14.5.2.2 - Operational Steps for ADD Instruction
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Understanding Immediate Mode
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Today, we start with the immediate mode of instruction execution. Can anyone tell me what the immediate mode means?
Does that mean the value is provided directly in the instruction?
Exactly, it’s when the operand is specified in the instruction itself. For example, in `LOAD R1, 32`, the value `32` is directly loaded into register `R1`. What steps do you think we need to execute this?
We first fetch the instruction from memory, right?
Correct! It takes 4 micro-steps. Let's remember them as `Fetch-Increment-Load`. Can you repeat that?
Fetch, Increment, Load!
Great! These steps form the foundation of the ADD instruction in the immediate mode.
Exploring Direct Mode
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Now, let’s move on to the direct mode. How does it differ from the immediate mode?
In the direct mode, the operand is not in the instruction itself, but the instruction indicates the memory address?
Exactly! It specifies the location from which to retrieve the operand. Can someone outline the steps for direct mode?
We still have the fetch, but we also need to read the value from that address.
Right! And we need `6 micro-steps` for this process. Remember, `Fetch + Read + Load`. Can one of you explain that?
We fetch the instruction, read the address, and then load the operand!
Perfect summary! This additional read is what distinguishes the direct mode from the immediate mode.
Understanding Indirect Mode
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Let’s discuss the indirect addressing mode, which is more complex. Who can explain what makes it different?
In indirect mode, the instruction references a memory location that contains the address of the actual operand.
Excellent point! This requires extra steps. How many micro-steps do you think it would take?
Seven steps for indirect mode?
Correct! We need steps to fetch the instruction, read the pointer, and then access the operand. Let's summarize: `Fetch + Point Read + Read Operand`. Can you remember that?
Fetch, Point Read, Read Operand!
Great job! Understanding these distinctions is vital in computer architecture.
Control Signals in Different Modes
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As we’ve seen, different addressing modes require varied control signals. What can you tell me about the control signals for the immediate mode?
I think we only need signals for fetching and loading the immediate value.
Exactly! It’s straightforward in immediate mode. Now, what about the direct mode?
We need signals for fetching the instruction, reading the address, and then loading the operand.
Right! A bit more complicated. Indirect mode? Any thoughts?
We’ll have even more signals because we have to access two memory locations.
Correct! It’s critical to understand how each addressing mode requires a unique set of control signals.
Summary of Instruction Execution Steps
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Now that we've covered different modes, let’s summarize what we’ve learned about executing the ADD instruction.
We explored immediate, direct, and indirect modes, and how many steps each requires.
Exactly! Immediate mode requires `4`, direct mode requires `6`, and indirect mode requires `7` steps. Each has its unique characteristics. What’s essential here?
Understanding which signals correspond to each mode!
Well stated! Knowing this allows you to design control steps accurately. Always remember the relationship between steps and control signals.
Introduction & Overview
Read summaries of the section's main ideas at different levels of detail.
Quick Overview
Standard
In this section, we examine the procedural steps required for executing an ADD instruction, comparing the operational steps across different addressing modes such as immediate, direct, and indirect. The focus is on understanding how control signals are generated and how they differ based on the addressing mode.
Detailed
Operational Steps for ADD Instruction
In this section, we delve into the operational steps required for executing the ADD instruction, particularly focusing on different addressing modes. The discussion begins with a recap of instruction fetch, which involves loading the program counter into the memory address register and retrieving the instruction from memory. We explore the varying steps involved depending on the addressing mode:
Addressing Modes Overview
- Immediate Mode:
- The operand is included within the instruction itself.
- Steps involve fetching the instruction and directly loading the immediate value into the designated register.
- This process requires only 4 micro steps.
- Direct Mode:
- The instruction specifies the memory address from which to read the operand.
- It involves fetching the instruction and requires 6 micro steps, as the operand must be read from memory.
- Indirect Mode:
- The instruction specifies a memory address that points to another address where the operand is stored.
- This mode increases complexity, requiring 7 micro steps due to the additional memory access needed to fetch the operand.
Throughout the section, control signals and micro-instructions involved in each step are discussed in detail, highlighting the importance of understanding how different architectures handle instruction execution.
