Different types of Addressing Modes | Direct addressing | Indirect addressing | Immediate addressing mode | Register Direct addressing mode | Register Indirect addressing mode | Index addressing mode

Addressing mode in simple words, is a way to access operand(data) from memory and CPU registers.
Different types of Addressing Modes are:

  1. Direct addressing :  Here, address field contains address of operand.
    Single memory access is required .
    Limited address space.
    No extra calculations are required to mainpulate effective address.
    Example : ADD B
  2. Indirect addressing : Use of pointer is required, which points to the address of the operand.
    Larger address space.
    Example : ADD (B) // adds contents of cell pointed to by contents of B to accumulator.
    It may be nested , multilevel.
    Example : ((B))
    slower way of accessing operand as compared to direct addressing mode as multiple memory accesses are required.
    Example: ADD @210 // adds contents of memory location 210 to accumulator.
  3.  Immediate addressing mode : operand is part of instruction.
    Example : ADD #12 // adds 12 to the contents of accumulator.
    Fastest mode.
    No memory access is required.
  4. Register Direct addressing mode : operand is present in register mentioned in addressing field.
    no memory access required.
    fastest way of execution.
    very limited address space.
    Example : ADD R1 // adds contents of Register R1 to the contents of accumulator.
  5. Register Indirect addressing mode : Here, address is present in register.
    operand is in memory cell pointed to by contents of register.
    larger address space.
    one fewer memory access than indirect addressing mode.
    Example : ADD @R1
    can be written as, ACC <- [ACC] + [[R1]]
  6. Index addressing mode : effective address is obtained by adding an index value to the address given in the instruction.
    Used for accessing arrays.
    EA = A + I // here, I is index and A is address.
  7. Relative addressing mode : EA is obtained by adding contents of program counter to the constant value specified in the instruction given.
    It is also known as Displacement addressing mode.
  8. Base register addessing mode : EA = A + BR // BR is base register and A is address.
    Mostly used in segmentation.

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