In 8085 microprocessor the first machine cycle of every instruction is:

Opcode Fetch Cycle in 8085 Microprocessor

Definition: The opcode fetch cycle is the first machine cycle of every instruction in the 8085 microprocessor. During this cycle, the microprocessor fetches the opcode (operation code) of the instruction from memory. The opcode is a part of the instruction that tells the microprocessor what operation to perform. It is essential for the execution of any instruction as it determines the subsequent processes in the instruction cycle.

Working Principle:

When the 8085 microprocessor begins executing an instruction, it starts with the opcode fetch cycle. This cycle involves the following steps:

• Memory Access: The microprocessor places the memory address of the instruction onto the address bus. This address is stored in the Program Counter (PC), which points to the memory location of the next instruction to be executed.
• Control Signals: The microprocessor sends the appropriate control signals, such as the MEMR (Memory Read) signal, to indicate that it wants to read data from the specified memory location.
• Opcode Retrieval: The memory responds by placing the opcode stored at the specified address onto the data bus. The microprocessor then reads this opcode and stores it in the Instruction Register (IR).
• Decoding: After fetching the opcode, the microprocessor decodes it to determine the type of instruction and the subsequent steps required for execution.

Importance: The opcode fetch cycle is crucial because it initiates the execution of every instruction. Without fetching the opcode, the microprocessor cannot determine what operation to perform, making this cycle the foundation of instruction execution in the 8085 microprocessor.

Timing Details:

The opcode fetch cycle typically consists of four T-states (clock cycles) in the 8085 microprocessor. It is longer than other machine cycles because it involves additional operations such as decoding the opcode. The duration of the opcode fetch cycle depends on the clock frequency of the microprocessor.

Correct Option Analysis:

The correct option is:

Option 1: Opcode Fetch Cycle

This option correctly identifies the first machine cycle of every instruction in the 8085 microprocessor. The opcode fetch cycle is the initial step in executing any instruction, as it retrieves the operation code (opcode) from memory and prepares the microprocessor for further execution steps. Without this cycle, the microprocessor cannot proceed with the instruction execution process.

Additional Information

To further understand the analysis, let’s evaluate the other options:

Option 2: Memory Read Cycle

This option is incorrect. The memory read cycle is used to read data or instructions from memory, but it is not specific to fetching the opcode. While the opcode fetch cycle involves a memory read operation, it is distinct because it is specifically designed to retrieve the opcode of an instruction. The memory read cycle, on the other hand, can occur at any point during the execution of an instruction when data needs to be read from memory.

Option 3: Memory Write Cycle

This option is incorrect. The memory write cycle is used to write data into memory. It is not related to fetching the opcode of an instruction. The memory write cycle occurs during certain instructions, such as STA (Store Accumulator), where the microprocessor writes data to a specified memory location. It is not the first machine cycle of every instruction.

Option 4: I/O Read Cycle

This option is incorrect. The I/O read cycle is used to read data from an input/output device. It is not related to fetching the opcode of an instruction. The I/O read cycle occurs during instructions that involve input operations, such as IN, and is not the first machine cycle of every instruction.

Option 5: (No Option Provided)

No explanation is required for this option as it is not relevant to the question.

Conclusion:

The opcode fetch cycle is the first and most important machine cycle of every instruction in the 8085 microprocessor. It ensures that the microprocessor retrieves the operation code (opcode) of the instruction from memory, laying the foundation for the subsequent execution steps. While other machine cycles, such as memory read, memory write, and I/O read, are essential for specific operations, they are not the first machine cycle of every instruction. Understanding the opcode fetch cycle is critical for comprehending the working of the 8085 microprocessor and its instruction execution process.