Manav Goyal

Addressing Modes - Microprocessor And Interfacing

  • Basic
    • The way in which the operand of an instruction is specified
  • Categories
    • Addressing modes for data
    • Addressing modes for branch
  • Instruction consists of
    • Opcode
    • Operand
      • Starting Address
      • Effective address or Offset -=> Determined by adding any combination of three address elements
        • Displacement -=> It is an 8 bit or 16 bit immediate value given in the instruction
        • Base -=> Contents of base register, BX or BP
        • Index -=> Content of index register SI or DI
  • Different Modes used by 8086
    • Implied mode
      • In implied addressing the operand is specified in the instruction itself
      • In this mode the data is 8 bits or 16 bits long and data is the part of instruction
      • Zero address instruction are designed with implied addressing mode
      • Example -=> CLC (Used to reset Carry flag to 0)
    • Immediate addressing mode (symbol #)
      • In this mode data is present in address field of instruction
      • Example -=> MOV AL,35H (Move the data 35H into AL register)
    • Register mode
      • In register addressing the operand is placed in one of 8 bit or 16 bit general purpose registers
      • The data is in the register that is specified by the instruction
      • Here one register reference is required to access the data
      • Example -=> MOV AX,CX (Move the contents of CX register to AX register)
    • Register Indirect mode
      • In this addressing the operand’s offset is placed in any one of the registers BX,BP,SI,DI as specified in the instruction
      • The effective address of the data is in the base register or an index register that is specified by the instruction
      • Here two register reference is required to access the data
      • Exam ple -=> MOV AX,[BX] (Move the contents of memory location addressed by the register BX to the register AX)
    • Auto Indexed (Increment mode)
      • Effective address of the operand is the contents of a register specified in the instruction
      • After accessing the operand, the contents of this register are automatically incremented to point to the next consecutive memory location
      • Here one register reference,one memory reference and one ALU operation is required to access the data
      • Useful for stepping through arrays in a loop
      • Example -=> Add R1, (R2)+
    • Auto indexed (Decrement mode)
      • Can be used to implement a stack as push and pop and LIFO
      • Example -=> Add R1,-(R2)
    • Direct addressing/Absolute addressing Mode (symbol [])
      • The operand’s offset is given in the instruction as an 8 bit or 16 bit displacement element
      • Only one memory reference operation is required to access the data
      • Example -=> ADD AL,[0301]
    • Indirect addressing Mode (symbol @ or ())
      • In this mode address field of instruction contains the address of effective address
      • Here two references are required
        • 1st reference to get effective address
        • 2nd reference to access the data
      • Types based on the availability of Effective address
        • Register Indirect
          • In this mode effective address is in the register, and corresponding register name will be maintained in the address field of an instruction
          • Here one register reference,one memory reference is required to access the data
        • Memory Indirect
          • In this mode effective address is in the memory, and corresponding memory address will be maintained in the address field of an instruction
          • Here two memory reference is required to access the data
    • Indexed addressing mode
      • The operand’s offset is the sum of the content of an index register SI or DI and an 8 bit or 16 bit displacement
      • Example -=> MOV AX,[SI +05]
    • Based Indexed Addressing
      • The operand’s offset is sum of the content of a base register BX or BP and an index register SI or DI
      • Example -=> ADD AX,[BX+SI]
  • Based on Transfer of control
    • PC relative addressing mode
    • Base register addressing mode
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