The document discusses the organization and architecture of computers. It covers topics such as the definition of computer organization and architecture, the functional units of a computer including input, output, memory, and the central processing unit. It also discusses the evolution of computers through different generations from the use of vacuum tubes to today's integrated circuits. The basic terminology related to computer hardware, software, and functions are also introduced.

1. COMPUTER
ORGANIZATION AND
ARCHITECTURE
K.PRASHANTH
Asst. Professor

2. UNIT-I
•Digital Computers: Introduction, Block diagram of Digital
computer
•Definition of Computer Organization , Computer Design
and Architecture
•Computer Types
•Functional units
•Basic operational concepts
•Bus structures
•Register Transfer Language
•Register transfer
•Bus and Memory Transfers
•Micro Operations (Arithmetic, logical, shift)
•Arithmetic Logic Shift Unit
•Instruction Codes
•Computer Registers
•Computer instructions
•Timing and Control
•Memory Reference instructions
•Input-Output instructions
•Interrupt

3. 3
What is a computer?
• a computer is a sophisticated electronic calculating
machine that:
– Accepts input information,
– Processes the information according to a list of internally
stored instructions and
– Produces the resulting output information.
• Functions performed by a computer are:
– Accepting information to be processed as input.
– Storing a list of instructions to process the information.
– Processing the information according to the list of
instructions.
– Providing the results of the processing as output.

4. Block Diagram of a Digital Computer

5. COMPUTER ORGANISATION AND
ARCHITECTURE
• The components from which computers are built,
i.e., computer organization.
• In contrast, computer architecture is the science of
integrating those components to achieve a level of
functionality and performance.
• It is as if computer organization examines the
lumber, bricks, nails, and other building material
• While computer architecture looks at the design of
the house.

6. Brief History of Computer Evolution
Two phases:
1. before VLSI 1945 – 1978
• ENIAC
• IAS
• IBM
• PDP-8
2. VLSI 1978  present day
• microprocessors !
VLSI = Very Large
Scale Integration

7. Evolution of Computers
FIRST GENERATION (1945 – 1955)
• Program and data reside in the same memory
(stored program concepts – John von Neumann)
• ALP was made used to write programs
• Vacuum tubes were used to implement the functions
(ALU & CU design)
• Magnetic core and magnetic tape storage devices are
used
• Using electronic vacuum tubes, as the switching
components

8. SECOND GENERATION (1955 – 1965)
• Transistor were used to design ALU & CU
• HLL is used (FORTRAN)
• To convert HLL to MLL compiler were used
• Separate I/O processor were developed to operate in
parallel with CPU, thus improving the performance
• Invention of the transistor which was faster, smaller
and required considerably less power to operate

9. • IC technology improved
• Improved IC technology helped in designing low cost, high
speed processor and memory modules
• Multiprogramming, pipelining concepts were incorporated
• DOS allowed efficient and coordinate operation of computer
system with multiple users
• Cache and virtual memory concepts were developed
• More than one circuit on a single silicon chip became
available
THIRD GENERATION (1965-1975)

10. FOURTH GENERATION (1975-1985)
• CPU – Termed as microprocessor
• INTEL, MOTOROLA, TEXAS,NATIONAL
semiconductors started developing microprocessor
• Workstations, microprocessor (PC) & Notebook
computers were developed
• Interconnection of different computer for better
communication LAN,MAN,WAN
• Computational speed increased by 1000 times
• Specialized processors like Digital Signal Processor
were also developed

11. • E-Commerce, E- banking, home office
• ARM, AMD, INTEL, MOTOROLA
• High speed processor - GHz speed
• Because of submicron IC technology lot of
added features in small size
BEYOND THE FOURTH GENERATION
(1985 – TILL DATE)

12. Slide 12
Generations of Progress
Table 3.2 The 5 generations of digital computers, and their ancestors.
Generation
(begun)
Processor
technology
Memory
innovations
I/O devices
introduced
Dominant
look & fell
0 (1600s) (Electro-)
mechanical
Wheel, card Lever, dial,
punched card
Factory
equipment
1 (1950s) Vacuum tube Magnetic
drum
Paper tape,
magnetic tape
Hall-size
cabinet
2 (1960s) Transistor Magnetic
core
Drum, printer,
text terminal
Room-size
mainframe
3 (1970s) SSI/MSI RAM/ROM
chip
Disk, keyboard,
video monitor
Desk-size
mini
4 (1980s) LSI/VLSI SRAM/DRAM Network, CD,
mouse,sound
Desktop/
laptop micro
5 (1990s) ULSI/GSI/
WSI, SOC
SDRAM,
flash
Sensor/actuator,
point/click
Invisible,
embedded

13. COMPUTER TYPES
Computers are classified based on the
parameters like
• Speed of operation
• Cost
• Computational power
• Type of application

14. DESK TOP COMPUTER
• Processing &storage units, visual display &audio uits,
keyboards
• Storage media-Hard disks, CD-ROMs
• Eg: Personal computers which is used in homes and offices
• Advantage: Cost effective, easy to operate, suitable for general
purpose educational or business application
NOTEBOOK COMPUTER
• Compact form of personal computer (laptop)
• Advantage is portability

