This document discusses various addressing modes used in computer organization. It defines addressing modes as rules for interpreting or modifying the address field of an instruction before referencing the operand. The main types discussed are immediate, direct, indirect, register, and register indirect addressing. Each mode has advantages like reducing instruction size, providing flexibility, or allowing efficient memory access through pointers. Understanding addressing modes helps write efficient assembly code and promotes faster instruction execution.

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