22.4.2.2. Instruction Execution Modes

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

So, as I told you that for a pedagogical perspective. So, we are first going to see what is the performance or what is the functions which are performed by a CPU for executing an instruction.

In this chunk, we introduce the core objective of the module, emphasizing understanding how the CPU performs instruction execution. The focus will be on the functions of the CPU, especially in the context of instruction sets and execution flow, laying the groundwork for further exploration of CPU operations.

Imagine a chef preparing a meal. Before they can cook, they need to understand the recipe instructions clearly. Similarly, the CPU must understand how to execute instructions effectively, akin to a chef executing a set of cooking steps to create a dish.

Generally we call this whole thing in a very few steps instruction fetch, instruction decode, instruction execute. So, in decoding we generally involve decoding these instructions as well as bringing the values from the memories or if the instructions are immediate addressing mode then take the values from the instruction itself.

This chunk outlines three essential steps involved in instruction execution: fetching, decoding, and executing the instruction. During the fetching phase, the instruction is retrieved from memory. The decoding phase involves interpreting what the instruction needs to do, including fetching necessary data. Finally, the execution phase carries out the operation defined by the instruction.

Consider a librarian retrieving a book (fetch), reading the title to determine its subject matter (decode), and then delivering it to a patron for reading (execute). Each step is crucial for ensuring the right book reaches the right person for the intended purpose.

After decoding you have to find out whether you want one operand to be fetched, whether you want 2 operands to be fetched or whether the operand is given itself with the instruction.

This chunk discusses the role of operands in instructions. It highlights that after decoding an instruction, the next step is to identify how many operands are needed to perform the operation. An operand can be a value provided directly in the instruction (immediate addressing mode) or a value fetched from memory (direct or indirect addressing mode).

Think of a simple math operation: if the instruction is 'add 3 and 5,' the operands (3 and 5) are provided in the instruction itself. But in cases like 'add the number from a specific page of a book,' the operand refers to data stored elsewhere, which needs to be retrieved first.

After that is done you have to do the operation of the data and then you have to store back the data in the memory, if it is required.

In this chunk, we explore the execution phase of instruction processing. After fetching and decoding the operands, the CPU carries out the operation specified by the instruction. Once this operation is complete, the result may be stored back in memory, ensuring data consistency and availability for future operations.

Consider the action of a barista at a coffee shop. They take the order (fetch), understand what coffee to make (decode), brew the coffee (execute), and finally, place the finished cup on the counter for the customer to pick up (store back). Each step is critical for effective service.

But there is also very special stuff which is actually called the interrupt... Every time the PPT is running means some code are executing and it’s being displayed in the screen.

This chunk introduces the concept of interrupts during instruction execution. Interrupts allow the CPU to pause the current task and respond to important events, like user inputs. After addressing the interrupt, the CPU must return to the previous task, effectively managing multiple operations simultaneously by temporarily saving its state.

Imagine a multitasking parent cooking dinner while also helping a child with homework. If the child needs urgent assistance, the parent will stop cooking (interrupt), assist the child, and then return to finish dinner (resume work). This ability to juggle multiple tasks illustrates how interrupts function in computing.

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