Addressing Modes

Addressing modes specify the way in which the address of the operand is given in the instruction. Addressing mode gives the flexibility of writing programs.

Opcode Operand

eg MVI B, 05 : Here MVI (Move Immediate) is the opcode part and the rest is the operand part.

MOV A , B : Here MOV is the opcode part and the rest is the operand part.

ADD B : Here ADD is the opcode part and the rest is the operand part.

The following are the various addressing modes used very commonly.

Immediate Addressing Mode
Direct Addressing Mode
Indirect Addressing Mode
Displacement Addressing Mode
Auto increment / Auto Decrement Addressing Mode
Scaled Addressing Mode

Immediate Addressing Mode

In Immediate Addressing mode the operand itself is present in the instruction. It is generally used for accessing constants.
eg MVI B , 05 The advantage of Immediate Addressing mode is that no extra memory reference needed since the value of the operand is given itself in the instruction. So Immediate Addressing mode is the fastest addressing mode.

Direct Addressing Mode

The format of Direct Addressing Mode is : Opcode Effective_Address_of_Operand

In Direct Addressing mode the effective address of the operand is given directly in the instruction. So it require one extra memory reference to access the operand. The direct addressing mode is slow compared to immediate addressing mode. It is generally used to access static variables.

Indirect Addressing Mode

The format of Indirect Addressing Mode is : Opcode Reference_to_Effective_Address_of_Operand

In Indirect addressing mode the reference for effective address of the operand is given in the instruction .In indirect addressing mode the operand part tells where the effective address of the operand is stored in memory.It is the slowest addressing mode. It is generally used for accessing local variables(eg: Arrays,Pointers) The indirection in Indirect Addressing mode can be of varied level, For example Single Indirection,Double indirection etc.

Single Indirection : The format is Opcode Reference_to_Effective_Address_of_Operand

Double Indirection: The format is Opcode

Reference_to_Reference_to_Effective_Address_of_Operand

Displacement Addressing Mode

In Displacement addressing mode the Effective address of the operand is given by

Effective Address = Implicit Processor Register + Displacement

The following addressing modes comes in the category of Displacement Addressing mode.

• Base Addressing mode
• Index Addressing mode
• Relative Addressing mode
• Relative Base Addressing mode

Base Addressing mode : Base addressing mode is used in relocatable programs. Effective Address =Base Register + Displacement

Index Addressing mode : Index Addressing mode is used to stepping through array. Effective Address =Index Register + Displacement

Relative Addressing mode: Relative Addressing mode is used in intrasegment branching (Example GOTO).Effective Address =Program Counter + Displacement

Rlative Base Addressing mode : Effective Address = PC + BR + displacement . Where PC stands for program counter and BR stands for base register.

Auto Increment / Auto Decrement Addressing Mode

It is generally used to increment or decrement the array pointer. For example while executing a loop the processor may require to increment or decrement the pointer to the adjacent address at each iteration. So it can be used to increment or decrement file pointers, or it can be used to implemet stack in which the top can be incremented (TOP++) or decremented(TOP--).

Also in Auto increment or Auto decrement addressing mode the contents of the register is automatically incremented or decremented before or after the execution of the instruction . For Example MOV (R2)+, R1

Scaled Addressing Mode

It allows the portion of the operand to be extracted for manipulation depended on the scaling factor . It may access 1st byte , 2ndbyte , or the entire operand. For example in 8086 , in the instruction MOV BL , AH . Here the content of A move to B.