Addressing Modes with examples

To reference a large range of locations in main memory, a variety of addressing technique are used. The common addressing modes are:

• Immediate
• Direct
• Indirect
• Register
• Register indirect
• Displacement (Indexed)
• Stack

Types of Addressing modes:

Immediate Addressing:
   Instruction

• operand is a 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

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

Indirect Addresssing:

• 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
• Large address space
• 2n where n = word length
• May be nested, multilevel, cascaded
  —e.g. EA = (((A)))
• Multiple memory accesses to find operand
• Hence slower

Register Addressing:

• 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
• 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;

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

Displacement Addressing:

• EA = A + (R)
• Address field hold two values
  —A = base value
  —R = register that holds displacement
  —or vice versa

Stack Addressing:

• Operand is (implicitly) on top of stack
• e.g.
  —ADD  Pop top two items from stack  and add
• The stack mode of addressing is a form of implied addressing
• the machine instructions need not include a memory reference but implicitly operate on top of stack.