15. WORK STATIONS
• More computational power than PC
•Costlier
•Used to solve complex problems which arises in engineering
application (graphics, CAD/CAM etc)
ENTERPRISE SYSTEM (MAINFRAME)
•More computational power
•Larger storage capacity
•Used for business data processing in large organization
•Commonly referred as servers or super computers

16. SERVER SYSTEM
• Supports large volumes of data which frequently need to be
accessed or to be modified
•Supports request response operation
SUPER COMPUTERS
•Faster than mainframes
•Helps in calculating large scale numerical and algorithm
calculation in short span of time
•Used for aircraft design and testing, military application and
weather forecasting

17. HANDHELD
• Also called a PDA (Personal
Digital Assistant).
• A computer that fits into a
pocket, runs on batteries, and
is used while holding the unit
in your hand.
• Typically used as an
appointment book, address
book, calculator, and notepad.
• Can be synchronized with a
personal microcomputer as a
backup.

18. Basic Terminology
• Computer
– A device that accepts input,
processes data, stores data, and
produces output, all according to
a series of stored instructions.
• Hardware
– Includes the electronic and
mechanical devices that process
the data; refers to the computer
as well as peripheral devices.
• Software
– A computer program that tells
the computer how to perform
particular tasks.
• Network
– Two or more computers and
other devices that are
connected, for the purpose of
sharing data and programs.
• Peripheral devices
– Used to expand the computer’s
input, output and storage
capabilities.

19. Basic Terminology
• Input
– Whatever is put into a computer system.
• Data
– Refers to the symbols that represent facts, objects, or ideas.
• Information
– The results of the computer storing data as bits and bytes; the words,
numbers, sounds, and graphics.
• Output
– Consists of the processing results produced by a computer.
• Processing
– Manipulation of the data in many ways.
• Memory
– Area of the computer that temporarily holds data waiting to be processed,
stored, or output.
• Storage
– Area of the computer that holds data on a permanent basis when it is not
immediately needed for processing.

20. Basic Terminology
•Assembly language program (ALP) – Programs are written using
mnemonics
•Mnemonic – Instruction will be in the form of English like form
•Assembler – is a software which converts ALP to MLL (Machine
Level Language)
•HLL (High Level Language) – Programs are written using English
like statements
•Compiler - Convert HLL to MLL, does this job by reading source
program at once

21. Basic Terminology
•Interpreter – Converts HLL to MLL, does this job
statement by statement
•System software – Program routines which aid the user
in the execution of programs eg: Assemblers, Compilers
•Operating system – Collection of routines responsible
for controlling and coordinating all the activities in a
computer system

22. Computing Systems
Computers have two kinds of components:
• Hardware, consisting of its physical devices
(CPU, memory, bus, storage devices, ...)
• Software, consisting of the programs it has
(Operating system, applications, utilities, ...)

23. Functions of computer
• ALL computer functions are:
– Data PROCESSING
– Data STORAGE
– Data MOVEMENT
– CONTROL
Data = Information
Coordinates How
Information is Used

24. Functional Units of a Computer
Control
ALU
Memory
Processor
Input
Output
Since 1946 all computers have had 5 components!!!

25. 25
Functional units of a computer
I/O Processor
Output
Memory
Instr1
Instr2
Instr3
Data1
Data2
Input
Control
Arithmetic
& Logic
Input unit accepts
information:
•Human operators,
•Electromechanical devices (keyboard)
•Other computers
Output unit sends
results of processing:
•To a monitor display,
•To a printer
Arithmetic and logic unit(ALU):
•Performs the desired
operations on the input
information as determined
by instructions in the memory
Control unit coordinates
various actions
•Input,
•Output
•Processing
Stores
information:
•Instructions,
•Data

26. FUNCTIONAL UNITS OF COMPUTER
• Input Unit
• Output Unit
• Central processing Unit (ALU and Control Units)
• Memory
• Bus Structure

27. INPUT UNIT:
•Converts the external world data to a binary format, which can
be understood by CPU
•Eg: Keyboard, Mouse, Joystick etc
OUTPUT UNIT:
•Converts the binary format data to a format that a common
man can understand
•Eg: Monitor, Printer, LCD, LED etc

28. 28
Input unit
Input Unit
Processor
Memory
Keyboard
Audio input
……
Binary information must be presented to a computer in a specific format. This
task is performed by the input unit:
- Interfaces with input devices.
- Accepts binary information from the input devices.
- Presents this binary information in a format expected by the computer.
- Transfers this information to the memory or processor.
Real world Computer

29. 29
Output unit
•Computers represent information in a specific binary form. Output units:
- Interface with output devices.
- Accept processed results provided by the computer in specific binary form.
- Convert the information in binary form to a form understood by an
output device.
Output Unit
Processor
Memory
Computer Real world
Printer
Graphics display
Speakers
……

30. CPU
•The “brain” of the machine
•Responsible for carrying out computational task
•Contains ALU, CU, Registers
•ALU Performs Arithmetic and logical operations
•CU Provides control signals in accordance with some
timings which in turn controls the execution process
•Register Stores data and result and speeds up the
operation

31. 31
Arithmetic and logic unit (ALU)
• Operations are executed in the Arithmetic and
Logic Unit (ALU).
– Arithmetic operations such as addition, subtraction.
– Logic operations such as comparison of numbers.
• In order to execute an instruction, operands need
to be brought into the ALU from the memory.
– Operands are stored in general purpose registers available in the ALU.
– Access times of general purpose registers are faster than the cache.
• Results of the operations are stored back in the
memory or retained in the processor for
immediate use.

