Computer Achitecture and org- Addressing Modes, Study notes of Computer Architecture and Organization

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Addressing Modes

Introduction

The way the operands are chosen during

the program execution is dependent on

the addressing mode of the instruction.

Implied Addressing Mode

• (^) No address field is required
• (^) Operand is implied / implicit
• (^) Ex:
  • (^) Complementing Accumulator
  • (^) Set or Clearing the flag bits (CLC, STC etc.)
• (^) 0 – address instructions in a stack organized computer are implied mode instructions.
• (^) Effective Address (EA) = AC or Stack[SP]
• (^) Ex: Tomorrow, I am on leave (implies that there is no CAO class)
• (^) Come to my cabin (implies to come to 316B- SJT)

Immediate Addressing Mode

• (^) Operand is specified in the instruction itself
• (^) Useful for initializing the registers with constant value
• (^) Operand = address field
• (^) Ex: Mov Dx, #0034H
• (^) Advantage: No memory Reference, fast
• (^) Disadvantage: Limited operand magnitude
• (^) Ex: Come to my cabin: 316B-02 SJT Opcode Operand Instruction

Indirect Addressing Mode

• (^) The address field of the instruction gives the

address of the effective address of the

operand stored in the memory.

• (^) EA = (A)
• (^) Ex: Mov CX, [4200H]
• (^) Advantage: Large address space, may be

nested, multilevel or cascaded

• (^) Disadvantage: Multiple memory accesses to

find the operand, hence slower

• (^) Ex: Anil, please bring my laptop from my

cabin (cabin is not known to Anil)

Indirect Addressing Mode Diagram

Register Addressing Diagram

Register Indirect Addressing Mode

• (^) Address part of the instruction specifies

the register which gives the address of the

operand in memory

• (^) Special case of indirect addressing
• (^) EA = (R)
• (^) Ex: Mov BX, [DX]
• (^) Advantage: Large address space
• (^) Disadvantage: Extra memory reference

Displacement Addressing Mode

• (^) EA = A + (R)
• (^) Address field holds two values
  • (^) A = Base value
  • (^) R = register that holds displacement
  • (^) Or vice-versa

Relative Addressing Mode

• (^) Version of the displacement addressing
• (^) R = program counter, PC
• (^) Content of PC is added to address part of the instruction to obtain the effective address of the operand
• (^) EA = A + (PC)
• (^) Ex: JC next
• (^) It is often used in branch (conditional and unconditional) instructions, locality of reference and cache usage
• (^) Advantage: Flexibility
• (^) Disadvantage: Complexity

Base Register Addressing Mode

• (^) The content of the base register is added

to the address part of the instruction to

obtain the effective address of the operand.

• (^) Used to facilitate the relocation of

programs in memory.

• (^) EA = A + (BX)
• (^) Ex: Mov 2345H [BX], 0AC24H
• (^) Advantage: Flexibility
• (^) Disadvantage: Complexity

Auto Increment and Auto Decrement Addressing Modes

• (^) This addressing mode is used when the

address stored in the register refers to a

table of data in memory, it is necessary to

increment or decrement the register after

every access to the table.

• (^) Ex: Mov AX, (BX)+, Mov AX, -(BX)
• (^) Used mostly in Motorola 680X0 series of

computers

Problems

• (^) Find the effective address and the content of AC for the given data.

Addressing Mode Effective Address Content of AC Direct Address Immediate operand Indirect address Relative address Indexed address Register Register Indirect Autoincrement Autodecrement AC ← (500) 800 AC ← 500 AC ← ((500)) AC ← (PC + 500) AC ← (XR + 500) AC ← R AC ← (R1) AC ← (R1)+ AC ← -(R1) 500 300 325 900 400 700 700 450 500 201 702 600 400 400 399 800

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