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Instruction Fetching
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Today, we're going to discuss how a CPU fetches instructions from memory. To start, can anyone tell me what happens when we want to execute an instruction?
I think we need to know the address of the instruction first?
Exactly! We calculate the address from the Program Counter, which tells us where the instruction is stored in memory. This is the first step in the fetch phase. Can anyone tell me what comes next?
The instruction is fetched from that address and stored into the Instruction Register?
Correct! So we fetch the instruction and load it into the IR. Let's remember this process with the acronym ‘FIR’ — Fetch, Instruction Register. Now, let's move on to what happens after we fetch the instruction.
Instruction Decoding
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Now that we've fetched the instruction, we need to decode it. Can anyone explain what that means?
Decoding is figuring out what the instruction is telling the CPU to do, right?
Exactly! The decoding phase involves identifying the opcode and any operands. Remember, the opcode is like a command that tells the CPU what operation to perform. Anyone remember how many operands we might use?
It can be one or two, depending on the instruction!
Right! If it’s an immediate addressing mode, the operand may be included in the instruction itself. This brings us to our next key point.
Instruction Execution
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Once the instruction is decoded, we execute it. Can anyone explain what steps are involved in execution?
We fetch the operand if needed and then carry out the instruction!
Correct! Depending on whether we’re using direct or indirect addressing, we may need to fetch the operand from memory or use one specified in the instruction. Now, what about interrupts?
Interrupts can pause the execution so that the CPU can handle urgent tasks.
Yes! Handling interrupts properly allows the CPU to respond to events, maintaining system performance. Let’s summarize: fetching involves locating the instruction, decoding is interpreting the instruction, and execution is performing the task.
Understanding Addressing Modes
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Let’s discuss addressing modes. What are the two types we covered?
Direct and indirect?
Exactly! In direct addressing, the operand's location is provided explicitly, while in indirect addressing, it points to another memory address that contains the actual operand. This is significant because it allows access to larger data sets. Can someone give an example?
If I have a memory address that points to another address where my actual data is stored.
Great example! Addressing modes are essential because they impact how efficiently we access data. Remember to think about how each mode affects the program's performance!
Introduction & Overview
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Quick Overview
Standard
The section outlines the principles of instruction execution within a CPU, focusing on the steps of fetching instructions from memory, decoding them to understand their purpose, and executing the specified operations. It also discusses the concepts of addressing modes and interrupts.
Detailed
Summary of the Unit
In this section, we explore the crucial processes involved in instruction execution in a CPU. Starting from an understanding of the CPU's architecture and memory organization, we delve into the three main phases of instruction execution: fetching, decoding, and executing.
- Instruction Fetch: The CPU calculates the address of the instruction, retrieves it from memory, and stores it in the Instruction Register (IR).
- Instruction Decode: In this phase, the CPU interprets the fetched instruction, identifying the operation (opcode) and determining the operand requirements. The instruction may instruct the CPU to perform various operations ranging from arithmetic calculations to data manipulations.
- Instruction Execute: After decoding, the necessary operands are fetched (either directly from memory or provided within the instruction itself), and the operation is carried out. If there are interrupts, the CPU must also handle these appropriately, temporarily suspending its current operation to service the interrupt.
Additionally, we discuss addressing modes, including direct and indirect addressing, which define how operands are accessed. The section ultimately emphasizes the performance functions and characteristics of CPUs, providing foundational knowledge for understanding how instructions guide the CPU's operations.
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Overview of Instruction Execution
Chapter 1 of 5
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Chapter Content
In this case 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.
Detailed Explanation
The instruction execution involves the CPU performing specific functions to carry out commands stored in memory. In a Von Neumann architecture, instructions and data share the same memory space. This means that the CPU accesses both types of information from this unified memory. The performance of the CPU during instruction execution is key to its overall efficiency.
Examples & Analogies
Think of a library where all books (instructions) and study materials (data) are stored on the same shelf. When a student (CPU) decides to read (execute) a particular book (instruction), they need to go to the shelf (memory) to find it.
Fetching and Decoding Instructions
Chapter 2 of 5
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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. Secondly, the instruction will be fetched, now it will be fetched and loaded into a special register called instruction register IR.
Detailed Explanation
To execute an instruction, the CPU first determines where the instruction is located in memory by calculating its address. Next, it retrieves (fetches) this instruction from the memory and stores it in a special register known as the Instruction Register (IR). This register holds the instruction that is currently being executed, allowing the CPU to process it appropriately.
