Instead of punched cards and printouts, users interacted with third
generation computers through keyboards and monitors and interfaced
with an operating system.
 Allowed the device to run many different applications at one time.
 E.g PDP-11, IBM 370

Advantages :
 ICs are very small in size
 Improved performance
 Production cost cheap
Disadvantages :
 ICs are sophisticated

Fourth Generation Computers
(1971-present)
 The microprocessor brought the fourth generation of computers, as
thousands of integrated circuits were built onto a single silicon chip.
 The Intel 4004 chip, developed in 1971, located all the components of the
computer.
 From the central processing unit and memory to input/output controls—
on a single chip.
 Fourth generation computers also saw the development of GUIs,
the mouse and handheld devices.

Fifth Generation Computers
(present and beyond)
 Fifth generation computing devices, based on artificial intelligence.
 Are still in development, though there are some applications, such as voice
recognition.
 The use of parallel processing and superconductors is helping to make
artificial intelligence a reality.
 The goal of fifth-generation computing is to develop devices that respond
to natural language input and are capable of learning and self-
organization.

Moore’s Law
1965; Gordon Moore – co-founder of Intel
Observed number of transistors that could be
put on a single chip was doubling every year
The pace slowed to a
doubling every 18
months in the 1970’s
but has sustained that
rate ever since
Consequences of Moore’s law:
The cost of
computer logic
and memory
circuitry has
fallen at a
dramatic rate
The electrical
path length is
shortened,
increasing
operating speed
Computer becomes
smaller and is more
convenient to use in
a variety of
environments
Reduction in
power and
cooling
requirements
Fewer interchip
connections

Types of Computers
 A computer is a programmable electronic device that accepts raw data as input and
processes it with a set of instructions (a program) to produce the desired result as
output.
 We can categorize computer in two ways: on the basis of data handling capabilities
and size.
 Based on the data type handling, computers can be categorized as Digital, Analog,
and Hybrid.

 Analogue Computer
• Analog Computers are particularly designed to process analog data.
• Analogue data is continuous data that changes continuously and cannot have
discrete values. We can say that analogue computers are used where we don't
need exact values always such as speed, temperature, pressure and current.
• Analogue computers directly accept the data from the measuring device without
first converting it into numbers and codes. They measure the continuous
changes in physical quantity and generally render output as a reading on a dial
or scale.
• Speedometer and mercury thermometer are examples of analogue
computers.

 Digital
• Personal computers are an example of a digital computer.
• These computers accept input in the form of 0s and 1s. The computer processes
binary input and provides the output.
• These computers perform all the logical & arithmetical operations.
• Any input given in any language is first converted into binary language and then
the computer processes the information.
• Examples – laptops, PCs, mobile phones, desktops, etc.

 Hybrid
• Hybrid computers are a mix of both analog and digital computers. These
computers perform a high level of calculations.
• Hybrid computers are quick and efficient. They take input in analog form,
convert it into digital form, and then process it to produce an output.
• For example, in hospitals to measure the heartbeat of the patients, and at
research institutes to measure earthquakes and other natural calamities.
• a processor is used in petrol pumps that converts the measurements of
fuel flow into quantity and price.

 On the basis of size, the computer can be of five types:
 Supercomputers
• a powerful computer that can process large amounts of data and do a great
amount of computation very quickly.
• Supercomputers are particularly used in scientific and engineering applications
such as weather forecasting, scientific simulations and nuclear energy research.
The first supercomputer was developed by Roger Cray in 1976
• NASA uses supercomputers for launching space satellites and monitoring and
controlling them for space exploration.

 Mainframe computer
• Mainframe computers are designed to support hundreds or thousands of users
simultaneously.
• At their core, mainframes are high-performance computers with large amounts of
memory and data processors that process billions of simple calculations and
transactions in real time.
• Examples – IBM z Series, System z9, etc.

 Mini-computers
• Mini-computers are also known as "Midrange Computers." They are
not designed for a single. They are multi-user computers designed to
support multiple users simultaneously.
• they are generally used by small businesses and firms. Individual
departments of a company use these computers for specific purposes.
• For example, the admission department of a University can use a Mini-
computer for monitoring the admission process.

