Difference between Indirect and Implied Addressing Modes

Addressing modes are the techniques used by the CPU to identify where the data needed for an operation is stored. They provide rules for interpreting or modifying the address field in an instruction before accessing the operand.

Indirect and implied addressing modes are two common techniques used to specify the memory address of an operand for an instruction in computer architecture. Indirect addressing mode the address of effective address is specified by the instruction whereas in implied addressing mode , address is not specified explicitly.

Indirect Addressing Mode

Indirect addressing involves specifying a memory address that contains the actual address of the operand. This can be useful when the actual address of the operand is not known at the time the instruction is executed.

• In Indirect Addressing, the address field of the instruction contains a pointer to the memory location where the actual operand (data) is stored.
• That means the instruction doesn’t directly point to the data. It points to a location that holds the address of the data.

In indirect addressing, the address of the operand is obtained by accessing the memory location specified by the indirect address. For example, in the instruction "MOV AX, [BX]", the memory location specified by the contents of the BX register is accessed to obtain the actual address of the operand.

There are 2 types of Indirect Addressing Mode

• Memory Indirect
• Register Indirect 

Memory Indirect

In this type, we directly mention the address of the memory location in the instruction either enclosed by parenthesis or preceded by the '@' character. 

Example:

LOAD R, (1005)
or
LOAD R, @1005

Register Indirect

In this type, the address of the target memory location will be stored in the register and the register will be mentioned in the instruction. 

Example:

MOV R1, @R2
LOAD R1, (R2)

Implied Addressing Mode 

Implied addressing mode , on the other hand involves not explicitly specifying an address for the operand, but instead relying on the instruction's operation code (opcode) to implicitly determine the location of the operand.

• In Implied Addressing, the operand is implicitly defined by the instruction itself. There’s no need to specify an address.
• The operation is typically performed on a predefined register or on a specific memory location known to the CPU.

This is typically used when the operand is a register or a fixed memory location that is known in advance. For example, in the instruction "INC AX", the operand is the AX register, which is implicitly specified by the opcode of the instruction.

This mode of addressing is normally used in zero address (e.g., Stack operations) and one address (e.g., MUL AL) instructions. Hence the operand is implied inside the instruction, it is called Implied Addressing Mode. 

Example:

Example:  CLC
(used to reset Carry flag to 0)

Method Of Indirect Vs Implied Addressing Mode Figure

Difference Between Indirect and Implied Addressing Modes

The table below represents the difference between indirect and implied addressing modes.