In the 8085 microprocessor, every instruction is stored in memory in the form of bytes. A byte is simply 8 bits of data. Depending on how much information an instruction carries, the total length of an instruction can be 1 byte, 2 bytes, or 3 bytes. This classification is very important because it helps us understand how much memory space an instruction will take and how the microprocessor reads and executes it. Figure 1 shows the memory usage for 1, 2, and 3 bytes instructions.

Fig. 1: memory usage for 1, 2, and 3 bytes instructions

One-Byte Instructions

An instruction is called one-byte when the entire instruction fits in a single 8-bit byte. That one byte holds the opcode and, if needed, the information about operands when those operands are registers or implied by the instruction.

• These instructions are only 1 byte long.
• The single byte contains the complete information: the opcode (operation code) as well as the operands (if any are registers or implied).
• The microprocessor does not need to fetch anything extra from memory.

Examples:

• MOV B, C → Copies content of register C into register B.
• ADD B → Adds the content of register B to the accumulator.
• CMA → Complements the content of the accumulator.

Format of Instruction:

Opcode + Operand (if register-based) are inside a single byte.

Two-Byte Instructions

A two-byte instruction is one that occupies exactly 2 bytes in memory. The first byte contains the opcode (the actual operation to be done). The second byte carries an 8-bit operand, which may be Immediate data (a constant number provided with the instruction), or 8-bit port address (for input/output operations).

• These instructions are 2 bytes long.
• The first byte contains the opcode.
• The second byte contains the operand, usually an 8-bit immediate data or an 8-bit port address.
• Since the operand is provided along with the instruction, the microprocessor must fetch one extra byte from memory.

Examples:

• MVI A, 32H → Load the hexadecimal value 32 into the accumulator.
• ADI 45H → Add immediate data 45H to the accumulator.

Format of Instruction:

Opcode (1 byte) + Immediate Data/Address (1 byte).

Three-Byte Instructions

A three-byte instruction occupies 3 consecutive bytes in memory. First byte Contains the opcode (the operation to be done). Second and third bytes together form a 16-bit operand, usually a memory address.

• These instructions are 3 bytes long.
• The first byte contains the opcode.
• The second and third bytes together represent a 16-bit address (low-order byte first, then high-order byte).
• These instructions usually involve memory locations or jumps.

Examples:

• LXI H, 2500H → Load the register pair HL with the address 2500H.
• JMP 2050H → Jump to the memory location 2050H.
• LDA 4500H → Load the accumulator with data from memory location 4500H.

Format of Instruction:

Opcode (1 byte) + 16-bit address (2 bytes).

Classification of Instructions Based on Length (8085 Microprocessor)

Type of Instruction	Length (in bytes)	Structure	Examples	Explanation
One-Byte Instruction	1 byte	Opcode + Register/Implicit Operand	MOV B, C ADD B CMA	Entire instruction fits in a single byte. No extra memory is needed for data or address.
Two-Byte Instruction	2 bytes	1st byte → Opcode 2nd byte → 8-bit Data/Address	MVI A, 32H ADI 45H	First byte tells the operation, second byte provides immediate data or an 8-bit address.
Three-Byte Instruction	3 bytes	1st byte → Opcode 2nd + 3rd byte → 16-bit Address (Low-order first, then High-order)	LXI H, 2500H JMP 2050H LDA 4500H	Used when the instruction needs a full 16-bit memory address. The processor fetches two extra bytes.