 Microcomputer
• Microcomputers are nothing but personal computers.
• These are single-chip systems.
• These are useful for personal use and can perform all the basic functions of the
computer.
• Microcomputers require very little space and are comparatively inexpensive. Such
computers have the most minimalistic requirement in terms of I/O devices. And
have all the circuitry mounted on a single PCB.
• For example tablets, I pads, smartwatches, laptops, desktops

 Workstations
• Workstation computers are for single usage and professional purposes.
• These are like our basic laptops and desktops but with added superior features.
For example, double-processor motherboard, added graphic card, ECC RAM, etc.
• The workstations are more powerful as compared to generic PCs.
• These can handle heavy-duty functions. Like animation, CAD, audio & video
editing, professional gaming, etc.
Examples: Apple PowerBook G4, SPARC CPU, MIPS CPU, etc.

Von Neumann Architecture
 Named after John von Neumann, he designed a computer architecture whereby data and
instructions would be retrieved from memory, operated on by an ALU, and moved back to
memory (or I/O)
 This architecture is the basis for most modern computers .
 It has three building blocks.
• The Memory
• I/O devices
• The CPU
 Instructions in memory are executed sequentially unless a program instruction explicitly
changes the order
 Contains a single path, between the main memory system and the control unit of the CPU

Computer System Architecture INTRODUCTION

 The von Neumann architecture operates on the fetch-
execute cycle
 Fetch an instruction from memory as indicated by the Program
Counter register
 Decode the instruction in the control unit
 Data operands needed for the instruction are fetched from
memory
 Execute the instruction in the ALU storing the result in a register
 Move the result back to memory if needed

 There is a single pathway used to move both data and
instructions between memory, I/O and CPU
 the pathway is implemented as a bus
 the single pathway creates a bottleneck known as the von Neumann
bottleneck
 A variation of this architecture is the Harvard architecture
which separates data and instructions into two pathways
 Another variation, used in most computers, is the system bus version
in which there are different buses between CPU and memory and
memory and I/O.

Input Device
 Devices which are used to feed programs and data to the
computer.
 These devices convert the input data into a digital form
that is acceptable by the computer system.
 Examples: keyboard, mouse, scanner, touch screen,
punch cards, light pen, joy stick, track ball, voice
recognition systems.

Output Devices
 The device that receives data from a computer
system for display, physical production, etc. is called
output device.
 It converts digital information into human
understandable form.
 Example: monitor, projector, headphone, speaker,
printer, etc.

Memory Unit
 Memory unit is that part of the computer system which is used to store
the data and instructions to be processed.
 Types:
i. Main memory (primary memory, internal memory)
ii. Auxiliary storage (secondary memory)
 Characteristics of memory units:
 Access time: time required to locate and retrieve a particular data from
the storage unit.
 Storage capacity: amount of data that can be stored by a memory unit.
 Cost
 Faster access time, higher storage capacity and low costs are desirable.

Main Memory
 It is characterized by the faster access time, less storage capacity and
higher costs as compared to auxiliary storage units.
 Programs and data are loaded into the main memory before
processing.
 The CPU interacts directly with the main memory to perform read or
write operation.
 It is of two types: i) RAM and ii) ROM.

RAM (Random Access Memory)
 It is a volatile memory. Volatile memory stores information based on the
power supply.
 If the power supply fails/ interrupted/stopped, all the data and information
on this memory will be lost.
 It temporarily stores programs/data which has to be executed by the
processor.
 RAM is further classified into two parts
• SRAM
• DRAM

 Static RAM (SRAM)
• The word static indicates that the memory retains its contents as long as power is being
supplied. However, data is lost when the power gets down due to volatile nature.
• SRAM chips use transistors and no capacitors. Transistors do not require power to
prevent leakage, so SRAM need not be refreshed on a regular basis.
 Dynamic RAM (DRAM)
• DRAM, unlike SRAM, must be continually refreshed in order to maintain the data.
• This is done by placing the memory on a refresh circuit that rewrites the data several
hundred times per second.
• DRAM is used for most system memory as it is cheap and small.
• All DRAMs are made up of memory cells, which are composed of capacitors and
transistors.

