Addressing Modes in Intel 8085

Addressing Mode is the method by which the operand (data) is specified in an instruction. It tells the processor where to find the data to be operated upon.

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Types of Addressing Modes in 8085

a) Immediate Addressing Mode

• The operand (data) is directly given in the instruction itself.
• Data is part of the instruction (2nd or 3rd byte).

Example: MVI A, 32H — Moves the value 32H immediately into register A.

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b) Register Addressing Mode

• The operand is in a register and the instruction specifies the register name.
• No memory access is needed to fetch data.

Example: MOV A, B — Moves the content of register B into register A.

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c) Direct Addressing Mode

• The address of the operand is directly specified in the instruction.
• The instruction contains the 16-bit memory address where data resides.

Example: LDA 2500H — Loads the content of memory location 2500H into accumulator.

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d) Indirect Addressing Mode (Register Indirect)

• The register pair contains the address of the memory location where data is stored.
• The instruction refers to a register pair that acts as a pointer.

Example: MOV A, M — Moves data from memory location pointed by HL pair into A.

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e) Implied (Implicit) Addressing Mode

• The operand is implied by the instruction itself. No operand is explicitly mentioned.
• The instruction inherently operates on a specific register.

Example: CMA — Complements the accumulator (operand is always A, implied).

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Analysis of Instruction: STA 4000H

Feature	Detail
Instruction	STA 4000H
Full Form	Store Accumulator
Opcode	STA
Operand	4000H (16-bit address)
Size	3 bytes (1 byte opcode + 2 bytes address)

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Function of STA 4000H

• STA stands for Store Accumulator Direct.
• It copies the content of the accumulator (A) into the memory location whose address is specified in the instruction.
• After execution: Memory[4000H] ← Content of A
• The content of the accumulator remains unchanged.

Example: If A = 45H, then after STA 4000H, the memory location 4000H will contain 45H.

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Addressing Mode of STA 4000H

The instruction STA 4000H uses Direct Addressing Mode.

Reason:

• The 16-bit memory address (4000H) is directly specified within the instruction itself.
• The processor does not need to look into any register to find the address — it is explicitly provided as part of the instruction.
• The data flows from the accumulator to the memory address 4000H directly.
• This is the defining characteristic of Direct Addressing Mode — the instruction contains the exact address of the operand location.

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Conclusion

Addressing modes provide flexibility in programming by allowing different ways to access operands. The instruction STA 4000H is a classic example of Direct Addressing Mode where the 16-bit address is embedded directly in the instruction, making it simple and straightforward for storing accumulator data into a specific memory location.