addressing modes in computer architecture
- 1. Addressing Modes • The term addressing modes refers to the way in which the operand of an instruction is specified. The addressing mode specifies a rule for interpreting or modifying the address field of the instruction before the operand is actually executed.
- 2. Addressing Modes • Implied • Immediate • Direct • Indirect • Register • Register Indirect • Displacement (Indexed) • Stack
- 3. Implied Addressing • Implied addressing refers to instructions that comprise only an opcode without an operand; for example, the INCA(“increment accumulator”)Instruction. • Zero address instruction are designed with implied addressing mode. • ADD (it takes previous two values from the stack and ADD’s them). • CLC (used to reset Carry flag to 0)
- 4. Immediate Addressing • Operand is part of instruction • Operand = address field • e.g. ADD 5 —Add 5 to contents of accumulator —5 is operand • No memory reference to fetch data • Fast • Limited range
- 6. 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
- 7. Direct Addressing Diagram Address A Opcode Instruction Memory Operand
- 8. Indirect Addressing (1) • Memory cell pointed to by address field contains the address of (pointer to) the operand • 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
- 9. Indirect Addressing (2) • Large address space • 2n where n = word length • May be nested, multilevel, cascaded —e.g. EA = (((A))) – Draw the diagram yourself • Multiple memory accesses to find operand • Hence slower
- 10. Indirect Addressing Diagram Address A Opcode Instruction Memory Operand Pointer to operand
- 11. Register Addressing (1) • Operand is held in register named in address filed • EA = R • Limited number of registers • Very small address field needed —Shorter instructions —Faster instruction fetch
- 12. Register Addressing (2) • No memory access • Very fast execution • Very limited address space • Multiple registers helps performance —Requires good assembly programming or compiler writing —N.B. C programming – register int a; • c.f. Direct addressing
- 13. Register Addressing Diagram Register Address R Opcode Instruction Registers Operand
- 14. Register Indirect Addressing • C.f. 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 Addressing Diagram Register Address R Opcode Instruction Memory Operand Pointer to Operand Registers
- 18. Displacement Addressing • EA = A + (R) • Address field hold two values —A = base value —R = register that holds displacement —or vice versa
- 19. Displacement Addressing Diagram Register R Opcode Instruction Memory Operand Pointer to Operand Registers Address A +