The cycle required to fetch and execute an instruction in 8085 microprocessor is which one of the fo

Question

The cycle required to fetch and execute an instruction in 8085 microprocessor is which one of the following?

Examoogle · Accepted Answer

Instruction cycle

Explanation: The instruction cycle is the complete sequence of events required to fetch an instruction from memory, decode it, and execute it within the microprocessor. In the 8085 architecture, this cycle is composed of one or more machine cycles.

Working Principle: The CPU performs a fetch operation (Opcode Fetch machine cycle) to retrieve the instruction from memory, followed by decoding the opcode. Depending on the instruction type, it may perform additional machine cycles like Memory Read, Memory Write, or I/O Read/Write to complete the execution. Each machine cycle consists of a specific number of T-states, which are periods of the system clock.

Key Formulas: $Instruction\ Cycle = \sum (Machine\ Cycles)$; $Machine\ Cycle = \sum (T-states)$

Key Points: An instruction cycle consists of multiple machine cycles.; A machine cycle is the time required to complete one operation (like memory read or write).; A T-state represents one clock period of the microprocessor.; The 8085 opcode fetch machine cycle typically takes 4 to 6 T-states.

Advantages: Modular design allows complex instructions to be broken into simpler operations.; Provides synchronization between the CPU, memory, and I/O devices.

Disadvantages: Inefficient memory access can lead to increased T-states and slower execution.; Higher complexity in timing control logic.

Applications: Embedded system design.; Real-time control systems.; Assembly language programming for 8085.

Additional Info: One Instruction Cycle can contain 1 to 5 Machine Cycles.. Option A (Clock cycle) is the fundamental unit (T-state), while Option C (Machine cycle) is a subset of the Instruction cycle.

Answer: D is correct — The instruction cycle is the fundamental sequence representing the full fetch-decode-execute process.

Exam Tip: Always remember the order of hierarchy: T-state

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