32. 32
Control unit
• Operation of a computer can be summarized as:
– Accepts information from the input units (Input unit).
– Stores the information (Memory).
– Processes the information (ALU).
– Provides processed results through the output units (Output unit).
• Operations of Input unit, Memory, ALU and
Output unit are coordinated by Control unit.
• Instructions control “what” operations take place
(e.g. data transfer, processing).
• Control unit generates timing signals which
determines “when” a particular operation takes
place.

33. MEMORY
•Stores data, results, programs
•Two class of storage
(i) Primary (ii) Secondary
•Two types are RAM or R/W memory and ROM read only memory
•ROM is used to store data and program which is not going to change.
•Secondary storage is used for bulk storage or mass storage

34. 34
Memory unit
• Memory unit stores instructions and data.
– Recall, data is represented as a series of bits.
– To store data, memory unit thus stores bits.
• Processor reads instructions and reads/writes
data from/to the memory during the
execution of a program.
– In theory, instructions and data could be fetched one bit at a time.
– In practice, a group of bits is fetched at a time.
– Group of bits stored or retrieved at a time is termed as “word”
– Number of bits in a word is termed as the “word length” of a computer.
• In order to read/write to and from memory, a
processor should know where to look:
– “Address” is associated with each word location.

35. 35
Memory unit (contd..)
• Processor reads/writes to/from memory
based on the memory address:
– Access any word location in a short and fixed amount of time based on the
address.
– Random Access Memory (RAM) provides fixed access time independent of
the location of the word.
– Access time is known as “Memory Access Time”.
• Memory and processor have to
“communicate” with each other in order to
read/write information.
– In order to reduce “communication time”, a small amount of RAM (known as
Cache) is tightly coupled with the processor.
• Modern computers have three to four levels of RAM units with different speeds
and sizes:
– Fastest, smallest known as Cache
– Slowest, largest known as Main memory.

36. 36
Memory unit (contd..)
• Primary storage of the computer consists of RAM units.
– Fastest, smallest unit is Cache.
– Slowest, largest unit is Main Memory.
• Primary storage is insufficient to store large amounts of
data and programs.
– Primary storage can be added, but it is expensive.
• Store large amounts of data on secondary storage
devices:
– Magnetic disks and tapes,
– Optical disks (CD-ROMS).
– Access to the data stored in secondary storage in slower, but take advantage of the fact that
some information may be accessed infrequently.
• Cost of a memory unit depends on its access time,
lesser access time implies higher cost.

37. MEMORY LOCATIONS AND ADDRESSES
•Main memory is the second major subsystem in a
computer. It consists of a collection of storage locations,
each with a unique identifier, called an address.
•Data is transferred to and from memory in groups of
bits called words. A word can be a group of 8 bits, 16
bits, 32 bits or 64 bits (and growing).
•If the word is 8 bits, it is referred to as a byte. The term
“byte” is so common in computer science that
sometimes a 16-bit word is referred to as a 2-byte word,
or a 32-bit word is referred to as a 4-byte word.

38. Figure 5.3 Main memory

39. Memory addresses are defined using unsigned
binary integers.
i

40. Basic Operational Concepts
Basic Function of Computer
• To Execute a given task as per the appropriate program
• Program consists of list of instructions stored in
memory

41. Interconnection between Processor and Memory

42. Registers
Registers are fast stand-alone storage locations that hold data
temporarily. Multiple registers are needed to facilitate the
operation of the CPU. Some of these registers are
 Two registers-MAR (Memory Address Register) and
MDR (Memory Data Register) : To handle the data
transfer between main memory and processor. MAR-
Holds addresses, MDR-Holds data
 Instruction register (IR) : Hold the Instructions that is
currently being executed
 Program counter: Points to the next instructions that is
to be fetched from memory

43. •(PC) (MAR)( the contents of
PC transferred to MAR)
•(MAR) (Address bus) Select a
particular memory location
•Issues RD control signals
•Reads instruction present in memory
and loaded into MDR
•Will be placed in IR (Contents transferred
from MDR to IR)

44. •Instruction present in IR will be decoded by
which processor understand what operation it has
to perform
•Increments the contents of PC by 1, so that it
points to the next instruction address
•If data required for operation is available in
register, it performs the operation
•If data is present in memory following sequence
is performed

45. •Address of the data MAR
•MAR Address bus select memory
location where is issued RD signal
•Reads data via data bus MDR
•From MDR data can be directly routed to ALU or it
can be placed in register and then operation can be
performed
•Results of the operation can be directed towards
output device, memory or register
•Normal execution preempted (interrupt)

46. BUS STRUCTURE
Connecting CPU and memory
The CPU and memory are normally connected by three
groups of connections, each called a bus: data bus, address
bus and control bus
Connecting CPU and memory using three buses

