Addressing Modes | Types of Addressing Modes

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-

 

 

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-

 

In this 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-

 

In this 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-

 

In this 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-

 

In this 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-

 

In this 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.

 

 

Example-

 

• ADD R will increment the value stored in the accumulator by the content of register R.

AC ← AC + [R]

 

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-

 

In this addressing mode,

• The address field of the instruction refers to a CPU register that contains the effective address of the operand.
• Only one reference to memory is required to fetch the operand.

 

 

Example-

 

• ADD R will increment the value stored in the accumulator by the content of memory location specified in register R.

AC ← AC + [[R]]

 

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-

 

In this 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-

 

In this 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-

 

In this 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

 

In this addressing mode,

• 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.
• Only one reference to memory is required to fetch the operand.

 

Example-

 

 

Assume operand size = 2 bytes.

Here,

• After fetching the operand 6B, the instruction register R_AUTO will be automatically incremented by 2.
• Then, updated value of R_AUTO 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 R_AUTO 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

 

In this addressing mode,

• 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.
• Only one reference to memory is required to fetch the operand.

 

Example-

 

 

Assume operand size = 2 bytes.

Here,

• First, the instruction register R_AUTO will be decremented by 2.
• Then, updated value of R_AUTO will be 3302 – 2 = 3300.
• At memory address 3300, the operand will be found.

 

NOTE-

 

In auto-decrement addressing mode,

• First, the instruction register R_AUTO 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

 

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Watch this Video Lecture

 

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