Addressing Modes, Instruction Set and Instruction Execution Flow
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Instruction Execution Steps
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Today we'll discuss how instructions are executed in a CPU. It all begins with the instruction cycle, which consists of three main steps: fetch, decode, and execute. Can anyone tell me what fetching means?
Fetching means getting the instruction from memory.
Exactly! The CPU uses the Program Counter, or PC, to find out where the instruction is stored. Can someone tell me what happens after the instruction is fetched?
After fetching, the instruction gets decoded to understand what it should do.
Correct! During decoding, the CPU identifies the opcode and determines the operands it needs. Now, let’s summarize these three steps: fetch, decode, execute. What aids our memory for this?
Acronym FDE could help remember the steps!
Great suggestion! Remembering FDE makes it simpler. Let’s recap: F for fetching, D for decoding, and E for execution.
Addressing Modes
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Now let’s explore addressing modes. What do we mean by addressing modes?
They define how the CPU addresses operands in memory.
Exactly! For instance, in immediate addressing mode, the operand is part of the instruction itself. Can anyone give an example?
If an instruction says 'INCR 5', then 5 is the immediate operand.
Perfect! Now, what about direct addressing?
It specifies the memory address of the operand directly.
That's right! And indirect addressing further complicates things. Student_3, can you explain that?
In indirect addressing, the instruction points to a memory location that contains the address of the operand.
Exactly! Understanding addressing modes is crucial for grasping how instructions operate. Let's summarize: immediate, direct, and indirect modes.
Interrupts in Instruction Execution
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Finally, let's delve into interrupts. How do interrupts affect the flow of instruction execution?
They can stop the current execution to handle immediate tasks, like keyboard input.
Right! When an interrupt occurs, what does the CPU need to do first?
It needs to save the current state, like the Program Counter and other registers.
Exactly! This is crucial as it allows the CPU to return to the interrupted state later. Can anyone recall how this is accomplished?
Through a stack, which temporarily stores status information before handling the interrupt.
Great job! Remember, interrupts are vital for multitasking and efficient CPU performance. Let’s recap: save state and handle interrupts!
Introduction & Overview
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Quick Overview
Standard
The section provides an in-depth understanding of instruction execution in CPUs, including the steps of fetching, decoding, and executing instructions. It also discusses various addressing modes and the significance of interrupts during instruction execution.
Detailed
Addressing Modes, Instruction Set and Instruction Execution Flow
This section delves into the intricacies of how instructions are executed in computer systems, primarily focusing on the instruction cycle that consists of fetching, decoding, and executing operations. There are three essential steps involved in this cycle:
- Instruction Fetch: The CPU fetches instructions from memory, utilizing the Program Counter (PC) to keep track of the memory address. During this process, the instruction is stored in the Instruction Register (IR) after being loaded from the Memory Buffer Register (MBR).
- Instruction Decode: The fetched instruction is decoded to understand the operation (opcode) it represents, determining whether it involves data transfer, arithmetic, logic, or control operations. It may require fetching one or two operands based on the instruction type and addressing mode employed.
- Instruction Execute: Once decoded, the operation is executed, which could involve performing calculations, data transfer, reading from, or writing to memory.
Additionally, the discussion includes various addressing modes like immediate, direct, and indirect addressing that influence how the operands are sourced, impacting the execution flow. Also highlighted is the importance of the interrupt process, which allows the CPU to handle unexpected conditions like user interactions (e.g., mouse movements) while maintaining the integrity of ongoing instructions. This comprehensive understanding is integral for appreciating the interaction between instruction execution flow and computer architecture.
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Introduction to Instruction Execution
Chapter 1 of 7
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Chapter Content
So, welcome to the module on addressing modes, instruction set and instruction execution flow, lecture number 3, so that is unit number 3. Since the last 2 units we have basically seen what is the basic structure and the components of a CPU and external interface and we have seen basically the black box architecture of a memory. So, with this we have built enough background to understand the basic stuff which we are going to cover in this module that is, what is an instruction, how it is executed, what are the different types and formats, instruction set design and what are the typical use of instructions to call functions and return procedure.
Detailed Explanation
This section introduces the purpose of the module, which is to explain instructions and their execution within a computer. It builds off previous lessons about CPU structure and memory architecture to lay a foundation for understanding instructions, their execution, and how they are structured. The specific focuses include instruction types, execution flow, and instruction set design.
Examples & Analogies
Think of a computer like a chef preparing a meal. Previous lessons (or units) taught us about the kitchen layout (the CPU and memory), and now we're learning about the recipes (instructions) that the chef follows, how to prepare them (execution), and the different styles of meals they can make (instruction formats and types).
Instruction Fetch and Execution Steps
Chapter 2 of 7
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Chapter Content
So, as I told you that for a pedagogical perspective, 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. So, basically as it is a Von Neumann architecture so the instruction as well as data are in the memory. So, first what happens? So, whenever you want to execute an instruction, first you have to calculate the address of the instruction and in which memory location and at what address the instruction is there. So, that is the first step. Secondly, the instruction will be fetched, now it will be fetched and loaded into a special register called instruction register (IR).
Detailed Explanation
This chunk outlines the steps taken to execute an instruction. In the Von Neumann architecture, both instructions and data are stored in memory. The process starts with calculating the address of the instruction to be executed. After determining the address, the instruction is fetched and loaded into the Instruction Register (IR), where it is prepared for decoding and execution.
Examples & Analogies
Imagine if the chef (CPU) needs to grab a recipe (instruction) from a cookbook (memory). The first step is figuring out which page the recipe is on (calculating the address), then they turn to that page and pick up the recipe (fetching it into the Instruction Register).