47. BUS STRUCTURE
•Group of wires which carries information form CPU to peripherals or vice
– versa
•Single bus structure: Common bus used to communicate between
peripherals and microprocessor
SINGLE BUS STRUCTURE
INPUT MEMORY PROCESSOR OUTPUT

48. Continued:-
• To improve performance multibus structure can be used
•In two – bus structure : One bus can be used to fetch
instruction other can be used to fetch data, required for
execution.
•Thus improving the performance ,but cost increases

49. A2 A1 A0 Selected
location
0 0 0 0th Location
0 0 1 1st Location
0 1 0
0 1 1
1 0 0
1 0 1
1 1 0
1 1 1
D0
D0 D7 D7
RD
PROCESSOR
CS
W/R
A0
A1
A2
CONTROL BUS
ADDRESS BUS
DATA BUS

50. Cont:-
•23 = 8 i.e. 3 address line is required to select 8 location
•In general 2x = n where x number of address lines
(address bit) and n is number of location
•Address bus : unidirectional : group of wires which
carries address information bits form processor to
peripherals (16,20,24 or more parallel signal lines)

51. Cont:-
•Databus: bidirectional : group of wires which
carries data information bit form processor to
peripherals and vice – versa
•Controlbus: bidirectional: group of wires which
carries control signals form processor to
peripherals and vice – versa
•Figure below shows address, data and control bus
and their connection with peripheral and
microprocessor

53. PERFORMANCE
•Time taken by the system to execute a program
•Parameters which influence the performance are
•Clock speed
•Type and number of instructions available
•Average time required to execute an instruction
•Memory access time
•Power dissipation in the system
•Number of I/O devices and types of I/O devices connected
•The data transfer capacity of the bus

54. 54
Information in a computer -- Instructions
• Instructions specify commands to:
– Transfer information within a computer (e.g., from
memory to ALU)
– Transfer of information between the computer and I/O
devices (e.g., from keyboard to computer, or computer to
printer)
– Perform arithmetic and logic operations (e.g., Add two
numbers, Perform a logical AND).
• A sequence of instructions to perform a task is
called a program, which is stored in the memory.
• Processor fetches instructions that make up a
program from the memory and performs the
operations stated in those instructions.

55. 55
Information in a computer -- Data
• Data are the “operands” upon which
instructions operate.
• Data could be:
– Numbers,
– Encoded characters.
• Data, in a broad sense means any digital
information.
• Computers use data that is encoded as a string
of binary digits called bits.

56. 56
How are the functional units connected?
•For a computer to achieve its operation, the functional units need to
communicate with each other.
•In order to communicate, they need to be connected.
Memory
Output Processor
Input
Bus
•Functional units may be connected by a group of parallel wires.
•The group of parallel wires is called a bus.
•Each wire in a bus can transfer one bit of information.
•The number of parallel wires in a bus is equal to the word length of
a computer

57. 57
Organization of cache and main
memory
Main
memory Processor
Bus
Cache
memory
Why is the access time of the cache memory lesser than the
access time of the main memory?

58. Computer Components: Top-Level View

59. Basic Operational Concepts

60. Slide 60
Computer Systems and Their Parts
The space of computer systems, with what we normally mean by the
word “computer” highlighted.
Computer
Analog
Fixed-function Stored-program
Electronic Nonelectronic
General-purpose Special-purpose
Number cruncher Data manipulator
Digital

61. REGISTER TRANSFER LANGUAGE
• The symbolic notation used to describe the micro-operation transfer
among registers is called RTL (Register Transfer Language).
• The information transformed from one register to another register is
represented in symbolic form by replacement operator is called Register
Transfer.
•
The use of symbols instead of a narrative explanation provides an
organized and concise manner for listing the micro-operation sequences in
registers and the control functions that initiate them.
•
A register transfer language is a system for expressing in symbolic form the
microoperation sequences among the registers of a digital module.
• It is a convenient tool for describing the internal organization of digital
computers in concise and precise manner.

62. • Registers:
•
• Computer registers are designated by upper case letters (and
optionally followed by digits or letters) to denote the function of
the register.
• For example, the register that holds an address for the memory unit
is usually called a memory address register and is designated by the
name MAR.
• Other designations for registers are PC (for program counter), IR
(for instruction register, and R1
• (for processor register).
• The individual flip-flops in an n-bit register are numbered in
sequence from 0 through n-1, starting from 0 in the rightmost
position and increasing the numbers toward the left.

64. Register Transfer Logic
• Register Transfer:
• Information transfer from one register to another
is designated in symbolic form by means of a
• replacement operator.
• The statement R2← R1 denotes a transfer of the
content of register R1 into register R2.
• It designates a replacement of the content of R2
by the content of R1.
• By definition, the content of the source register R
1 does not change after the transfer.

65. • If we want the transfer to occur only under a predetermined control
condition then it can be shown by an if-then statement.
•
if (P=1) then R2← R1
•
P is the control signal generated by a control section.
• We can separate the control variables from the register transfer operation
by specifying a Control Function.
• Control function is a Boolean variable that is equal to 0 or 1.
• control function is included in the statement as
• P: R2← R1
• Control condition is terminated by a colon implies transfer operation
be executed by the hardware only if P=1.
• Every statement written in a register transfer notation implies a hardware
construction for implementing the transfer.

