Various Addressing Modes

1. A R U N A D E V I P .
A S S I S T A N T P R O F E S S O R , D E P T . O F C S
V . V . V A N N I A P E R U M A L C O L L E G E F O R W O M E N
Computer Organisation
Addressing Modes

2. Addressing Modes
 It specifies a rule for
 interpreting or modifying the address field of the
instruction before the operand is actually
referenced.

3. Purpose of Addressing Mode
• Give programmers the facility of using pointers,
counters and indexing of data.
• To reduce the number of bits in the addressing
field of the instruction.

4. Advantages of Addressing Modes
 It gives experienced programmers the flexibility of
writing assembly language
 Reduced no. of instruction
 Efficient execution time

5. Types of Addressing Mode
 Immediate
 Direct
 Indirect
 Register
 Register Indirect
 Displacement (Indexed)

6. Immediate Addressing
 Operand part of instruction itself.
 Address field of the instruction is specified by the
operand
 No memory reference
 Fast execution
 Ex. ADD 5

7. Diagram
 Instruction
OperandOpcode

8. Direct Addressing
 Address field contains address of operand
 Effective address (EA) = address field (A)
 e.g. ADD A
 Add contents of cell A to accumulator
 Look in memory at address A for operand
 Single memory reference to access data
 No additional calculations to work out effective
address
 Limited address space

9. Direct Addressing

10. Indirect Addressing
 Memory cell pointed to by address field contains the
address of (pointer to) the operand
 Large address space
 Slower since multiple memory access needed
 EA = (A)
 Look in A, find address (A) and look there for operand
 e.g. ADD (A)
 Add contents of cell pointed to by contents of A to accumulator

11. Indirect Addressing
Address AOpcode
Memory
Operand
Pointer to operand

12. Register Addressing
 Operand is held in register named in address field
 Needs no Memory Reference
 Very fast Execution
 EA = R
 Limited number of registers
 Requires good Assembly Programming
 Very small address field needed
 Shorter instructions
 Faster instruction fetch

13. Register Addressing Diagram
Register Address ROpcode
Registers
Operand

14. Register Indirect Addressing
 Indirect addressing
 EA = (R)
 Operand is in memory cell pointed to by contents of
register R
 Large address space (2n)
 One fewer memory access than indirect addressing

15. Register Indirect Diagram
Register Address ROpcode
Instruction
Memory
OperandPointer to Operand
Registers

16. Conclusion
 All these addressing modes helps
programmer write efficient piece of code.
 Helps in better understanding of instruction
 Promotes faster instruction execution