Instruction-Fetch-and-Execute-Cycle.pptx

Instruction-Fetch-and-Execute-Cycle

1. Instruction Fetch and Execute Cycle The instruction cycle is fundamental to how a CPU operates. It consists of fetching an instruction from memory, decoding it to understand what operation needs to be performed, executing the instruction, and storing the result if necessary. This cycle repeats continuously, driving the execution of programs. Understanding this process is crucial for grasping the basics of computer architecture and how software interacts with hardware.

2. The Fetch Cycle: Retrieving Instructions The fetch cycle is the initial step in the instruction execution process. The program counter (PC) holds the memory address of the next instruction to be executed. The CPU sends this address to memory and retrieves the instruction, loading it into the instruction register (IR). After the instruction is fetched, the PC is incremented to point to the subsequent instruction, ensuring a sequential flow of program execution. PC points to next instruction Instruction fetched Load into IR PC Incremented

3. Decoding Instructions for Execution During the decode cycle, the control unit (CU) deciphers the instruction's operation code (opcode) stored in the instruction register (IR). This process determines the type of operation to be performed, such as arithmetic, logic, load, store, or branch operations. The CU also identifies the operands and their locations, whether they are in registers or memory. The control signals are then generated to direct the subsequent execution phase. IR holds instruction The fetched instruction is stored here. CU deciphers opcode Determines the operation type. Identify operands Find their locations.

4. Executing Operations in the CPU The execute cycle is where the CPU carries out the instruction. The CPU, having decoded the instruction, determines the operation type (arithmetic, logical, memory, or I/O). The arithmetic logic unit (ALU) performs the required operation using the specified operands. Results are then stored either in registers for quick access or in memory for long-term storage, depending on the instruction's requirements. Decode Instruction 1 ALU Operation 2 Store Results 3

5. Storing Results: The Store Cycle The store cycle occurs when the results of an executed instruction need to be saved. The destination address, either a memory location or a register, is determined. The ALU or I/O result is transferred to this designated location. The system prepares for the next instruction by updating the necessary registers. For example, in an addition instruction (ADD R1, R2, R3), the result of R2 + R3 is stored in R1. 1 Determine destination 2 Transfer result 3 Update registers

6. Interrupts: Boosting CPU Efficiency Interrupts significantly enhance CPU efficiency by enabling multitasking. Instead of waiting for I/O operations to complete, the CPU can perform other tasks. Common types of interrupts include program interrupts (e.g., division by zero), timer interrupts (generated periodically), I/O interrupts (device ready signals), and hardware failures (power failure, memory errors). This mechanism prevents the CPU from being idle during I/O waits. Program Interrupts Division by zero. Timer Interrupts Generated periodically. I/O Interrupts Device ready signals.

7. Interrupt Handling: A Detailed Mechanism When an interrupt occurs, the CPU first saves the current execution state to preserve its context. It then jumps to the Interrupt Service Routine (ISR) to process the interrupt. The ISR handles the specific interrupt, and upon completion, the CPU restores the previous state and resumes execution from where it left off. This process ensures seamless multitasking and efficient utilization of the CPU. Save State ISR Processing Restore State

8. I/O Module: Connecting Peripherals The I/O module serves as the critical interface connecting peripherals to the CPU. It manages control and timing for device communication, handles data transfer between the CPU and peripherals, and provides error detection and status reporting. Common modes of I/O operations include programmed I/O (CPU directly controls data transfer), interrupt-driven I/O (uses interrupts to signal the CPU), and direct memory access (DMA), which transfers data without CPU intervention. Control Timing for device communication. Transfer Data between CPU and peripherals. Error Detection Reporting.

9. Importance of Interrupts in I/O Operations Without interrupts, the CPU would waste significant time waiting for slow I/O devices to complete their tasks. Interrupts allow for the concurrent execution of CPU and I/O tasks, greatly enhancing system efficiency. For instance, without interrupts, printing a document would require the CPU to wait for the printer. With interrupts, the CPU can continue executing other tasks while printing occurs in the background, optimizing overall system performance. 10x Efficiency Increased system performance. 0 Wait No Waiting Concurrent CPU and I/O. Background Background Tasks run simultaneously.