Before going into details, let's talk informally. Cool?
Assume that your girlfriend told you to meet at A. and you don't know what A is?
So there are few possibilities what A may be:
- It might be the address of some restaurants.
- It may be the address of some Hotel or maybe some Hill station name.
So before going to meet your girlfriend, you must know what A is?
The same case happens here.
I say "Add 8"
here we don't know what 8 represent.
- It may be the id_number of some particular register where our operand is present.
- It may be the operand itself.
- It may be the address of our operand directly.
So the Entire purpose of reading Addressing mode is to calculate Effective address.
An addressing mode: specifies how to calculate the effective memory address of an operand by using information held in registers and/or constants contained within a machine instruction or elsewhere.
Effective Address: The actual address of our operand.