Question

Describe the steps in the fetch/decode/execute cycle.

Short answer

Question: Briefly describe the fetch/decode/execute cycle for a computer processor. Answer: The fetch/decode/execute cycle involves three main steps: 1) fetching the instruction from memory and storing it in the Instruction Register (IR), 2) decoding the instruction to determine the operation and operands required, and 3) executing the specified operation using the operands. The cycle repeats as long as there are instructions to execute.

Step-by-step solution

Fetching the Instruction

In the fetch step, the processor retrieves the instruction from memory. The address of the memory location where the current instruction is stored is called the Program Counter (PC). The processor reads the instruction from the memory at the address specified by the PC and stores it in the Instruction Register (IR). After fetching the instruction, the PC is incremented to point to the next instruction in memory.

Decoding the Instruction

In the decode step, the processor interprets the instruction stored in the Instruction Register (IR) to determine what operation it should perform. This involves reading the OpCode (Operation Code) that specifies the type of operation to be performed, and any operands (data) required for the operation. The OpCode and operands are typically represented as binary values within the instruction. The processor also identifies which registers will be involved in the operation and verifies whether any additional data must be fetched from memory.

Executing the Instruction

In the execute step, the processor performs the operation specified by the instruction's OpCode, using the operands determined in the decoding step. This may involve performing arithmetic or logical operations, moving data between registers, or updating memory. After the operation is executed, the processor may write the result to a specified register or memory location. Finally, the processor proceeds to the next instruction, repeating the fetch/decode/execute cycle as long as there are instructions to execute.

Key concepts

Instruction Fetching

The first step of the fetch/decode/execute cycle is known as Instruction Fetching. During this phase, a computer's processor starts by locating the instruction it needs to execute next. This is akin to finding a recipe before starting to bake a cake. Imagine the processor having a 'to-do' list that tells it where the next set of instructions is; this 'to-do' list is called the Program Counter (PC). Just as you would turn the pages of a cookbook, the PC 'turns' to the next instruction to be executed.

The processor reaches into the memory, using the PC like an address, and picks up the instruction. This instruction is temporarily stored in a space called the Instruction Register (IR). It's like copying the recipe onto a notepad to refer to as you bake. After grabbing the instruction, the processor prepares to move on by updating the PC, much like you would mark the page for the next recipe. The process ensures that no instruction is missed and the flow of operations is maintained steadily.

Instruction Decoding

Once the instruction fetching is complete, the Instruction Decoding phase commences. Continuing with our baking analogy, decoding an instruction is like reading through the recipe you've fetched to understand what ingredients and tools you need. In the processor's case, the ingredients and tools are referred to as the OpCode (which stands for Operation Code) and operands (the data it works on).

The processor examines the binary values in the IR, looking for the OpCode to figure out what kind of operation it's going to perform. Will it add or subtract numbers, or perhaps execute a comparison? This is akin to knowing whether you're going to be mixing, baking, or frosting. It also looks for operands, which can be regarded as the specifics, such as 'how much' flour or 'which' bowl. This step is crucial because, without a proper understanding of the OpCode and operands, the processor wouldn't be able to carry out the right action. Additionally, this step includes a check on which registers, or temporary storage areas, will be needed and whether more data must be fetched from the memory; much like checking if you need to preheat the oven or greasing the pan.

Instruction Execution

With the decoded instruction ready, the processor moves on to the Instruction Execution phase, which is just like when you begin to follow the steps of your recipe. During this stage, the computer's CPU gets busy, diving into the actual work. It takes the operation identified by the OpCode, alongside the operands, and begins to mix, slice, and combine them — only in a digital sense. Arithmetic computations, logical assessments, data transfers, and more are all in a day's work for the CPU.

Imagine performing the act of stirring the batter or chopping the nuts. In binary terms, it might be adding or shifting bits. The outcome of this operation, just like the consistency of the dough or the pile of walnut pieces, will be placed somewhere specified by the instruction — either back into a register or into the memory for longer-term storage. After this, much like rinsing off the spoon before starting on the next step of the recipe, the processor cleans up and sets up for the next round, repeating the whole fetch/decode/execute process until the dish, or in the computer's case, the program, is complete.