Direct Addressing Mode
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Fetch Cycle
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Let's start with the fetch cycle. What do we mean by fetching an instruction?
Doesn't it involve getting the instruction from memory?
Exactly! The CPU calculates the memory address using the Program Counter, and then fetches the instruction from that address. Can anyone tell me what happens after the instruction is fetched?
It goes into the Instruction Register, right?
Correct! The Instruction Register holds the instruction until it's ready to be decoded for execution. Remember, we refer to this as the fetch phase. Let's summarize: the fetch phase involves calculating the address and reading the instruction into the IR.
Decode Cycle
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Now, moving on to the decode cycle, why is decoding essential?
It's needed to understand the action required by the opcode.
Exactly! The decoding phase breaks down the instruction to identify the opcode and operands. What can you tell me about operands?
They can be immediate, direct, or indirect!
Right! An immediate operand is given directly in the instruction, while direct and indirect operands involve accessing memory—either directly or through another address. Good job remembering that!
Execute Cycle
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The last step is execution. What happens during the execution cycle?
The CPU carries out the operation defined by the instruction!
Correct! The CPU performs operations like arithmetic or data transfer. Can anyone provide an example of an arithmetic operation?
Adding two numbers together, like ADD A, B.
Great example! Also, keep in mind that during this step, if an interrupt is received, it needs to be handled. This leads back to the initial instruction flow after completing the interrupt service. Let’s conclude this session with a quick recap: the execution phase follows the decode phase and implements the instruction.
Interrupt Handling
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Let’s discuss interrupts. What is the purpose of an interrupt in an instruction cycle?
It allows the CPU to respond to external stimuli, right?
Exactly! An interrupt can pause the current instruction, save the program's state, and then allow the CPU to handle the interrupt. Can someone tell me what needs to be stored before managing an interrupt?
The current program status and registers have to be saved.
Yes, once the interrupt is serviced, the CPU returns to where it left off, reinstating the saved state. This is vital for maintaining smooth operation. To summarize: interrupts temporarily halt instructions, allowing responsive operations while preserving the CPU's state.
Introduction & Overview
Read summaries of the section's main ideas at different levels of detail.
Quick Overview
Standard
In this section, understanding the execution of CPU instructions is highlighted, focusing on the fetch-decode-execute cycle, addressing modes, and how interrupts affect execution. Key concepts such as immediate addressing, direct addressing, and indirect addressing are explored alongside their implications for instruction execution.
Detailed
Detailed Summary
This section focuses on how the CPU processes instructions through a series of defined steps in the execution cycle known as fetch-decode-execute.
- Fetch: The first step involves calculating the memory address of the instruction to be executed. The CPU uses the Program Counter (PC) to access this information. The instruction is then fetched from memory and loaded into the Instruction Register (IR).
- Decode: After fetching the instruction, the CPU decodes it to understand what operation needs to be conducted, identifying the opcode and the number of operands required. The types of addressing modes significantly influence how the instruction interacts with memory.
- Immediate Addressing: The operand is part of the instruction, thus no additional memory access is needed.
- Direct Addressing: The instruction specifies the memory address where the operand is stored.
- Indirect Addressing: The instruction provides a pointer to another address, which contains the operand.
- Execute: The CPU executes the instruction based on the decoded information, performing operations such as arithmetic, data transfer, or control changes, which may involve reading from or writing to memory.
- Interrupt Handling: The section also discusses how interrupts are managed within this cycle; an interrupt can temporarily halt the execution of the current instruction, store the current state, handle the interrupt, and then return to continue execution, showcasing the need for precise state management in complex computing tasks.
Overall, this section emphasizes the logical flow of instruction execution in a CPU within the framework of its architecture.
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Understanding Direct Addressing Mode
Chapter 1 of 4
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Chapter Content
Direct addressing mode means here I will put the value of the memory maybe I will write 30 H six bits. Now this is going to point to a memory location and that memory location will have the exact value.
Detailed Explanation
Direct addressing mode is a way to specify an operand directly within an instruction. This mode allows the instruction to contain the exact memory address of the operand where the data is stored. For example, if you have an instruction that points to memory address 30H, the system can directly access the data stored at that location without needing any additional calculations or steps.
Examples & Analogies
Imagine you want to send a letter to a friend. If you write your friend's exact address on the envelope, the postal service knows exactly where to deliver it. This is similar to how direct addressing mode works; the instruction contains the precise address of the data, allowing the CPU to access it directly.
