Computer Instruction Cycle

Instruction Cycle

• A single cycle of operation consists of Fetch, Decode, Execute, Memory Access, and Write Back phases
• Each instruction specifies an operation and the locations of its data operands
• A 32-bit word typically holds one encoded instruction

Instruction Types

• There are three basic instruction types: Load, Store, and Operate
• Load instruction: Read a data operand from memory or an input device into the processor
• Store instruction: Write a data operand from a processor register to memory or an output device
• Operate instruction: Perform an arithmetic or logic operation on data operands in processor registers

Program Execution

• A sequence of instructions, executed one after another, constitutes a program
• Both a program and its data are stored in the main memory
• A program counter (PC) register holds the memory address of the current instruction
• The instruction register (IR) holds the current instruction

Processor Components

• General-purpose registers hold data and addresses
• Control circuits and the arithmetic and logic unit (ALU) fetch and execute instructions

Fetching and Executing Instructions

• The processor control circuits send the address in PC to memory and issue a Read instruction
• The instruction is loaded from memory into IR
• The PC is incremented to point to the next instruction

Program Counter

• A program counter is a register in a computer processor that contains the address of the instruction being executed
• As each instruction gets fetched, the program counter increases its stored value by 1

Handling I/O Devices

• An application program can read data from an input device, write data to an output device, and sense the readiness of an I/O device

Performance

• Factors affecting performance: speed of electronic circuits, access times to cache and main memory, design of the instruction set, and number of operations that can be done in parallel
• Improvements in process technology lead to more transistors on a chip, resulting in more logic functionality and memory storage capacity
• Multicore processors can execute instructions in parallel, increasing performance

Test your understanding of the instruction cycle, including the fetch, decode, execute, memory access, and write back phases. Learn how instructions are encoded and executed in a program.