22.4. Handling Instruction Execution

So, welcome to the module on addressing modes, instruction set and instruction execution flow, lecture number 3, so that is unit number 3.

In this unit, we introduce the concepts related to instruction execution within a CPU. This involves understanding how the CPU handles instructions that are stored in memory and the basic processes that take place during the execution of these instructions.

Think of a CPU like a chef and the instructions as recipes. The chef follows the recipe (instruction) to prepare a meal (execute an instruction), gathering ingredients (data) from the pantry (memory). Just like a chef needs to know what the recipe says before they start cooking, the CPU needs to fetch and decode the instruction before executing it.

So, in a nutshell basically we store, fetch the instruction, decode the instruction, and then find out whether some operands has to be fetched from the memory...

The execution of an instruction typically involves several key steps: fetching the instruction, decoding it to understand what operation needs to be performed, fetching necessary operands (data), executing the operation, and finally storing any results back in memory if needed. This structured approach ensures clarity and efficiency in how the CPU operates.

Imagine ordering a pizza. First, you check the menu (fetch instructions), then decide what topping to choose (decode the instruction), call up the restaurant to place the order (fetch operands), and finally, enjoy your pizza once it arrives (execute the operation).

Then based on that it may have it may operate on 2 numbers, it may operate on 2 operands it may operate on single operand...

At a fundamental level, instructions are broken down into 'opcodes' (the operation to perform) and 'operands' (the data on which the operation acts). Depending on the instruction, it could require one or more operands, or even none at all, leading to varying complexity in execution.

Think of a math equation. The operator (like + or -) represents the opcode, while the values (like 2 and 3 in 2 + 3) represent the operands. Different equations (instructions) may require different numbers of operands depending on what you want to calculate.

For example, I may want to compute 5 + 5. So, this 5 + 5 is given in the instruction itself...

In an immediate addressing mode, the operand is embedded directly within the instruction (e.g., adding 5 + 5). In contrast, operand address calculation refers to when the instruction specifies the address of the operand in memory, and the CPU must go to that memory location to fetch the operand before executing the instruction.

It's like baking a cake. If the recipe says to use 3 eggs immediately (immediate addressing), you grab the eggs from the fridge. However, if it says to use the eggs listed in the pantry (operand address), you first go check the pantry to see how many are available.

So, we generally call this whole thing in a very few steps instruction fetch, instruction decode, instruction execute.

The cycle of fetching, decoding, and executing an instruction is often referred to as the fetch-decode-execute cycle. This cycle is foundational in CPU operations, as it ensures that each instruction is processed step-by-step without skipping any crucial stage.

Consider this as the routine of making a phone call. First, you pick up the phone and dial the number (fetch), listen to the ringing until someone answers (decode), and then talk with the person on the line (execute). Each step is essential for successful communication.

But there is also very special stuff which is actually called the interrupt, sometimes based on requirement you may want to interrupt the existing flow of instruction...

An interrupt allows the CPU to temporarily halt its current operations to address a different task that requires immediate attention. Once the interrupt is processed, the CPU can return to its previous operations, ensuring that important tasks are not overlooked.

Imagine you are reading a book when suddenly your phone rings. You set the book down (interrupt the current instruction) to answer the call. Once the call is done, you return to your book from where you left off (continue execution).

So, before addressing the interrupt or servicing interrupt you have to store the present status of the code...

When an interrupt occurs, the CPU must save the current status of the program, including the program counter and status registers, onto a stack. This action allows the CPU to resume the interrupted task accurately once the interrupt has been serviced.

Think of a worker using a whiteboard to track tasks. If they suddenly need to attend a meeting, they jot down their current task (saving the status) on a notepad and leave the whiteboard. After the meeting, they return to the whiteboard and can pick up exactly where they left off.

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