Addressing Modes in Computer Architecture

Addressing Modes in Computer Architecture-

 

Before you go through this article, make sure that you have gone through the previous article on Addressing Modes.

 

We have discussed-

• Addressing modes are the different ways of specifying the location of an operand.
• Various kinds of addressing modes and their applications.

 

 

In this article, we will discuss syntax used for addressing modes.

 

Syntax Of Addressing Modes-

 

• The computer instructions may use different addressing modes.
• Different addressing modes use different syntax for their representation.

 

The syntax used for addressing modes are discussed below-

 

1. Immediate Addressing Mode-

 

Syntax-

# Expression

 

Interpretation-

Operand = Expression

 

Examples-

 

• Load R1, #1000 is interpreted as R1 ← 1000
• ADD R2, #3 is interpreted as R2 ← [R2] + 3

 

2. Direct Addressing Mode-

 

Syntax-

Constant

 

Interpretation-

 

Effective address of the operand = Constant

Operand = Content of the memory location ‘Constant’

Operand = [Constant]

 

Examples-

 

• Load R1, 1000 is interpreted as R1 ← [1000]
• ADD R2, 3 is interpreted as R2 ← [R2] + [3]

 

3. Register Direct Addressing Mode-

 

Syntax-

R_n or [R_n]

 

Interpretation-

 

Operand = Content of the register R_n

Operand = [R_n]

 

Examples-

 

• Load R1, R2 is interpreted as R1 ← [R2]
• ADD R1, R2 is interpreted as R1 ← [R1] + [R2]

 

4. Indirect Addressing Mode-

 

Syntax-

@Expression or @(Expression) or (Expression)

 

Interpretation-

 

Effective address of the operand = Content of the memory location ‘expression’

Operand = [[ Expression]]

 

Examples-

 

• Load R1, @1000 is interpreted as R1 ← [[1000]]
• ADD R1, @(1000) is interpreted as R1 ← [R1] + [[1000]]
• ADD R1, (1000) is interpreted as R1 ← [R1] + [[1000]]

 

5. Register Indirect Addressing Mode-

 

Syntax-

@R_n or @(R_n) or (R_n)

 

Interpretation-

 

Effective address of the operand = Content of the register R_n

Operand = [[ R_n ]]

 

Examples-

 

• Load R1, @R2 is interpreted as R1 ← [[R1]]
• ADD R1, @(R2) is interpreted as R1 ← [R1] + [[R2]]
• ADD R1, (R2) is interpreted as R1 ← [R1] + [[R2]]

 

6. Displacement Addressing Mode-

 

This addressing mode may be-

• Relative addressing mode
• Index addressing mode
• Base register addressing mode

 

Syntax-

disp (R_n)

 

Interpretation-

 

Effective address of the operand = disp + Content of the register R_n

Operand = [disp + [ R_n ]]

 

Examples-

 

• Load R1, 100(R2) is interpreted as R1 ← [100 + [R1]]
• ADD R1, 100(R2) is interpreted as R1 ← [R1] + [100 + [R2]]

 

7. Auto-Increment Addressing Mode-

 

Syntax-

(R_n)+

 

Interpretation-

 

Effective address of the operand = Content of the register R_n

Operand = [[ R_n ]]

After fetching the operand, increment [R_n] by step size ‘d’

 

Examples-

 

• Load R1, (R2)+ is interpreted as R1 ← [[R2]] followed by R2 ← [R2] + d
• ADD R1, (R2)+ is interpreted as R1 ← [R1] + [[R2]] followed by R2 ← [R2] + d

 

8. Auto-Decrement Addressing Mode-

 

Syntax-

-(R_n)

 

Interpretation-

 

Before fetching the operand, decrement [R_n] by step size ‘d’

Then, Effective address of the operand = Content of the register R_n

Operand = [[ R_n ]]

 

Examples-

 

• Load R1, -(R2) is interpreted as R2 ← [R2] – d followed by R1 ← [[R2]]
• ADD R1, -(R2) is interpreted as R2 ← [R2] – d followed by R1 ← [R1] + [[R2]]

 

To gain better understanding about Syntax of Addressing Modes,

Watch this Video Lecture

 

Next Article- Practice Problems On Addressing Modes

 

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