Instruction Decoding
Chapter 3 of 7
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Chapter Content
So, now, the instructions will be decoded that what it has to do, sometimes it may be very simple like shift 2 numbers or add 2 numbers or do bitwise shifting and sometimes it can be very complicated like a matrix multiplication. So, that is actually called the instruction fetch. Then you have to find out that is the operation decoding that what is the instruction expected to do, that is all the instruction basically has 2 minimum characters, minimum fields one is the opcode that is the operation it has to do.
Detailed Explanation
Once the instruction is fetched into the IR, the next step is decoding it to understand what operation it will perform. Instructions can range from simple operations, like adding numbers, to complex ones, such as matrix multiplication. The decoding process involves identifying the opcode, which specifies the operation that the instruction will execute.
Examples & Analogies
Think of it like reading the title and instructions of the recipe. The title tells you what dish you're going to make (the opcode), and the detailed steps provide clarity on how to prepare the dish (what operations to perform).
Operand Fetching
Chapter 4 of 7
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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. Like for example, if the instruction just compares whether a number is greater than 0. So, it is just a single operand instruction or so add, but if it adds 2 numbers then it is a 2 operand instruction.
Detailed Explanation
After decoding the instruction, the next step is fetching the operands, which are the values that the operation will use. Depending on the specific instruction, the CPU may need to fetch one operand (for a comparison) or two operands (for an addition). Identifying how many operands are needed is crucial for executing the instruction correctly.
Examples & Analogies
Continuing with the chef analogy, it’s like determining how many ingredients are necessary for the dish. If the recipe calls for a single item (like a spice for flavor), that’s one ingredient. However, for a more complex dish requiring two main components (like meat and vegetables), both need to be gathered for the dish to be complete.
Immediate vs. Memory Addressing
Chapter 5 of 7
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Chapter Content
For example, I may want to compute 5 + 5. So, this 5 + 5 is given in the instruction itself. So, you need not go and bring the data from the memory itself...but sometimes I may see that I want to add 3 with the data which is available in the memory location may be the variable of the memory address is 0003 h. So, in that case you have to go for operand address calculation and then you have to fetch the operand from the memory.
Detailed Explanation
This section explains two methods of handling operands: immediate addressing and memory addressing. In immediate addressing, the operand (like 5 in 5 + 5) is included directly in the instruction, allowing for quick access without needing to fetch it from memory. In contrast, memory addressing requires the CPU to look up the operand's value in memory, which takes additional steps.
Examples & Analogies
Using our cooking analogy, immediate addressing is like having all your spices measured out and ready at your kitchen counter (like having '5' directly in the recipe). Meanwhile, memory addressing is akin to needing to go into the pantry to grab a specific spice from a jar before you can add it to your dish.
Instruction Execution and Storage
Chapter 6 of 7
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Chapter Content
So, 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. 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 and then fetch the operands do the operation and store it in a memory location.
Detailed Explanation
Once the operands are fetched, the CPU performs the specified operation (like adding numbers together). If needed, the result is then stored back in memory. The overall process of executing an instruction consists of fetching the instruction, decoding it, fetching any necessary operands, executing the operation, and finally storing the result where required.
Examples & Analogies
Imagine that after preparing your dish, you taste it (execute the operation) and then decide to put any leftovers back into the fridge (storing back the data). Each step in the recipe mirrors the instruction execution cycle in the CPU.
Understanding Interrupts
Chapter 7 of 7
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Chapter Content
So, generally from the users perspective we see that instruction 1, then instruction 2, then instruction 3 it will keep on going if there is a jump instruction you will jump to the position again come back and so forth. 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.
Detailed Explanation
In the flow of executing instructions, there are situations where the CPU may need to pause its current operations to handle an event that requires immediate attention—this is called an interrupt. For instance, while the CPU is executing a program, any action like moving the mouse or pressing a key can trigger an interrupt, causing the CPU to temporarily halt its current tasks to handle the new input.
Examples & Analogies
Think of it like a busy restaurant kitchen. A chef (CPU) preparing a meal (executing instructions) will pause to take a customer’s order (interrupt). Once the order is taken, they return to finishing the meal, just as the CPU returns to its original task after servicing the interrupt.
Key Concepts
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FDE Cycle: The Fetch-Decode-Execute cycle is essential for instruction processing.
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Addressing Modes: Different ways of referencing operands in memory, affecting efficiency.
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Interrupts: Critical for handling asynchronous events and multitasking.
Examples & Applications
Example of Immediate Addressing Mode: 'ADD 5' means adding 5 directly without fetching from memory.
Example of Direct Addressing Mode: 'LOAD 10' refers to loading data from memory location 10.
Example of Indirect Addressing Mode: 'LOAD (20)' means loading data from the memory location pointed to by address 20.
Memory Aids
Interactive tools to help you remember key concepts
Rhymes
Fetch, decode, and execute, that’s how we compute!
Stories
Imagine a librarian (the CPU) who fetches books (instructions) from shelves (memory), reads their titles (decodes), and tells stories (executes) to visitors. Sometimes, they must pause to answer a call (interrupt) before resuming the storytelling.
Memory Tools
FDE: 'First, Decode, Execute' to remember instruction processing steps.
Acronyms
DINE
Direct
Immediate
Negative
and Operand for types of addressing modes.
Flash Cards
Glossary
- Instruction Cycle
The process in which the CPU fetches, decodes, and executes an instruction.
- Opcode
The portion of an instruction that specifies the operation to be performed.
- Immediate Addressing
A mode where the operand is directly specified in the instruction itself.
- Direct Addressing
An addressing mode where the operand's memory address is specified directly in the instruction.
- Indirect Addressing
An addressing mode where the instruction points to a memory location containing the address of the operand.
- Interrupt
A mechanism by which a process or thread can be preempted to allow a higher-priority process to run.
Reference links
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