66. Basic symbols of the register transfer
notation
Symbol Description Examples
Letters(and
numerals)
Denotes a register MAR, R2
Parentheses ( )
Denotes a part of a
register R2(0-7), R2(L)
Arrow <-- Denotes transfer of
information
R2 <-- R1
Comma ,
Separates two
microoperations
R2 <- R1, R1<-R2

67. • A comma is used to separate two or more
operations that are executed at the same
time.
• The statement
(exchange
• T : R2← R1, R1← R2
operation)
• denotes an operation that exchanges the
contents of two rgisters during one common
clock pulse provided that T=1.

68. •
• Bus and Memory Transfers:
• A more efficient scheme for transferring information
between registers in a multiple-register configuration
is a Common Bus System.
• A common bus consists of a set of common lines, one
for each bit of a register.
Control signals determine which register is selected
by the bus during each particular register transfer.
• Different ways of constructing a Common Bus System
• Using Multiplexers
• Using Tri-state Buffers

69. • In general a bus system has
•
multiplex “k” Registers
•
each register of “n” bits
• to produce “n-line bus”
• no. of multiplexers required = n
• size of each multiplexer = k x 1
• When the bus is includes in the statement, the register transfer is symbolized as
follows:
• BUS← C, R1← BUS
• The content of register C is placed on the bus, and the content of the bus is loaded
into register R1 by activating its load control input. If the bus is known to exist in
the system, it may be convenient just to show the direct transfer.
• R1← C

70. • Three-State Bus Buffers:
•
• A bus system can be constructed with three-state gates instead of
multiplexers.
• A three-state gate is a digital circuit that exhibits three states.
• Two of the states are signals equivalent to logic 1 and 0 as in a
conventional gate.
• The third state is a high-impedance state.
• The high-impedance state behaves like an open circuit, which means that
the output is disconnected and does not have logic significance.
• Because of this feature, a large number of three-state gate outputs can be
connected with wires to form a common bus line without endangering
loading effects.
•
The graphic symbol of a three-state buffer gate is shown in Fig. 4-4.

72. • Memory Transfer:
• The transfer of information from a memory word to the outside
environment is called a read
• operation.
• The transfer of new information to be stored into the memory is
called a write operation.
• A memory word will be symbolized by the letter M.
• The particular memory word among the many available is selected
by the memory address during the transfer.
• It is necessary to specify the address of M when writing memory
transfer operations.
• This will be done by enclosing the address in square brackets
following the letter M.
•

73. • Consider a memory unit that receives the address from a register,
called the address register, symbolized by AR.
• The data are transferred to another register, called the data register,
symbolized by DR.
• The read operation can be stated as follows:
• Read: DR<- M [AR]
•
• This causes a transfer of information into DR from the memory
word M selected by the address in AR.
• The write operation transfers the content of a data register to a
memory word M selected by the address. Assume that the input
data are in register R1 and the address is in AR.
• The write operation can be stated as follows:
• Write: M [AR] <- R1

74. Micro-operations
• Types of Micro-operations:
• Register Transfer Micro-operations: Transfer binary information
from one register to another.
• Arithmetic Micro-operations: Perform arithmetic operation on
numeric data stored in registers.
• Logical Micro-operations: Perform bit manipulation operations on
data stored in registers.
• Shift Micro-operations: Perform shift operations on data stored in
registers.
Register Transfer Micro-operation doesn’t change the information
content when the binary information moves from source register to
destination register.
Other three types of micro-operations change the information
change the information content during the transfer.

75. Arithmetic Micro Operations
•
• The basic arithmetic micro-operations are
• Addition
• Subtraction
• Increment
• Decrement
• Shift
• The arithmetic Micro-operation defined by the statement below specifies the add
micro- operation.
• R3 ← R1 + R2
• It states that the contents of R1 are added to contents of R2 and sum is transferred
to R3.
To implement this statement hardware requires 3 registers and digital
component that performs addition
• Subtraction is most often implemented through complementation and addition.
• The subtract operation is specified by the following statement
• R3 ← R1 + R2 + 1

76. • instead of minus operator, we can write as
• R2 is the symbol for the 1’s complement of R2
• Adding 1 to 1’s complement produces 2’s complement
• Adding the contents of R1 to the 2's complement of R2 is
equivalent to R1-R2.
• Binary Adder:
• Digital circuit that forms the arithmetic sum of 2 bits and the
previous carry is called FULL ADDER.
• Digital circuit that generates the arithmetic sum of 2 binary
numbers of any lengths is called
• BINARY ADDER.
• Figure 4-6 shows the interconnections of four full-adders (FA) to
provide a 4-bit binary adder.

77. • The augends bits of A and the addend bits of B are designated by
subscript numbers from right to left, with subscript 0 denoting the
low-order bit.
• The carries are connected in a chain through the full-adders. The
input carry to the binary adder is Co and the output carry is C4. The
S outputs of the full-adders generate the required sum bits.
• An n-bit binary adder requires n full-adders.

