Computer organization basics

Computer Architecture
Deepak John
Department Of Computer Applicaions
SJCET-Pala

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

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

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

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

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.

Basic Terminology
• Input
– Whatever is put into a computer system.
• Data
– Refers to the symbols that represent facts, objects, or ideas.
• 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.

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

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

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

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
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

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

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
Interconnection between
Processor and Memory

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

Typical Operating Steps
• Programs reside in the memory through input
devices
• PC is set to point to the first instruction
• The contents of PC are transferred to MAR
• A Read signal is sent to the memory
• The first instruction is read out and loaded
into MDR
• The contents of MDR are transferred to IR
• Decode and execute the instruction

Typical Operating Steps (Cont’)
• Get operands for ALU
General-purpose register
Memory (address to MAR – Read – MDR to ALU)
• Perform operation in ALU
• Store the result back
To general-purpose register
To memory (address to MAR, result to MDR – Write)
• During the execution, PC is incremented to
the next instruction

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

Bus Structure
• Single-bus
Speed Issue
• Different devices have different transfer/operate speed.
• If the speed of bus is bounded by the slowest device
connected to it, the efficiency will be very low.
• A common approach – use buffers.

Memory Locations, Addresses,
and Operations

• Memory consists of
many millions of
storage cells, each of
which can store 1 bit.
• Data is usually
accessed in n-bit
groups. n is called
word length.
second word
first word
Figure Memory words.
n bits
last word
i th word
•
•
•
•
•
•

• A k-bit address memory has 2k memory locations, namely
0 – 2k-1, called memory space.
24-bit memory: 224 = 16,777,216 = 16M (1M=220)
32-bit memory: 232 = 4G (1G=230)
• byte-addressable memory.
• Byte locations have addresses 0, 1, 2, … If word length is
32 bits, the successive words are located at addresses 0,
4, 8,…

Big-Endian and Little-Endian Assignments
2
k
4- 2
k
3- 2
k
2- 2
k
1- 2
k
4-2
k
4-
0 1 2 3
4 5 6 7
00
4
2
k
1- 2
k
2- 2
k
3- 2
k
4-
3 2 1 0
7 6 5 4
Byte addressByte address
(a) Big-endian assignment (b) Little-endian assignment
4
W ord
address
•
•
•
•
•
•
Figure . Byte and word addressing.
Big-Endian: lower byte addresses are used for the most significant bytes of the
word
Little-Endian: opposite ordering. lower byte addresses are used for the less
significant bytes of the word

• Address ordering of bytes
• Word alignment
– Words are said to be aligned in memory if they
begin at a byte address. that is a multiple of the
num of bytes in a word.
• 16-bit word: word addresses: 0, 2, 4,….
• 32-bit word: word addresses: 0, 4, 8,….
• 64-bit word: word addresses: 0, 8,16,….

Instruction and Instruction
Sequencing

“Must-Perform” Operations
• Data transfers between the memory and the
processor registers
• Arithmetic and logic operations on data
• Program sequencing and control
• I/O transfers

Register Transfer Notation
• Identify a location by a symbolic name standing
for its hardware binary address (LOC, R0,…)
• Contents of a location are denoted by placing
square brackets around the name of the location
(R1←[LOC], R3 ←[R1]+[R2])
Assembly Language Notation
 Represent machine instructions and programs.
 Move LOC, R1 = R1←[LOC]
 Add R1, R2, R3 = R3 ←[R1]+[R2]

CPU Organization
• Single Accumulator
– Result usually goes to the Accumulator
– Accumulator has to be saved to memory quite often
• General Register
– Registers hold operands thus reduce memory traffic
– Register bookkeeping
• Stack
– Operands and result are always in the stack

Instruction Formats
• Three-Address Instructions
– ADD R1, R2, R3 R1 ← R2 + R3
• Two-Address Instructions
– ADD R1, R2 R1 ← R1 + R2
• One-Address Instructions
– ADD M AC ← AC + M
• Zero-Address Instructions
– ADD TOS ← TOS + (TOS – 1)
Opcode Operand(s) or Address(es)

Instruction Formats
Example: Evaluate (A+B) ∗ (C+D)
• Three-Address
1. ADD R1, A, B ; R1 ← M[A] + M[B]
2. ADD R2, C, D ; R2 ← M[C] + M[D]
3. MUL X, R1, R2 ; M[X] ← R1 ∗ R2

Instruction Formats
Example: Evaluate (A+B) ∗ (C+D)
• Two-Address
1. MOV R1, A ; R1 ← M[A]
2. ADD R1, B ; R1 ← R1 + M[B]
3. MOV R2, C ; R2 ← M[C]
4. ADD R2, D ; R2 ← R2 + M[D]
5. MUL R1, R2 ; R1 ← R1 ∗ R2
6. MOV X, R1 ; M[X] ← R1