Execution Steps in Direct Addressing Mode
Chapter 2 of 4
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Chapter Content
To do that? Some sub steps will be required. So again you have to take the value of the operand which is that is the address of the operand from the instruction put it in the memory address register and again the memory that is now the data part of the memory.
Detailed Explanation
When using direct addressing mode, the CPU will follow several steps to execute the instruction. First, it takes the address of the operand from the instruction. Next, this address is loaded into a memory address register (MAR). Once the address is set, the CPU can access the data from that location in memory. After retrieving the data, it can perform any operation specified in the instruction using this operand.
Examples & Analogies
Think of this process as searching for a book in a library. First, you get the number of the shelf where the book is located. You then go to that shelf (like loading the address into the MAR) and fetch the book (accessing the data) to read or reference it. Just like in the CPU, this process involves locating the exact spot before retrieving the necessary information.
Benefits of Direct Addressing Mode
Chapter 3 of 4
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Chapter Content
So, the main problem is that immediate addressing is very easy to handle, but then we will be limited by the range of data.
Detailed Explanation
Direct addressing mode is advantageous because it allows for straightforward access to operands, making the instruction easier to understand and implement. However, one limitation is that the range of data you can reference may be constrained by the size of the addressing field in the instruction. If an instruction only allows for a small number of bits to represent an address, it restricts how far into memory the CPU can access data.
Examples & Analogies
Consider a phone book with only a few contacts. If the book only lists 30 people, you can easily find anyone in it. However, if you need to look up a person not within those 30, you're out of luck. Similarly, direct addressing mode is easy to use but can only point to a limited number of addresses based on its structure.
Comparison with Indirect Addressing Mode
Chapter 4 of 4
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Chapter Content
In indirect mode, what happens you can find out that indirect mode, what indirect mode basically what happens that is in the last instruction what we have seen? In the last instruction we said that the data will be present the address of the data will be present in the instruction.
Detailed Explanation
In direct addressing mode, the CPU obtains the data from an address supplied in the instruction. In contrast, indirect addressing mode does not provide the data directly but instead gives an address that points to another address where the actual data resides. This method of addressing allows for a wider range of memory because one can reference larger data sets by chaining addresses.
Examples & Analogies
Imagine a treasure hunt where you find a map (the instruction) that doesn't show you the treasure directly but provides directions to another map. Once you follow the directions and find the second map, that map will then tell you exactly where the treasure is buried. This reflects the essence of indirect addressing mode where the initial instruction leads you to the true data location.
Key Concepts
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Fetch-Decode-Execute Cycle: The series of steps a CPU follows to process instructions.
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Immediate Addressing: Operand is part of the instruction, reducing memory access.
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Direct Addressing: The instruction specifies where to find the operand in memory.
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Indirect Addressing: The instruction points to another address that contains the operand.
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Interrupt Handling: Management of high-priority tasks that may preempt the current instruction flow.
Examples & Applications
If an instruction reads 'ADD A, B', the CPU fetches this instruction, decodes the operation to add the operands A and B, and then executes the addition.
In immediate addressing, 'LOAD 5' means the operand 5 is directly provided in the instruction, and the CPU can use it without additional memory access.
Memory Aids
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Rhymes
Fetch, Decode, Execute—an instruction relay, interrupts can sway the CPU's play.
Stories
Imagine a postman (CPU) checking addresses (PC), picking up letters from a mailbox (memory), decoding the message, and delivering it to the right house (instruction execution). Sometimes, an urgent letter (interrupt) can divert the postman to another task before completing his rounds.
Memory Tools
FDE (Fetch-Decode-Execute) - Fetch the letter, Decode the message, Execute the delivery.
Acronyms
IDE (Immediate, Direct, Indirect) - Three ways to find an operand in cooking (fetching data)!
Flash Cards
Glossary
- Program Counter (PC)
A register that holds the address of the next instruction to be executed within a CPU.
- Instruction Register (IR)
A special register that temporarily holds the instruction being executed.
- Opcode
The portion of an instruction that specifies the operation to be performed.
- Immediate Addressing
A mode in which the operand is specified directly within the instruction itself.
- Direct Addressing
A mode where the instruction contains the address of the operand in memory.
- Indirect Addressing
A mode where the instruction contains a pointer to a memory location that holds the address of the operand.
- FetchDecodeExecute Cycle
The process through which a CPU retrieves an instruction from memory, decodes it, and then executes it.
- Interrupt
A signal that temporarily halts the CPU's current operations to allow for a higher priority task to be executed.
Reference links
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