78. • Binary Adder – Subtractor:
• The addition and subtraction operations can be combined into one common circuit
by including an exclusive-OR gate with each full-adder.
• A 4-bit adder-subtractor circuit is shown in Fig. 4-7.
• The mode input M controls the operation. When M = 0 the circuit is an adder and
when M = 1 the circuit becomes a subtractor.
• Each exclusive-OR gate receives input M and one of the inputs of B
• When M = 0, we have B xor 0 = B. The full-adders receive the value of B, the
input carry is 0, and the circuit performs A plus B.
• When M = 1, we have B xor 1 = B' and Co = 1.
• The B inputs are all complemented and a 1 is added through the input carry.
• The circuit performs the operation A plus the 2's complement of B.

79. Logical Micro Operations
• Logic Micro-operations:
• Logic microoperations specify binary operations for strings
of bits stored in registers.
• These operations consider each bit of the register
separately and treat them as binary variables.
• For example, the exclusive-OR microoperation with the
contents of two registers RI and R2 is symbolized by the
statement
• It specifies a logic microoperation to be executed on the
individual bits of the registers provided that the control
variable P = 1.

81. • Hardware Implementation:
• The hardware implementation of logic microoperations
requires that logic gates be inserted for each bit or pair of
bits in the registers to perform the required logic function.
• Although there are 16 logic microoperations, most
computers use only four--AND, OR, XOR (exclusive-OR), and
complement from which all others can be derived.

82. Shift Micro Operations
• Shift microoperations are used for serial transfer of
data.
• The contents of a register can be shifted to the left or
the right.
• During a shift-left operation the serial input transfers a
bit into the rightmost position.
• During a shift-right operation the serial input transfers
a bit into the leftmost position.
• There are three types of shifts: logical, circular, and
arithmetic.
• The symbolic notation for the shift microoperations is
shown in Table 4-7.

84. • Logical Shift:
• A logical shift is one that transfers 0 through the serial input.
• The symbols shl and shr for logical shift-left and shift-right
microoperations.
The microoperations that specify a 1-bit shift to the left of the
content of register R and a 1-bit shift to the right of the content of
register R shown in table 4.7.
• The bit transferred to the end position through the serial input is
assumed to be 0 during a logical shift.
• Circular Shift:
• The circular shift (also known as a rotate operation) circulates the
bits of the register around the two ends without loss of
information.
• This is accomplished by connecting the serial output of the shift
register to its serial input.
• We will use the symbols cil and cir for the circular shift left and
right, respectively.

85. • Arithmetic Shift:
• An arithmetic shift is a microoperation that shifts
a signed binary number to the left or right.
• An arithmetic shift-left multiplies a signed binary
number by 2.
• An arithmetic shift-right divides the number by 2.
• Arithmetic shifts must leave the sign bit
unchanged because the sign of the number
remains the same when it is multiplied or divided
by 2.

86. Arithmetic Logic Shift Unit
• Instead of having individual registers performing the
microoperations directly, computer systems employ a number of
storage registers connected to a common operational unit called an
arithmetic logic unit, abbreviated ALU.
• The ALU is a combinational circuit so that the entire register
transfer operation from the
• source registers through the ALU and into the destination register
can be performed during one clock pulse period.
• The shift microoperations are often performed in a separate unit,
but sometimes the shift unit is made part of the overall ALU.
• The arithmetic, logic, and shift circuits introduced in previous
sections can be combined into one ALU with common selection
variables. One stage of an arithmetic logic shift unit is shown in Fig.
4- 13.

88. • Particular microoperation is selected with inputs S1 and S0. A 4 x 1
multiplexer at the output chooses between an arithmetic output in
Di and a logic output in Ei.
• The data in the multiplexer are selected with inputs S3 and S2. The
other two data inputs to the multiplexer receive inputs Ai-1 for the
shift-right operation and Ai+1 for the shift-left operation.
• The circuit whose one stage is specified in Fig. 4-13 provides eight
arithmetic operation, four logic operations, and two shift
operations.
• Each operation is selected with the five variables S3, S2, S1, S0 and
Cin.
• The input carry Cin is used for selecting an arithmetic operation
only.

90. 90
Instruction Codes
• Instruction:
• Instructions specify commands to:
– Transfer information within a computer (e.g., from memory to
ALU)
– Transfer of information between the computer and I/O devices
(e.g., from keyboard to computer, or computer to printer)
– Perform arithmetic and logic operations (e.g., Add two numbers,
Perform a logical AND).
• A sequence of instructions to perform a task is called a
program, which is stored in the memory.
• Processor fetches instructions that make up a program
from the memory and performs the operations stated
in those instructions.

91. Instruction Codes
● The organization of the computer is defined by its
internal registers, the timing and control structure,
and the set of instructions that it uses.
● A computer instruction is a binary code that
specifies a sequence of microoperations for the
computer.
● An instruction code is a group of bits that instruct
the computer to perform a specific operation.
● Instruction code is usually divided into two parts.
○ Operation part - Group of bits that define such
operations as add, subtract, multiply, shift, and
complement.
○ Address part - Contains registers or memory words
where the address of operand is found or the result is to
be stored.
● Each computer has its own instruction code format.

