ADDRESSING MODES

ADDRESSING MODES

The different ways in which the location of an Operand is specified in an instruction are addressing modes.

  

INSTRUCTION FORMAT 

  


Effective Address   =   Address of Operand

     Operand Value    =   Value at Effective Address

TYPES OF ADDRESSING MODES

There are various types of addressing modes -

 

DIRECT ADDRESSING

If the second part of instruction specifies the address of an Operand , then it is said that the instruction has a direct address or direct operand .

 Example 

                              Add    r  ,  2000

                             

                      *  value at E.A. (2000) is 5

                            

                         Therefore  r + 5  →  r

* Single Memory reference only.

USE :  used to implement variables. 

        

                                                               

INDIRECT ADDRESSING

If the second part of an instruction code specifies an address of memory in which the address of the operand is obtained , then it is called indirect addressing.                        

Effective Address   =   2000  =  [1000]
Operand Value        =   [ E.A.]   =  [[1000]]
                                  =   [2000]
                                  =   5
             

* It requires two memory references.

USE : used to implement pointer.