ROM (Read Only Memory)
 The memory from which we can only read but cannot write on it. This
type of memory is non-volatile.
 The information is stored permanently in such memories during
manufacture.
 A ROM stores such instructions that are required to start a computer. This
operation is referred to as bootstrap.
 ROM chips are not only used in the computer but also in other electronic
items like washing machine and microwave oven.
 Mainly there are three types of ROM:
• PROM
• EPROM
• EEPROM

 PROM
• Programmable Read Only Memory
• This read-only memory is modifiable once by the user.
• The user purchases a blank PROM and uses a PROM program to put the required
contents into the PROM. Its content can’t be erased once written.
 EPROM
• Erasable Programmable Read Only Memory
• EPROM is an extension to PROM where you can erase the content of ROM by
exposing it to Ultraviolet rays for nearly 40 minutes.

 EEPROM
• EEPROM is programmed and erased electrically.
• It can be erased and reprogrammed about ten thousand times. Both erasing and
programming take about 4 to 10 ms (millisecond).
• In EEPROM, any location can be selectively erased and programmed. EEPROMs
can be erased one byte at a time, rather than erasing the entire chip. Hence, the
process of reprogramming is flexible but slow.

Cache Memory
 Cache memory is an extremely fast memory type that acts as a buffer
between RAM and the CPU.
 Cache Memory holds frequently requested data and instructions so that
they are immediately available to the CPU when needed.
 Cache memory is costlier than main memory or disk memory but more
economical than CPU registers.
 Cache Memory is used to speed up and synchronize with a high-speed
CPU.

Auxiliary Memory
 Main memory has limited storage capacity and is either volatile
(RAM) or read-only (ROM). Thus, a computer system needs auxiliary
storage to permanently store the data or instructions for future use.
 It is non-volatile and has larger storage capacity than the main
memory.
 Slower and cheaper than the main memory.
 It can not be accessed directly by the CPU.
 Contents of auxiliary storage need to be first brought into the main
memory for the CPU to access.
 Example: Hard Disk Drive (HDD), CD/DVD, Memory Card, etc.

Central Processing Unit (CPU)
 It is the electronic circuitry of a computer that carries out the actual
processing and usually referred as the brain of the computer.
 The CPU is given instructions and data through programs.
 The CPU performs arithmetic and logic operations as per the given
instructions.

 The CPU comprises of three parts:
i) the control unit,
ii) the arithmetic & logic unit, and
iii) the main memory.
 Control Unit (CU):
• It controls the operations of the entire computer system.
• The control unit gets the instructions from the programs stored in the
main memory, interpret these instructions and subsequently directs the
other units to execute the instructions.
• The control unit is a component of a computer's central processing unit
that coordinates the operation of the processor. It tells the computer's
memory, arithmetic/logic unit and input and output devices how to
respond to a program's instructions.

Arithmetic & Logic Unit (ALU):
 Most of all the arithmetic and logical operations of a
computer are executed in the ALU (Arithmetic and Logical
Unit) of the processor.
 It performs arithmetic operations like addition, subtraction,
multiplication, division and also the logical operations like
AND, OR, NOT operations.

Data Transfer
 Data Transfer between Memory and CPU:
• Data are transferred between different components of a
computer system using physical wires called bus.
 Bus is of three types:
• Data bus – to transfer data between different components.
• Address bus – to transfer addresses between CPU and main
memory.
• Control bus – to communicate control signals between
different components of a computer.

..contd
 All these three buses collectively make the system bus.

…contd
 The CPU places on the address bus, the address of main memory
location from which it wants to read data or to write data.
 While executing the instructions, the CPU specifies the read/write
control signal through the control bus.
 The CPU may require to read data from main memory or write data
to main memory, a data bus is bidirectional.
 Control bus and address bus – unidirectional.
 In case of read operation, the CPU specifies the address, and the data
is placed on the data bus by a dedicated hardware, called memory
controller.

Computer System Architecture INTRODUCTION

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