Examples & Analogies
Imagine you are in a restaurant and you have to order food. First, you check the menu (calculating the address). Once you decide what to order, you tell the waiter (fetching) who then writes it down (Instruction Register) to ensure they deliver the correct dish.
Instruction Execution Flow
Chapter 3 of 5
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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.
Detailed Explanation
Once the instruction has been fetched and decoded, the CPU proceeds to execute the operation defined by the instruction. This could involve performing calculations or data manipulations. If the instruction requires data (operands), the CPU must fetch these from memory, execute the required operation, and, if necessary, store the result back in memory. This process encapsulates the cycle of instruction execution: fetch, decode, execute.
Examples & Analogies
Consider a chef in a kitchen. Upon receiving a cooking order (instruction), the chef gathers necessary ingredients (operands), cooks the meal (executes the operation), and finally serves it (stores the result in memory) to be enjoyed.
Handling Interrupts
Chapter 4 of 5
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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 that will be based on requirement may be a code is executing, at that time you want to move the mouse.
Detailed Explanation
Interrupts are signals that temporarily halt the current flow of execution in the CPU. They can occur due to external events, such as user input (e.g., moving the mouse), requiring the CPU to pause its current task, save the current state, and handle the interrupt. After addressing the interrupt, the CPU returns to where it left off, resuming normal operations. This mechanism allows the CPU to respond to real-time events effectively.
Examples & Analogies
Think of a student studying at a desk. If their friend drops by (the interrupt), they must pause their studies (current instruction), greet their friend, and then return to studying where they left off.
Understanding Operand Fetching
Chapter 5 of 5
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Chapter Content
Therefore, that every time the PPT is running means some code are executing and it’s being displayed in the screen. So now when I move a mouse the servicing the instruction by interrupt service routine and after servicing it is coming back.
Detailed Explanation
Operand fetching is an essential part of the instruction execution process where the CPU retrieves data needed to perform operations. This might involve fetching from memory locations based on direct or indirect addressing modes. It ensures that the CPU has all necessary data at hand before executing an instruction.
Examples & Analogies
Imagine an artist painting a landscape. Before starting the painting (executing the instruction), the artist needs to gather their supplies (fetch operands). If they need a specific paint color that's not on their palette, they must go back to the storage area to get it (fetch from memory).
Key Concepts
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Instruction Cycle: The sequence of fetching, decoding, and executing an instruction in the CPU.
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Fetching: The process of retrieving instructions from memory via the Program Counter.
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Decoding: The interpretation of the instruction to determine the operation and its operands.
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Executing: Carrying out the instruction’s operation, which may involve data transfer or arithmetic processing.
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Direct Addressing: Accessing operands using a specified memory location.
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Indirect Addressing: Accessing operands via a pointer found in a specified memory address.
Examples & Applications
Example of fetching: The CPU uses the PC to get the instruction located at address 0x0000.
Example of decoding: The instruction in the IR indicates an addition operation with two operands.
Example of executing: The CPU performs the addition of two numbers stored in specified memory locations.
Memory Aids
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Rhymes
To fetch the code, the PC we trust, / From memory it brings, that’s a must.
Stories
Imagine the PC is a mail carrier, fetching letters (instructions) from various houses (memory addresses) and delivering them to the IR for sorting and action.
Memory Tools
When remembering instruction phases: 'F-D-E': Fetch first, Decode next, Execute last.
Acronyms
Remember ‘FIRE’ for Fetch, Immediate (or Decode), Read, Execute—your steps in instruction execution!
Flash Cards
Glossary
- Instruction Register (IR)
A special register in the CPU that holds the instruction being executed.
- Program Counter (PC)
A register that contains the address of the next instruction to be fetched.
- Opcode
Part of the instruction that specifies the operation to be performed.
- Operand
Data that the instruction operates on, which can be a specific value or a memory address.
- Fetching
The process of retrieving an instruction from memory.
- Decoding
The process of interpreting the fetched instruction to determine the operation and operands.
- Executing
The process of performing the operation specified by the instruction.
- Direct Addressing
An addressing mode where the operand's location is directly specified in the instruction.
- Indirect Addressing
An addressing mode where the instruction points to a memory location that contains the address of the operand.
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