Instruction Execution and Straight-
Line Sequencing
R0,C
B,R0
A,R0
Movei + 8
Begin execution here Movei
ContentsAddress
C
B
A
the program
Data for
segment
program
3-instruction
Addi + 4
Figure A program for C ← [Α] + [Β].
For executing the pgm,address of
first instruction placed in PC and
the processor control circuits use
the information in the PC to fetch
and execute instructions, one at a
time ,in the order of increasing
addresses is called straight line
sequencing
Three-phase procedure
-Instruction fetch
-Instruction decode
-Instruction execute

Instruction Execution
Instruction execution is a 3-phase process:
instr fetch:
select an instr. from MM based on the current value of
the PC & place it in the IR.
instr decode:
decode and fetch the operand values.
instr execute:
perform the appropriate data transformations and
store the results. In the case of sequencing/branch
instructions, update the PC if necessary.

BRANCHING
A straight line program for adding n
numbers Using a loop to add n numbers

BRANCHING
• Branch instruction are those which changes the normal
sequence of execution.
• Sequence can be changed either conditionally or
unconditionally.
• Accordingly we have conditional branch instructions and
unconditional branch instruction.
• Conditional branch instruction changes the sequence
only when certain conditions are met.
• Unconditional branch instruction changes the sequence
of execution irrespective of condition of the results.

Condition Codes
• Condition code flags
• Condition code register / status register
• N (negative)
• Z (zero)
• V (overflow)
• C (carry)
• Different instructions affect different flags

Addressing Modes
• Immediate
• Direct
• Indirect
• Register
• Register Indirect
• Displacement (Indexed)
• Stack

Immediate Addressing
• Operand is part of instruction
• Operand = address field
• e.g. ADD 5
– Add 5 to contents of accumulator
– 5 is operand
• No memory reference to fetch data
• Fast,Limited range
OperandOpcode
Instruction

Direct Addressing
• Address field contains address of operand
• Effective address (EA) = address field (A)
• e.g. ADD A
– Add contents of cell A to accumulator
– Look in memory at address A for operand
• Single memory reference to access data
• No additional calculations to work out effective address
• Limited address space

Direct Addressing Diagram
Address AOpcode
Instruction
Memory
Operand

Indirect Addressing
• Memory cell pointed to by address field contains the address of
(pointer to) the operand
• EA = (A)
– Look in A, find address (A) and look there for operand
• e.g. ADD (A)
– Add contents of cell pointed to by contents of A to accumulator
• Large address space
• Multiple memory accesses to find operand
• Hence slower

Indirect Addressing Diagram
Address AOpcode
Instruction
Memory
Operand
Pointer to operand

Register Addressing
• Operand is held in register named in address filed
• EA = R
• Ex:add a,b
• The contents of register B are added to the accumulator ,a += b;
• Limited number of registers
• Very small address field needed
– Shorter instructions
– Faster instruction fetch
• No memory access
• Very fast execution
• Very limited address space

Register Addressing Diagram
Register Address ROpcode
Instruction
Registers
Operand

Register Indirect Addressing
• EA = (R)
• Operand is in memory cell pointed to by contents of register R.
• One fewer memory access than indirect addressing.
• Mov (R1),A
Register Address ROpcode
Instruction
Memory
OperandPointer to Operand
Registers

Displacement Addressing
• EA = A + (R)
• Address field hold two values
– A = base value
– R = register that holds displacement
– or vice versa

Register ROpcode
Instruction
Memory
OperandPointer to Operand
Registers
Address A
+

Relative Addressing
• A version of displacement addressing
• R = Program counter, PC
• EA = A + (PC)
• i.e. get operand from A cells from current location pointed to by PC
• c.f locality of reference & cache usage

Base-Register Addressing
• A holds displacement
• R holds pointer to base address
• R may be explicit or implicit
• e.g. segment registers in 80x86

Indexed Addressing
• A = base
• R = displacement
• EA = A + (R)
• Good for accessing arrays
– EA = A + (R)
– R++

Stack Addressing
• Operand is (implicitly) on top of stack
• e.g.
– ADD Pop top two items from stack
and add

• The different ways in which the location of an operand is specified in
an instruction are referred to as addressing modes.
Name Assem bler syntax Addressing function
Immediate #V alue Op erand = Value
Register R i EA = R i
Absolute(Direct) LOC EA = LOC
Indirect (Ri) EA = [Ri]
(LOC) EA = [LOC]
Index X(R i) EA = [Ri]+ X
Base with index (Ri,Rj) EA = [Ri]+ [Rj]
Base with index X(R i,Rj) EA = [Ri]+ [Rj] + X
and offset
Relative X(PC) EA = [PC] + X
Autoincrement (Ri)+ EA = [Ri];
Increment R i
Autodecrement (Ri) Decremen t R i ;
EA = [Ri]
−
Register Mode-Operand is
the contents of a processor
register
Absolute Mode -Operand
is in a memory Location
Effective Address:is any
operand to an instruction
which references memory.