92. Operation Code
● The operation code(op-code) of an
instruction is a group of bits that define such
operations as add, subtract, multiply, shift,
and complement.
● The number of bits required for the operation
code of an instruction depends on the total
number of operations available in the
computer.(n bits for 2n operations)
● An operation code is sometimes called a
macro-operation because it specifies a set
of micro-operations.

93. Stored Program Organization
● Simplest way to organize computer is to have
one processor register(Accumulator AC) and an
instruction code format with two parts.
○ First-Operation to be performed(Opcode)
○ Second – Address
● The memory address tells the control where to
find an operand in memory.
● This operand is read from memory and used as
the data to be operated on together with the
data stored in the processor register.

94. ● Instructions are stored
in one section of the
memory and data in
another.
● For a memory unit with
4096 words we need 12
bits to specify an
address since
212=4096.
● 4 bits are available for
opcode to specify one
out of 16 possible
operations.

95. ● The control reads a 16-bit instruction from
the program portion of memory.
● It uses the 12-bit address part of the
instruction to read a 16-bit operand from
the data portion of memory.
● It then executes the operation specified
by the operation code.
● The operation is performed with the
memory operand and the content of AC.

96. Computer Registers
Need of Registers?
● Computer instructions are normally stored in consecutive memory
locations and are executed sequentially one at a time.
● The control reads an instruction from a specific address in memory
and executes it. It then continues by reading the next instruction in
sequence and executes it, and so on.
● This type of instruction sequencing needs a counter to calculate
the address of the next instruction after execution of the current
instruction is completed.
● It is also necessary to provide a register in the control unit for
storing the instruction code after it is read from memory.
● The computer needs processor registers for manipulating data and
a register for holding a memory address.

97. List of basic Registers
Code Bits Name Purpose
DR 16 Data Register Holds memory operand
AR 12 Address register Holds address for memory
AC 16 Accumulator Processor register
IR 16 Instruction register Holds instruction code
PC 12 Program counter Holds address of instruction
TR 16 Temporary register Holds temporary data
INPR 8 Input register Holds input character
OUTR 8 Output register Holds output character

98. Common Bus System
Need of Common Bus System ?
● The basic computer has eight registers, a memory
unit, and a control unit.
● Paths must be provided to transfer information from
one register to another and between memory and
registers.
● The number of wires will be excessive if connections
are made between the outputs of each register and
the inputs of the other registers.
● Hence more efficient scheme with a common bus is
used.

99. ● The outputs of seven registers and memory are connected to
the common bus.
● The specific output that is selected for the bus lines at any
given time is determined from the binary value of the
selection variables S2, S1, and S0.
● The number along each output shows the decimal equivalent
of the required binary selection.
● For example, the number along the output of DR is 3. The
16-bit outputs of DR are placed on the bus lines when
S2S1S0 =011 since this is the binary value of decimal 3.
● The lines from the common bus are connected to the inputs
of each register and the data inputs of the memory.
● The particular register whose LD (load) input is enabled
receives the data from the bus during the next clock pulse
transition.
● The memory receives the contents of the bus when its write
input is activated.
● The memory places its 16-bit output onto the bus when the
read input is activated and S2S1S0 =111.

101. ● INPR is connected to provide information to the bus
but OUTR can only receive information from the bus.
● This is because INPR receives a character from an
input device which is then transferred to AC.
● OUTR receives a character from AC and delivers it to
an output device.
● There is no transfer from OUTR to any of the other
registers.
● The inputs of AC come from an adder and logic
circuit.
● This circuit has three sets of inputs.
● One set of 16-bit inputs come from the outputs of AC.
They are used to implement register micro-operations
such as complement AC and shift AC.
● Another set of 16-bit inputs come from the data
regisler DR. The inputs from DR and AC are used for
arithmetic and logic microoperations.
● A third set of 8-bit inputs come from the input register
INPR.

102. Computer
Instructions

103. Instruction Cycle
• The processing required for a single instruction is called an
instruction cycle.
• The CPU performs a sequence of micro operations for each
instruction. The sequence for each instruction of the Basic
Computer can be refined into 4 abstract phases:
• 1. Fetch instruction
• 2. Decode
• 3. Fetch operand
• 4. Execute

104. • ADD LocationA, LocationB
• This instruction adds the contents of memory
locationA and Contents of memory locationB in 3
instruction cycles.
• The program fragment shown adds the contents
of the memory word at address 940 to the
contents of the memory word at address 941 and
stores the result in the latter location.
• Three instructions, which can be described as
three fetch and three execute cycles, are required

105. INSTRUCTION CYCLE

106. INSTRUCTION CYCLE

107. • Program execution can be represented in above
figure:
• Instruction 1
step1. Fetch instruction1
step2. Execute instruction1
• Instruction 2
step3. Fetch instruction2
step4. Execute instruction2
• Instruction 3
step5. Fetch instruction3
step6. Execute instruction3

108. 1. The PC contains 300, the address of the first instruction.
This instruction (the value 1940 in hexadecimal) is
loaded into the instruction register IR, and the PC is
incremented.
2. The first 4 bits (first hexadecimal digit) in the IR indicate
that the AC is to be loaded. The remaining 12 bits (three
hexadecimal digits) specify the address (940) from
which data are to be loaded.
3. The next instruction (5941) is fetched from location 301,
and the PC is incremented.
4. The old contents of the AC and the contents of location
941 are added, and the result is stored in the AC.
5. The next instruction (2941) is fetched from location 302,
and the PC is incremented.
6. The contents of the AC are stored in location 941.

