This document discusses computer architecture and organization. It defines computer architecture as the attributes visible to the programmer and computer organization as the operational units and their interconnections. It then classifies computers based on size, cost, computational power, and application. The basic functional units of a computer are described as the input, output, memory, arithmetic logic unit, and control unit. Common computer components like the CPU, registers, and buses are also explained.

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UNIT-1
Computer architecture: Means those attributes of a system visible to the programmer.
Computer organization: Refers to the operational units and their inter connections that
realize the architectural specifications.
Computer: A computer is fast electronic calculating machine that accepts digitalized
information from the user and processes it according to a sequence of instructions stored in
the internal storage and provides the processed information to the user.
• A sequence of instructions stored in the internal storage is called computer program.
• An internal storage is called memory.
• According to the size, cost computational power application computers are
classified as:
- Micro computers Mini computers
- Desktop computers
- Personal computers
- Portable notebook computers
- Workstations
- Mainframes or enterprise systems
- Servers
- Super computers
Micro computers: Means smaller computers
• It contains only one CPU
• It is the integration of microprocessor and supporting peripherals(memory & i/o
devices)
• The word length is 8-32 bit
• Used in small industrial control, process control and where storage requirements are
moderate.
Mini computers:
• It is designed to process smaller data words 32-bit words
• These are used for scientific calculations, research, data processing applications
Desktop computers:
• Commonly used in home or office desk
• It is consists of processing units, storage unit, visual display and audio as output
units, keyboard and mouse as input-output units.
• Storage units consists of hard disks, CD-ROM , desketters.
Personal computers: Like desktop
• Used in homes, schools and business offices.
Portable network computers:
• Compact version of personal computers.
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Ex: laptops
Workstations: Have a higher computation power than personal computer.
• Have a higher resolution graphics terminals and improved input/output capabilities
• Used in engineering applications and in interactive graphics applications.
Mainframe or enterprise systems:
• These are implemented using two or more CPU’s
• Work at very high speeds with large data word length(64bit or more)
• Data storage capacity is very high
• Used in complex scientific calculations, large data processing applications, military
defense control complex graphics applications.
Servers:
• Having large storage unit and fast communication links.
• Large storage allows storing sizable database and fast communication links allow
faster communication of data blocks with computers connected in the n/w.
• They play major role in internet communication.
Super computers:
• Basically multi processing computers.
• Used in large scale numerical calculations required in applications such as whether
forecasting, robotic engineering, air craft designing and simulations.
FUNCTIONAL UNITS:
Functional Units
Figure 1.1. Basic f unctional units of a computer.
I/O Processor
Output
Memory
Input and
Arithmetic
logic
Control
• All the above activities are coordinated and controlled by the control unit.
• The arithmetic and logic unit in conjunction with control is commonly called CPU.
Input unit:
• A computer accepts digitally coded information through i/p unit using input devices
like keyboard and mouse.
• Keyboard take text and numeric information
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• Mouse take to position the screen cursor and there by enter the information by
selecting option
• Take ball, space ball, digitizers, scanners(i/p units)
Memory unit:
• Used to store programs and data.
• Memory devices are two types
• Primary storage memory device is also called main memory.
• Secondary storage memory device.
• Main memory is fast memory used for the storage of programs and active
data(current process data)
• Main memory is semi conductor memory.
• It consists of a large no of semi conductor storage cells.
• Each cell storing one bit of information
• These cells are read or written by CPU in a group of fixed size called word.
• One word contains n bits.
• Each word has distinct address.
• No of bits in word is word length.
• No of words are the sizes of memory (or) capacity of memory.
• Important characteristics of memory is an access time (required to access one word)
• It should be as small as possible 10 to 100 nano seconds.
• In rams, fixed time is required to access any word in the memory.
• In sequential access memory this time is not fixed.
• Main memory consists of RAMs
• These are fast but they are small in capacities and expensive.
• Computer uses secondary storage memories such as magnetis taps, magnetic disk
for storage of large amount of data.
Arithmetic and logical unit:
• It performing arithmetic operations such as add, subtracts division, and
multiplication.
• Logical operations such as ending, oring, inverting etc..
• To perform operations operands brought from main memory into the high speed
storage elements called registers of the processors.
• Each register can store one word of data and they are used to store frequently used
operands.
• Access times of registers are typically 5 to 10 times faster than access times to
memory.
• The result is either stored in the register or memory location.
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Output uint:
• It sends the proceeded results to the user using o/p devices such as video monitor,
printer, and plotter etc.
• Video monitor display the o/p on the CRT screen
• Printers and plotter give the hardcopy o/p.
• Printers will be classified as
1. Impact printers
2. Non impact printers
• Impact printer’s process formed character faces against inked printers.
• Non impact printers and plotters used laser techniques, inkjet sprays,
xerographically processes, electrostatic methods and electro thermal methods to get
images on the paper.
