Addressing Modes _ Types of Addressing Modes _ Gate Vidyalay.pdf

Computer Organization and Architecture
Addressing Modes | Types of Addressing Modes
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Addressing Modes-
The different ways of specifying the location of an operand in an instruction are called as addressing
modes.
Types of Addressing Modes-
In computer architecture, there are following types of addressing modes-
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1. Implied / Implicit Addressing Mode
2. Stack Addressing Mode
3. Immediate Addressing Mode
4. Direct Addressing Mode
5. Indirect Addressing Mode
6. Register Direct Addressing Mode
7. Register Indirect Addressing Mode
8. Relative Addressing Mode
9. Indexed Addressing Mode
10. Base Register Addressing Mode
11. Auto-Increment Addressing Mode
12. Auto-Decrement Addressing Mode
In this article, we will discuss about these addressing modes in detail.
1. Implied Addressing Mode-
In this addressing mode,
The definition of the instruction itself specify the operands implicitly.
It is also called as implicit addressing mode.
×
×
×

Examples-
The instruction “Complement Accumulator” is an implied mode instruction.
In a stack organized computer, Zero Address Instructions are implied mode instructions.
(since operands are always implied to be present on the top of the stack)
2. Stack Addressing Mode-
The operand is contained at the top of the stack.
Example-
×
×

ADD
This instruction simply pops out two symbols contained at the top of the stack.
The addition of those two operands is performed.
The result so obtained after addition is pushed again at the top of the stack.
3. Immediate Addressing Mode-
The operand is specified in the instruction explicitly.
Instead of address field, an operand field is present that contains the operand.
Examples-
ADD 10 will increment the value stored in the accumulator by 10.
MOV R #20 initializes register R to a constant value 20.
4. Direct Addressing Mode-
The address field of the instruction contains the effective address of the operand.
Only one reference to memory is required to fetch the operand.
It is also called as absolute addressing mode.
×

Example-
ADD X will increment the value stored in the accumulator by the value stored at memory location X.
AC ← AC + [X]
5. Indirect Addressing Mode-
The address field of the instruction specifies the address of memory location that contains the
effective address of the operand.
Two references to memory are required to fetch the operand.
×
×

Example-
ADD X will increment the value stored in the accumulator by the value stored at memory location
specified by X.
AC ← AC + [[X]]
6. Register Direct Addressing Mode-
The operand is contained in a register set.
The address field of the instruction refers to a CPU register that contains the operand.
No reference to memory is required to fetch the operand.
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Example-
ADD R will increment the value stored in the accumulator by the content of register R.
AC ← AC + [R]
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NOTE-
It is interesting to note-
This addressing mode is similar to direct addressing mode.
The only difference is address field of the instruction refers to a CPU register instead of main
memory.
7. Register Indirect Addressing Mode-
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The address field of the instruction refers to a CPU register that contains the effective address of
the operand.
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×

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Example-
ADD R will increment the value stored in the accumulator by the content of memory location
specified in register R.
AC ← AC + [[R]]
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···
···
×

NOTE-
It is interesting to note-
This addressing mode is similar to indirect addressing mode.
The only difference is address field of the instruction refers to a CPU register.
8. Relative Addressing Mode-
Effective address of the operand is obtained by adding the content of program counter with the
address part of the instruction.
Effective Address
= Content of Program Counter + Address part of the instruction
×

NOTE-
Program counter (PC) always contains the address of the next instruction to be executed.
After fetching the address of the instruction, the value of program counter immediately increases.
The value increases irrespective of whether the fetched instruction has completely executed or not.
9. Indexed Addressing Mode-
Effective address of the operand is obtained by adding the content of index register with the
address part of the instruction.
Effective Address
= Content of Index Register + Address part of the instruction
10. Base Register Addressing Mode-
×

Effective address of the operand is obtained by adding the content of base register with the address
part of the instruction.
Effective Address
= Content of Base Register + Address part of the instruction
11. Auto-Increment Addressing Mode-
This addressing mode is a special case of Register Indirect Addressing Mode where-
Effective Address of the Operand
= Content of Register
After accessing the operand, the content of the register is automatically incremented by step size
‘d’.
×

Step size ‘d’ depends on the size of operand accessed.
Example-
Assume operand size = 2 bytes.
Here,
After fetching the operand 6B, the instruction register RAUTO will be automatically incremented by 2.
Then, updated value of RAUTO will be 3300 + 2 = 3302.
At memory address 3302, the next operand will be found.
NOTE-
In auto-increment addressing mode,
First, the operand value is fetched.
Then, the instruction register RAUTO value is incremented by step size ‘d’. ×

12. Auto-Decrement Addressing Mode-
This addressing mode is again a special case of Register Indirect Addressing Mode where-
Effective Address of the Operand
= Content of Register – Step Size
First, the content of the register is decremented by step size ‘d’.
Step size ‘d’ depends on the size of operand accessed.
After decrementing, the operand is read.
Example-
Assume operand size = 2 bytes.
Here,
×

First, the instruction register RAUTO will be decremented by 2.
Then, updated value of RAUTO will be 3302 – 2 = 3300.
At memory address 3300, the operand will be found.
NOTE-
In auto-decrement addressing mode,
First, the instruction register RAUTO value is decremented by step size ‘d’.
Then, the operand value is fetched.
Also Read- Practice Problems On Addressing Modes
Applications of Addressing Modes-
Addressing Modes Applications
Immediate Addressing Mode To initialize registers to a constant value
Direct Addressing Mode
and
Register Direct Addressing Mode
To access static data
To implement variables
Indirect Addressing Mode
and
Register Indirect Addressing Mode
To implement pointers because pointers are
memory locations that store the address of
another variable
To pass array as a parameter because array
name is the base address and pointer is
needed to point the address
Relative Addressing Mode
For program relocation at run time i.e. for
position independent code
To change the normal sequence of
execution of instructions
For branch type instructions since it directly
updates the program counter
×

Index Addressing Mode
For array implementation or array
addressing
For records implementation
Base Register Addressing Mode
For writing relocatable code i.e. for
relocation of program in memory even at run
time
For handling recursive procedures
Auto-increment Addressing Mode
and
Auto-decrement Addressing Mode
For implementing loops
For stepping through arrays in a loop
For implementing a stack as push and pop
To gain better understanding about Addressing Modes,
Watch this Video Lecture
Next Article- Syntax Of Addressing Modes
Get more notes and other study material of Computer Organization and Architecture.
Watch video lectures by visiting our YouTube channel LearnVidFun.
Summary
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Article Name Addressing Modes | Types of Addressing Modes
Description In computer architecture, Addressing Modes specify the
location of an operand. Types of Addressing Modes-
Implied / Implicit Addressing Mode, Immediate Addressing
Mode, Direct Addressing Mode, Indirect Addressing
Mode, Register Direct Addressing Mode, Register Indirect
Addressing Mode, Relative Addressing Mode, Indexed
Addressing Mode, Base Register Addressing Mode, Auto-
Increment Addressing Mode, Auto-Decrement Addressing
Mode, Stack Addressing Mode
Author Akshay Singhal
Publisher Name Gate Vidyalay
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