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Computer Organisation - 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
- 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
- 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