Control unit:
• Control unit coordinates and controls the activities among the functional unit.
• Basic function of control unit is to fetch the instructions stored in the main memory.
• Identifying the operations and the devices involved in it.
• Generate control signals to execute the desired operations.
• It controls i/p o/p operations data transfers between the processor memory and i/p
and o/p devices using timing signals.
BASIC OPERATIONAL CONCEPTS
• The basic function of a computer is to execute program.
•
Figure 1.2. Connections between the processor and the memory.
Processor
Memory
PC
IR
MDR
Control
ALU
Rn 1­
R1
R0
MAR
n general purpose
registers
Fig: connections between the processor and main memory
To perform execution of information in addition to the arithmetic logic unit, and
control unit the processor contain a number of registers used for temporary storage of data
and some special function registers include pc, ir, mdr, mar.
Program counter (pc):
• A program is a series of instructions stored in the memory.
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• It is important that these instructions must be executed in a proper way to get the
correct result.
• Sequence of instructions execution is monitored by the program counter.
IR (Instruction Registers):
• Is used to hold the instruction that is currently being executed.
• The contents of IR are available to the control unit, which generate timing signals
that control the various processing elements involved in executing the instruction.
• MAR & MDR all used to handle the data transfer b/w the main memory and the
processor.
• MAR holds the address of main memory to or from which data is to be transferred.
• MDR contains the data to be written into or read from the addressed word of the
main memory.
• When ever you want to service from a device like keyboard,
• The processor can service these devices in one of the two ways polling routine &
interrupt.
• In polling processors s/w checks each of I/O devices every so often.
• Once the service is completed the processor would resume exactly where it left off.
This method is called interrupt method.
• If more than one i/p devices request I/O service simulation sly, i/o provides service
based on priority.
• Interrupt service routine itself can be interrupted by higher priority interrupt. These
interrupt is called nested interrupted.
Ex: 1.state the operations involved in the execution of ADD R1, R0 instruction.
2. Fetch the instruction from the memory into 1R register of the processor.
3. Add the contents of R1 and R0 and store the result in the R0.
BUS STRUCTURES:
• Paths are connecting the modules together for communicating together.
• The collection of paths connecting the various modules is called the interconnection
structure.
• The designs of this interconnection structure will depend on the exchanges that must
be made between modules.
• A group of wires is called bus.
• Bus provides necessary signals for communication between modules.
• A bus that connects major computer components/modules( cpu, memory, i/o) is
called system bus
• System bus is separated into 3 functional groups
Data bus
Address bus
Control bus
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Data bus:
• Data bus consists of 8, 16, 32 or more parallel signal lines.
• These lines are bidirectional
• The communication between peripheral and cpu is activated by giving output
enable pulse to the peripheral.
Address bus:
• It is a unidirectional bus.
• It consists of 16,20,24 or more parallel signal lines
• On these lines the CPU sends out the address of the memory location or i/o port that
is to be written to or read form.
Control bus:
• The control lines regulate the activity on the bus.
• The cpu sends signals on the control bus to enable the outputs of addressed memory
devices or port devices
• Typically control bus signals are:
Memory read (MEMR) Memories write (MEMW)
I/O read (IOR) I/O writes (IOW) Bus request (BR)
Bus grant (BG) Interrupt request (INTR),
Interrupt acknowledgement (INTA) Clock (CLK)
Reset Ready Hold, Hold acknowledgment (HLDA).
Signal bus structure:
• Interconnection of bus structure is called signal bus structure.
Figure 1.3. Single­bus structure.
MemoryInput Output Processor
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• In single bus structure all units (address bus, data bus, and control bus) are
connected to common bus called system bus.
• In single bus only two units can communicate with each other at a time.
• Advantage of single bus structure is its low cost and its flexibility for attaching
peripheral devices.
Multi bus structures:
The performance of computer systems suffers when large number of devices connected to
the bus this is because of two major reasons.
• When two or more devices are connected to the common bus we need to share the
bus amongst these devices the sharing mechanisms co ordinates the use of bus of
different devices. This coordination requires finite time called propagative delay.
When control of the bus passes from one device to another frequently these
propagation delays are noticeable and affect the performance of computer system.
• When the aggregate data transfers demand approaches the capacities of the bus, the
bus may become a bottleneck. In such situations we have to increase the data rate of
the bus.
• For this reason most computer systems use the multiple buses.
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System Software:
• Is a collection of programs.
• System software in micro computer allows one to develop applications/user
programs for micro processor based systems.
• System software includes operating system, text editors, assemblers, compilers and
interpreters an
• It is a collection of programs to creation, preparation and execution of other
programs.
• Basic functions provided by system software are as fallows:
Receive and interrupt user commands.
Enter and edit user application programs and store them as files in secondary
storage devices such as hard disk or floppy disk.