Assembly Language Instructions
• Built from two pieces
Add R1, R3, 3
Opcode
What to do with the data
(ALU operation)
Operands
Where to get data and
put the results
• An instruction has the following format:

Types of Instructions
• Data Transfer
InstructionsName Mnemonic
Load LD
Store ST
Move MOV
Exchange XCH
Input IN
Output OUT
Push PUSH
Pop POP
Arithmetic
Name Mnemonic
Increment INC
Decrement DEC
Add ADD
Subtract SUB
Multiply MUL
Divide DIV
Add with carry ADDC
Subtract with borrow SUBB
Negate NEG

Data Manipulation Instructions
• Logical & Bit Manipulation
• Shift
Name Mnemonic
Clear CLR
Complement COM
AND AND
OR OR
Exclusive-OR XOR
Clear carry CLRC
Set carry SETC
Complement carry COMC
Enable interrupt EI
Disable interrupt DI
Name Mnemonic
Logical shift right SHR
Logical shift left SHL
Arithmetic shift right SHRA
Arithmetic shift left SHLA
Rotate right ROR
Rotate left ROL
Rotate right through carry RORC
Rotate left through carry ROLC

Program Control Instructions
Name Mnemonic
Branch BR
Jump JMP
Skip SKP
Call CALL
Return RET
Compare
(Subtract)
CMP
Test (AND) TST
Conditional Branch Instructions
Mnemoni
c
Branch Condition
Tested
Condition
BZ Branch if zero Z = 1
BNZ Branch if not zero Z = 0
BC Branch if carry C = 1
BNC Branch if no carry C = 0
BP Branch if plus S = 0
BM Branch if minus S = 1
BV Branch if overflow V = 1
BNV
Branch if no
overflow
V = 0

Assembly Directive
• For an assembler to produce an object code, it
has to know the following:
1. How to interpret the name (sum = 200)
2. Where to place the instructions in the memory
3. Where to place the data operands in the memory

• Assembler Directives
Allows the programmer to specify other information needed
translate the source program into the object program
• EQU-simply equates a symbolic name to a numeric value.
• ORIGIN-tells the assembler where to load instructions and data
into memory.
• DATAWORD- states that the data value 100, is to be placed in the
memory (at address 204).
• RESERVE- declares that a memory block of 400 bytes, is to be
reserved for data.
• RETURN -directive indicates where the execution of the program
should be terminated.
• END -directive tells the assembler where is the end of the source
program.

Assembler
• The assembler is a program that replaces all symbols in
the source code with the binary codes.
• The assembler also replaces names and labels with the
actual values.
• The assembler scans through a source program, keep
tracks of all names and the numerical values in a symbol
table. When names appear, they are replaced with
values from this table.
• Normally use two pass assembler

• Stack is a list of data
elements that can be
added or removed at
one end only (top). It
is LIFO ( Last in first
out)
• Stack is usually used
to handle control
between a main
program and
subroutines
• SP-Stack pointer is a
processor register
used to keep track of
the address of the
element of the stack
that is at the Top
.
.
.
-28
17
739
.
.
.
43
.
.
.
Current
Top
element
Bottom
element
SP
Stack
Pointer
register
BOTTOM
0
2k-1
Stack
PUSH: inserting an Element
POP: removing an element

19
-28
17
739
.
.
.
43
19
-28
17
739
.
.
.
43
-28
Stack
SP
SP
NEWITEM ITEM
(a) After push from NEWITEM (b) After pop into ITEM

Subroutines
• Only one copy of subroutine is store in a memory to save space.
Any program that want to use a subroutine simply branches to its
starting location.
• When a subroutine returns, the assembler has to make sure that it
returns to the appropriate location. The simplest way is to use a link
register to save the content of the PC when a subroutine is called.
Memory
Location
Calling Program Memory
Location
Subroutine SUB
.
.
.
200 Call SUB 1000 first instruction
204 next instruction .
.
.
.
.
. Return

204
204
1000
Link
PC
Call Return

• The link register cannot support the nested subroutine calls (a
subroutine calling another subroutine).
• Using a stack concept, nested subroutine calls can be carried out at
any depth.
– Call instruction : pushes the content of the PC to the stack and
loads the beginning of subroutine address to the PC.
– Return instruction : pops the return address from a stack and
copy it to a PC.
• Parameter passing
1. Can be done using registers. The called program can place a
set of parameters in a set of registers before calling a
subroutine. The subroutine then uses those values and returns
the result via another register.
2. Can also be done by placing parameters in the stack together
with the return address.

Interrupt
• An interrupt is a request from I/O device for service by
processor
• 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

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

Basic Instruction Cycle
• Basic computer operation cycle
– Fetch the instruction from memory into a control register
(PC)
– Decode the instruction
– Locate the operands used by the instruction
– Fetch operands from memory (if necessary)
– Execute the operation in processor registers
– Store the results in the proper place
– Go back to step 1 to fetch the next instruction

Computer organization basics

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Computer organization basics