ADDRESSING MODE

WELCOME TO
OUR
PRESENTATION
10/17/2014 1

Define
Addressing
Mode
&
Different Types
of addressing
mode with
numerical
example
10/17/2014 2

Prepared By
NAME ID
Israt Jahan Mishu BKH1201028F
Asma Anika Shahabuddin BKH1201043F
Parmita Dewanjee Shalu BKH1201040F
10/17/2014 3

ADDRESSING MODES
The way the operands are chosen during
program execution is dependent on the
addressing mode of the instruction. The
addressing mode specifies a rule for interpreting
or modifying the address field of the instruction
before the operand is actually referenced.
Computers use addressing mode techniques for
the purpose of accomodating one or both of the
following:
10/17/2014 4

Computers use addressing mode techniques for
the purpose of accomodating one or both of the
following:
 TO give programming versatility to the user by
providing such facilities as pointers to
memory,counters for loop control,indexing of
data,and program relocation.
 To reduce the number of bits in the addressing
field of the instruction.
10/17/2014 5

DIFFERENT TYPES OF ADDRESSING
MODES
Implied mode
Immediate mode
Register mode
Register indirect mode
Autoincrement or Autodecrement mode
Direct address mode
Indirect address mode
Relative address mode
Indexed addressing mode
Base register addressing mode
10/17/2014 6

Implied Mode
In this mode the operands are specified implicitly
in the defination of the instruction.
For example, the instruction “complement
accumulator” is an implied-mode instruction.
Opcode Mode Address
Fig: Instruction format with mode field
10/17/2014 7

Immediate Mode
In this mode the operand is specified in
the instruction itself.
In other words, an immediate-mode
instruction has an operand field rather
than an address field.
10/17/2014
8

Immediate-mode instructions are useful for
initializing registers to a constant value.
The operand field contains the actual operand
to be used in conjunction with the operation
specified in instruction.
10/17/2014 9

Register Mode
In this mode the operands are in registers
that reside within the CPU.
The particular register is selected from register
field in the instruction.
A k-bit field can specify any one of 2ᴷ registers
10/17/2014 10

In this mode the instruction specifies a
register in the CPU whose contents give the
address of the operand in memory.
In other words, the selected register contains
the address of the operand rather than the
operand itself.
10/17/2014 11

The advantage of a register indirect
mode a instruction is that the
address field of the instruction uses
fewer bits to select a register than
would have been required to specify
a memory address directly
10/17/2014 12

AUTOINCREMENT OR
AUTODECREMENT MODE:
This is similar to the register indirect mode
except that the register is increment or
decrement after (or before) its value is used to
access memory.
When the address stored in the register refers to a
table of data in memory,it is necessary to increment
or decrement the register after every access to the
table.
10/17/2014 13

However, because it is such a
common requirement. Some
computers incorporate a special
mode that automatically increments
or decrements the content of the
register after data access.
10/17/2014 14

DIRECT ADDRESS MODE
Is this mode the effective address is equal to the
address part of the instruction.
The operand resides in memory and its address is
given directly by the address field of the instruction.
In a branch-type instruction the address field
specifies the actual branch address.
10/17/2014 15

INDIRECT ADDRESS MODE
In this mode the address field of the
instruction gives the address where the
effective address is stored in memory.
Control fetches the instruction from memory
and uses its address part to access memory
again to read the effective address.
10/17/2014 16

 A few addressing modes require that the address
field of the instruction be added to the content of
a specific register in the CPU.
The effective address in these modes is obtained
from the following computation:
Effective address=address part of instruction +
content of CPU register.
10/17/2014 17

RELATIVE ADDRESS MODE
 In this mode the content of the program
counter is added to the address part of the
instruction in order to obtain the effective
address.
 The address part of the instruction is usually
a signed number which can be either positive
or negative.
10/17/2014 18

When this number is added to the content of the
program counter , the result produces an effective
address whose position in memory is relative to
the address of the next instruction.
 It results in a shorter address field in the
instruction format since the relative address can
be specified with a smaller number of bits
required to designate the entire memory address.
10/17/2014 19

INDEXED ADDRESSING MODE
 In this mode the content of an index
register is added to the address part of
the instruction to obtain the effective
address.
The index register is a special CPU
register that contains an index value.
10/17/2014 20

 The address field of the instruction
defines the beginning address of a data
array in memory.
 Each operand in the array is stored in
memory relative to the beginning
address.
10/17/2014 21

BASE REGISTER ADDRESSING MODE
 In this mode the content of a base register is
added to the address part of the instruction to
obtain the effective address.
 This is similar to the indexed addressing mode
except that the register is now called a base
register instead of an index register.
10/17/2014 22

 The base register addressing mode is used in
computers to facilitate the relocation of
programs in memory.
 Only the value of the base register requires
updating to reflect the beginning of a new
memory segment.
10/17/2014 23

Numerical Example
To show the differences between the various modes,
we will show the effect of the addressing modes on
the instruction defined in fig:1. The two-word
instruction at address 200 and 201 is a “load to AC”
instruction with an address field equal to 500. The
first word of the instruction specifies the operation
code and mode, and the second word specifies the
address part. PC has the value 200 for fetching this
instruction. The content of the processor register R1
is 400,and the content of an index register XR is 100.
AC receives the operand after the instruction is
executed.
10/17/2014 24

Address Memory
PC=200
R1=400
XR=100
AC
Load to Ac Mode
Address=500
Next instruction
450
700
800
900
325
10/17/2014 1
300
Figure: Numerical example for
addressing modes
200
201
202
399
400
500
600
702
800

Addressing mode Effective Address Content of AC
Direct Address 500 800
Immediate
operand
201 500
Indirect Address 800 300
Relative Address 702 325
Indexed Address 600 900
Register 400
Register Indirect 400 700
AutoIncrement 400 700
Autodecrement 399 450
Table :Tabular List of Numerical Example
10/17/2014 26

Special Thanks To…………..
Our honourable Sir
Abul kalam Azad
Lecturer ,Dept. Of CSTE
Noakhali Science & Technology
University
10/17/2014 28

ADDRESSING MODE

More Related Content

What's hot (20)

Viewers also liked (14)

Similar to ADDRESSING MODE (20)

Recently uploaded (20)

ADDRESSING MODE