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Difference between Relative Addressing Mode and Direct Addressing Mode

Last Updated : 15 Jul, 2025

Addressing modes are methods utilise by a CPU in computer architecture to access data stored in memory. It is in this respect that different addressing modes are employed depending on the manner in which the data or the instruction is required to be accessed and/or dealt with. There are two basic addressing modes; Relative Addressing Mode and Direct Addressing Mode. These two modes are different and they differ in the manner they are deployed in business depending on the flexibility required in a certain task or project.

What is Relative Address Mode?

In this mode, the Effective Address (EA) of the operand is calculated by adding the content of the CPU register and the address part of the instruction word. The effective address is calculated by adding displacement (immediate value given in the instruction) and the register value. The address part of the instruction is usually a signed number, either positive or negative. The effective address thus calculated is relative to the address of the next instruction.

EA = CPU Register + Displacement

Figure - PC Relative Addressing Mode

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Relative Addressing Mode can be further classified into three types -

  • PC (Program Counter) Relative Addressing Mode.
  • Index Register Relative Addressing Mode or Indexed Addressing Mode.
  • Base Register Addressing Mode.

Advantages of Relative Addressing Mode

  • Compact code: This is because relatively smaller offsets can be used for giving instructions, hence it uses less memory.
  • Flexibility: Facilitates the branching off to other instructions that are nearby especially in loops and conditionals.
  • Position Independence: The basic form of the program may be located anywhere in storage since all the relative addressing is performed in all instructions, which adds portability.

Disadvantages of Relative Addressing Mode

  • Limited range: Another limitation of the operand address is that it is limited to some fixed range from the current instruction; this means that it cannot access far memory addresses.
  • Requires additional computation: The last address is calculated depend on the current program counter by adding an offset value so it has some computational complexity.

What is Direct Address Mode?

In Direct Address Mode, the effective address of the operand is equal to the address part of the instruction, i.e. the address part of the instruction indicates the memory location containing the operand. Example - ADD R1, 4000 where 4000 is the effective address of the location. 

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Figure - Direct Address Mode In this example, the memory location 4000 contains the operand 100 which gets added to the contents of R1 and gets stored in R1. 

Advantages of Direct Addressing Mode

  • Simplicity: It is simple because the address is normally called directly in the instruction and hence it is easy to comprehend.
  • Faster execution: Since there is no requirement for any address calculation or to find the location of the operand the CPU takes lesser time to reach and fetch the operand.

Disadvantages of Direct Addressing Mode

  • Lack of flexibility: Static means that the operand’s memory address is not changed in accordance with change in the memory layout or program relocation.
  • Increased instruction size: Because the actual address is a part of an instruction, it leads to increased size of the instructions, and hence in memory consumption.
  • Limited by address space: The size of the address field in the instruction defines the amount of memory which can be accessed directly.

Difference between Relative Addressing Mode and Direct Addressing Mode 

RELATIVE ADDRESSING MODEDIRECT ADDRESSING MODE
The effective address is calculated by adding the contents of the CPU Register with the address part of the instruction.The address for fetching the operand is already provided in the address part of the instruction. It is not calculated.
The relative mode has three types: PC Relative, Index Register Relative,  and Base Register Relative Addressing modes.Direct addressing mode has no types.
It results in a shorter address field in the instruction format as the relative address can be specified with a small number of bits.It results in a longer address field in the instruction format as it requires more bits to designate the entire memory address.
It is often used with branch-type instructions since it directly updates the program counter.It is not used in branch type of instructions.
It is used for writing relocatable code, and position independent code, i.e. for program relocation at run time.It is used to access static data.
It is used to implement array addressing, and records.It is used to implement variables.
It is used to handle recursive procedures.It cannot handle recursive procedures.

The PC Relative or Base Relative Addressing mode is favored over the direct addressing mode because-
i) Reduce the size of program storage.
ii) Reduce the instruction fetch queue's size.

iii) It will be simpler to relocate.

It is less preferable to use over PC Relative or Base Relative Addressing mode.
Advantage- FlexibleAdvantage- Simple to use as no intermediary is involved for referencing memory.
Disadvantage- ComplexityDisadvantage- Address space limit
Application- It aids in run-time program relocation and instruction sequence modification.Application- It assists in accessing static data and implementing variables.

Conclusion

Overall, Relative addressing mode and Direct addressing mode are two different ways of accessing memory in computer architecture. Direct addressing mode is simpler and more efficient, but has the limitation of not allowing relocatable code, while Relative addressing mode allows for relocatable code but requires additional calculation to determine the memory address.

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