Instruction Execution Cycle: First Step in the Cycle
In a typical CPU instruction execution cycle, the processor first fetches the next instruction from memory, then decodes it, then executes it, and finally stores/write-backs any results to the appropriate destination (register or memory). This means that for the choices given, the first step is (ii) Fetch.
To connect this to the common textbook phrasing: many references describe the instruction cycle as including a Fetch phase (getting the instruction), a Decode phase (interpreting it and generating control signals), an Execute phase (performing the operation), and a Write-back/Memory phase (storing results).2
Key idea: the CPU cannot decode or execute an instruction until it has fetched it from where instructions live (typically main memory or an instruction cache).2
Relevant terms: Instruction cycle Fetch Decode Execute Store
Footnotes
-
Fetch-Decode-Execute cycle description and phases (instruction cycle overview). https://en.wikipedia.org/wiki/Instruction_cycle - Explains the typical phases of instruction processing including fetch, decode, execute, and memory/write-back variants. ↩ ↩2
-
Basic CPU instruction cycle and micro-operations concept. https://en.wikipedia.org/wiki/CPU_instruction_cycle - Discusses the instruction cycle phases commonly presented in computer architecture courses. ↩
-
Instruction fetch concept and instruction register usage. https://en.wikipedia.org/wiki/Instruction_register - Covers the role of an instruction register in holding the fetched instruction for subsequent decode/execute phases. ↩
CPU Instruction Cycle (Fetch–Decode–Execute) Overview
Correct answer to the multiple-choice question
Given:
- Decode
- Fetch
- Execute
- Store
The first step in the instruction execution cycle is (ii) Fetch.2
Footnotes
-
Fetch-Decode-Execute cycle description and phases (instruction cycle overview). https://en.wikipedia.org/wiki/Instruction_cycle - Explains the typical phases of instruction processing including fetch, decode, execute, and memory/write-back variants. ↩
-
Basic CPU instruction cycle and micro-operations concept. https://en.wikipedia.org/wiki/CPU_instruction_cycle - Discusses the instruction cycle phases commonly presented in computer architecture courses. ↩
Instruction execution cycle (conceptual)
- 1Step 1
Retrieve the next instruction from instruction memory (often via instruction cache) into the instruction register.
- 2Step 2
Interpret the opcode/addressing mode and generate the control signals needed for execution.
- 3Step 3
Perform the operation using the ALU or other execution units; compute effective addresses if needed.
- 4Step 4
Write results back to the destination (register file and/or data memory), completing the instruction’s effects.
Why Fetch must come first
Decoding requires the instruction bits. Without fetching the next instruction, the CPU has nothing to interpret or act on—so Fetch precedes Decode.
“Store” vs “Write-back” terminology
Some sources use “Store”, others use “Write-back” or “Memory” to describe the final phase where results are saved. The role is the same: committing execution results.
From instruction stream to committed result
Fetch
Phase 1Next instruction is retrieved from memory (instruction side)."
Decode
Phase 2Opcode/control signals are determined."
Execute
Phase 3Operation occurs (ALU/unit action)."
Store / Write-back
Phase 4Result is written to register(s) and/or data memory."
Common confusion points
Knowledge Check
What is the first step in the instruction execution cycle (i) Decode (ii) Fetch (iii) Execute (iv) Store?