109. Instruction Format
● The basic computer has three instruction
code formats each having 16 bits
○ Memory reference instructions
○ Register reference instructions
○ I/O instructions
● The opcode part of the instruction contains
three bits and the meaning of the remaining
13 bits depends on the operation code
encountered.

110. Memory reference instructions
● Bits 0-11 for specifying address.
● Bits 12-14 for specifying opcode.
● 15th bit specifies addressing modes. (0 for direct
and 1 for indirect)
● Opcode=000 through 110
● I=0 or 1
● Eg:
○ AND - 0xxx(direct) or 8xxx(indirect)
○ ADD - 1xxx or 9xxx

111. Register reference instructions
● Recognized by the operation code 111 with a 0 in the
15th bit of the instruction.
● Specifies an operation on or a test of the AC register.
● An operand from memory is not needed.
● Therefore the 12 bits are used to specify the operation
to be executed.
● Eg:
○ CLA - 7800 : Clear AC
○ CLE - 7400 : Clear E

112. I/O instructions
● These instructions are needed for transfering
informations to and from AC register.
● Recognized by the opcode 111 and a 1 in the
15th bit.
● Bits 0-11 specify the type of I/O Operation
performed.
● Eg:
○ INP - F800 : Input characters to AC
○ OUT - F400 : Output characters from AC

114. Instruction Set Completeness
● The set of instructions are said to be
complete if the computer includes a sufficient
number of instructions in each of the
following categories
a. Arithmetic, logical, and shift instructions
b. Instructions for moving information to and from
memory and processor registers
c. Program control instructions
d. Input and output instructions

115. Timing and
Control

116. Timing and Control
● The timing for all registers in the basic computer
is controlled by a master clock generator.
● The clock pulses are applied to all flip-flops and
registers.
● The clock pulses do not change the state of a
register unless the register is enabled by a
control signal.
● Two major types of control organization:
○ hardwired control
○ microprogrammed control.

117. Example
Add R1, R2
T1
T2
T3
T4
Enable R1
Enable R2
Enable ALU for addition operation

118. •Control unit works with a
reference signal called
processor clock
•Processor divides the
operations into basic steps
•Each basic step is
executed in one clock
cycle
T1
T2
R1 R2
R2

119. Hardwired Control
● The control logic is
implemented with
gates, flip-flops,
decoders, and other
digital circuits.
● It can be optimized to
produce a fast mode of
operation.
● Requires changes in the
wiring among the
various components if
the design has to be
modified or changed.
Microprogrammed Control
● The control information
is stored in a control
memory.
● The control memory is
programmed to initiate
the required sequence
of microoperations.
● Required changes or
modifications can be
done by updating the
microprogram in
control memory.

120. ● Consists of two decoders, a sequence counter, and a
number of control logic gates.
● An instruction read from memory is placed in the
instruction register(IR).
● The IR is divided into three parts:
○ I bit, opcode, and Address bits.
● Op-code in 12-14 bits are decoded with a 3x8
decoder.
● 8 outputs of the decoder are designated by the
symbols D0 through D7.
● Bit 15 is transferred to a flip-flop I.
● Bits 0-11 are applied to the control logic gates.
Hardwired control unit

121. Hardwired control unit

122. ● The 4-bit sequence counter can count in binary from 0-
15.
● The outputs of the counter are decoded into 16 timing
signals T0-T15.
● The sequence counter SC can be incremented or
cleared synchronously.
● Mostly,SC is incremented to provide the sequence of
timing signals(T1,T2,...,T15)
● Once in a while, the counter is cleared to 0, causing
the next active timing signal to be T0.
● Eg: Suppose,at time T4, SC is cleared to 0 if decoder
output D3 is active. D3T4: SC ← 0.
Hardwired control unit

123. ● The SC responds to the positive transition of the
clock.
● Initially, the CLR input of SC is active.
● Hence it clears SC to 0, giving the timing signal T0
out of the decoder.
● T0 is active during one clock cycle and will trigger
only those registers whose control inputs are
connected to timing signal T0.
● SC is incremented with every positive clock
transition, unless its CLR input is active.
● This produces the sequence of timing signals T0, T1,
T2, T3, T4 up to T15 and back to T0.
Hardwired control unit

124. Interrupt
• When the external device becomes ready to be
serviced— then the external device sends an interrupt
request signal to the processor. The processor
responds by suspending operation of the current
program
• With interrupts, the processor can be engaged in
executing other instructions while an I/O operation is
in progress and resumes the original execution after
the device is serviced.
• Processor provides requested service by executing
interrupt service routine (ISR)
• Contents of PC, general registers, and some control
information are stored in memory .
• When ISR completed, processor restored, so that
interrupted program may continue

126. • It suspends execution of the current program
being executed and saves its context. This
means saving the address of the next
instruction to be executed

127. END OF UNIT 1