I/O handling using standard device drivers.
Translation of programs from assembly language to machine language or high-level
language to machine language.
Editor:
• The editor is programs is used to creat and modify source program/text.
• The editor has commands to change, delete or interest lines or characters.
Assemblers:
• In the 1st
pass the assembler performs the fallowing operations:
1. Reading the source program instructions.
2. Creating the symbol table in which all symbols used in the program together with
their attributes are stored.
3. Replacing all mnemonic codes by their binary codes.
4. Detecting any syntax error in the source program
5. Assigning relative address to instructions and data.
Interpreter and Compiler:
• Advantage of using an interpreter is that if an error is found, you can just correct the
source program and immediately return it.
• Disadvantage, interpreted program runs 5 to 25 times slower than the same program
will run after being completed.
• Each statement must be translated to machine code every time the program is run.
• Disadvantage, when an error is found, it usually must be corrected in the source
program and the entire compile-load sequence repeated.
Operating system:
• An OS performs resource management and provides an interface between the user
and program.
• Resource-microprocessor, memory, I/O devices.
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• As a collection of system programs that tell the machine what to do under a verity
of conditions.
• DOS, Unix, Windows
PERFORMANCE:
• The time between start and completion of the program or event is reducing
execution time (or) response time.
• Reduction in response time increases the throughput(the total amount of work done
in a given time)
• The performance of a computer is directly related to throughput and hence it is
reciprocal of execution time.
• The idle performance of a computer system is achieved when we have a perfect
match between the machine capability and the program behavior.
• Factors for projecting the performance of a computer.
Processor clock:
• The processor is driven by a clock with a constant cycle time called processor
clock.
• The time period of processor clock is denoted by P.
• The period ’p’ of clock cycle is an important parameter that effects processor
performance.
• The clock rate is given by R=1/p which measured in cycles per second (cps).
• The computer having clock rate of 800 MHZ have 800 million cycles per second.
Basic performance equation:
• To execute a program a processor has to execute number of machine language
instructions.
• This number is denoted by N.
• The number N is actual number of instructions executed by processor and is not
necessary equal to the number of machine instructions in the machine language
program.
• Each machine instruction takes one or more cycle time for execution.
• The average no of basic steps required to execute one machine instruction is
denoted by ‘S’
• Each basic step is completed in one clock cycle.
• The program execution time is given by
T=N*S/R.
• R clock rate measured in clocks per second.
• S the average no of steps needed to execute one machine instruction.
• The above equation is known as basic performance equation.
• When machine instruction execution time is measured in terms of cycles per
instruction(cpi) the program execution time is given as T=N*CPL/R.
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• The CPI of an instruction type can be divided into 2 components terms
corresponding to the total processor cycles and memory cycles need to complete the
execution of the instruction.
• We can rewrite the eq(2)
T=N* (P+M*K)/R.
Where P-no of processor cycles required for the instruction decodes and
executes.
M-no of memory references needed.
K-ratio between memory cycle and processor cycle.
N-machine instruction count.
R-clock rate.
Throughput rate:
• It indicates a no of programs a system can execute per unit time.
• Throughput can be measured separately for the system (ws) and for the processor
(wp).
• Wp = no of machine instructions executed per second/no of machine instructions
per program.
Wp= MIPS rate*106
/N.
• If the processor kept busy in a perfect program interleaving fashion, then
Ws=Wp
Pipelining and superscalar operation:
• Clock cycles are required to perform various steps in the instruction execution.
• S1-fetch(f) S2-decode (d)
S3-execute (e) S4-store(s)
• These stages for several instructions are performed simultaneously to reduce overall
processing time, the processing is called instruction pipeline.
• To improve performance is to achieve high degree of concurrency.
• High degree of concurrency is achieved by implementing multiple instruction
pipelines in the processor.
• To implement multiple instruction pipelines processor has multiple functional units
and they are capable of executing multiple instructions at a time creating parallel
execution paths such a processor.
• Is known as superscalar processor and such an operation is known as superscalar
execution.
Multiprocessor and multicomputer:
• In a large computer systems multiple processor are used. Such systems are known
as multiprocessor systems.
• This system executes a no of different application tasks in parallel, or they execute
sub tasks of a single large task in parallel.
• In this system memory shared between all processors.
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• Such multi processor system is known as shared memory multi processor systems.
• A group of systems or computers inter connected to achieve high total
computational power such systems are called multicomputer.
• In multi computer systems each computer has its own memory unit.
• When the tasks are needed to execute, communication between computers are used
message passing mechanisms’ to exchange information.
• Message passing mechanisms exchanges the messages over a communication
network. such computer system are known as message passing multi computers.
NOTE: For remaining topics in unit-1 and for
complete unit-2
Refer your notes and soft copy of text book pdf file
which is attached along with this file.
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