The instruction life cycle refers to the series of steps that a CPU takes to fetch, decode, and execute an instruction. This process is often referred to as the instruction execution cycle or fetch-decode-execute cycle and forms the foundation of a computer's operation.
The instruction life cycle refers to the series of steps that a CPU takes to fetch, decode, and execute an instruction. This process is often referred to as the instruction execution cycle or fetch-decode-execute cycle and forms the foundation of a computer's operation.
1. Fetch:
The CPU retrieves the next instruction from memory.
The Program Counter (PC) holds the address of the instruction to be fetched.
The instruction is loaded into the Instruction Register (IR).
Key steps:
- Memory address in PC is placed on the address bus.
- The instruction is read from memory and transferred to IR.
- PC is incremented to point to the next instruction.
2. Decode:
The control unit interprets the fetched instruction in the IR.
The opcode is extracted and analyzed to determine the operation to perform.
The addressing mode is identified to locate the operand(s).
Key outputs:
- Control signals for various CPU components.
- Operand addresses or immediate values.
3. Execute:
The CPU performs the operation specified by the instruction.
Operations may include arithmetic/logic operations, memory access, or I/O operations.
Execution actions:
- The ALU processes data for arithmetic/logic instructions.
- Data is moved to/from memory or registers as required.
- Input/output devices are activated for I/O instructions.
4. Memory Access (Optional):
Some instructions involve accessing data from memory.
If required, the effective address is calculated, and the data is fetched or written.
Examples: Load and Store instructions.
5. Write Back (Optional):
The result of the execution phase is written back to a destination register or memory.
Ensures the output of the instruction is available for subsequent operations.
6. Interrupt Handling (If Applicable):
If an interrupt occurs during the cycle, the current execution is paused.
The processor saves the current state and services the interrupt before resuming.
Control Unit (CU): Directs the flow of the cycle by generating control signals.
Program Counter (PC): Keeps track of the instruction address.
Instruction Register (IR): Stores the current instruction being processed.
Arithmetic Logic Unit (ALU): Executes arithmetic and logic operations.
Registers: Provide temporary storage for data and instructions.