What is addressing mode?

Addressing mode is a way that tells the CPU how to choose or access the operand (data) for an instruction in computer architecture. It gives the CPU directions on where to find the data.

Program Counter

Program Counter (PC) = Next Instruction.
It keeps track of which instruction the computer will execute next.

Categories of addressing mode?

The most well known addressing mode are:

Implied Addressing Mode
Immediate Addressing Mode
Register Addressing Mode
Register Indirect Addressing Mode
Auto-increment or Auto-decrement Addressing Mode
Direct Addressing Mode · Indirect Addressing Mode
Displacement Address Addressing Mode
Relative Addressing Mode · Index Addressing Mode

Example problem

Find out the effective address of operand and operand value by considering different addressing modes.

The solution is presented in a table format to show how different addressing modes determine the effective address and operand value.

Addressing Mode	Effective Address	Operand	Notes
Immediate Addressing Mode	201	500	As instruction contains immediate number 500 It is stored as address 201
Register Addressing Mode	-	400	Register R1 contains 400 As operand is in register so no any memory location
Register Indirect Addressing Mode	400	700	Register R1 contains 400. So effective address of operand is 400. The data stored at 400 is 700
Direct Addressing Mode	500	800	Instruction contains the address 500. So effective address of operand is 500. The data stored at 500 is 800.
Indirect Addressing Mode	800	300	Instruction contains the address 500. Address at 500 is 800. So effective address of operand is 800. The data stored at 800 is 300.
Relative Addressing Mode	702	325	PC = 200, Offset = 500, Instruction is of 2 bytes. So effective address = PC + 2 + offset = 200 + 500 +2 =702 The data stored at 702 is 325
Index Addressing Mode	600	900	XR = 100, Base = 500. So effective address = Base + XR = 500 + 100 = 600. The data stored at 600 is 900.
Auto-increment Addressing Mode	400	700	It is same as register indirect addressing mode except the contents of R1 are incremented after the execution. R1 contains 400. So effective address of operand is 400. The data stored at 400 is 700
Auto-decrement Addressing Mode	399	450	t is same as register indirect addressing mode except the contents of R1 are decremented before the execution. R1 contains 400. R1 is first decremented to 399. So effective address of operand is 399. The data stored at 399 is 450.