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Instruction Fetch Operations
Chapter 1 of 4
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Chapter Content
The first 3 steps are similar for all types of instructions because they correspond to instruction fetch. In step 1, the program counter value is loaded into the bus. The program counter is read into the memory address register. Memory is set to read mode, and the ALU adds the program counter value, which is now in the bus, with a constant value, which is the next memory location. This is then waited to be loaded into the program counter in the next iteration. In the second step, we wait until the memory indicates that it has dumped the values or the instruction into the memory data register. In step 3, the memory signals readiness, and the memory data register transfers the instruction to the instruction register via the bus. The instruction register now has the instruction to be executed.
Detailed Explanation
The first three steps in executing any instruction are crucial as they are consistent across instruction types. First, the program counter (PC) points to the location of the next instruction in memory. This value is transferred to the memory address register so that the memory can fetch the instruction. Next, we read from memory, enabling the ALU to prepare the PC for the next instruction by incrementing its value. This process is standardized before any actual instruction is executed. Once the memory indicates it is ready (i.e., the instruction has been fetched), this instruction is loaded into the instruction register where it can be accessed for execution.
Examples & Analogies
Think of the instruction fetch process like ordering food at a restaurant. The program counter is like your order number; it tells the chefs (memory) which meal (instruction) to prepare next. When you place your order (load the address), you are waiting for the kitchen to finish preparing your meal. Once they let you know your meal is ready, they bring it to your table (instruction register) so you can start eating (executing the instruction).
Processing Direct Mode Instructions
Chapter 2 of 4
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Chapter Content
Starting from the fourth step and onwards, instructions change depending on the addressing mode. In the case of a direct instruction, you take the value of M from the instruction register, indicating a specific memory location from which to read data. This value M is loaded into the memory address register. The memory is then set to read mode, and the instruction waits for memory to respond. Once the response is ready, the operand at memory location M is transferred into the memory data register. Finally, in the last step, the operand is moved to the appropriate register.
Detailed Explanation
In the direct addressing mode, starting from step four, we specify the memory location directly in the instruction itself. The instruction stored in the instruction register provides this address. After moving this address to the memory address register, we set the memory to read mode to access the data at the specified address. Once the memory completes the read operation, it transfers the value to the memory data register. Finally, we execute the instruction by moving the operand from the memory data register to the designated register where it can be utilized in further computations.
Examples & Analogies
Consider this like you having a grocery list (the instruction) that specifies exactly which ingredients (data) to buy from a specific aisle in the store (memory location). Once you get to the aisle (reading mode activated), you take the listed items (operand) off the shelf (memory data register) and place them in your cart (the register). The journey from the list itself to the cart symbolizes fetching the instruction and then the data needed for your recipe.
Handling Indirect Addressing
Chapter 3 of 4
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In an indirect addressing mode, the memory location M points to another memory location that contains the actual operand. After the first three common steps, in step 4, the value of M is loaded into the memory address register. In the fifth step, the algorithm waits for the memory to indicate readiness, transferring the intermediary address value of M to the memory data register. Step 6 requires us to load this intermediary value into the memory address register again, which then points to the actual operand. Finally, in step 7, the ALU retrieves the operand from the second memory location to execute the instruction.
Detailed Explanation
Indirect addressing means that instead of directly working with the final data, we first retrieve an address that points to the actual data. In the indirect mode, after fetching the instruction and determining the first address (M), we load that into the memory address register. When we read the data at that address, it might not be the final operand, but another address. Hence, we need to load this new address into memory address register to access the real operand. The process is a chain of operations that ultimately leads us to the data we need for execution.
Examples & Analogies
This can be compared to a postal delivery process. Instead of receiving a package (the operand) directly, you first receive a letter (address) which tells you where to pick up the actual package. You read the address from the letter, head to that location (first memory access), retrieve another letter (new address) from there, and then finally go to the new address to collect your package (the operand), illustrating how complex addressing modes can be.
Immediate Addressing Mode Simplified
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Chapter Content
In immediate addressing mode, the operand is directly included in the instruction itself. After the first three fetching steps, the fourth step is straightforward: the value (for example, 32) is transferred directly from the instruction register to the designated register. This mode requires the fewest steps as there's no need for memory access for the actual operand.
Detailed Explanation
Immediate addressing is the simplest form of execution since the operand is embedded within the instruction. For example, if you have an instruction such as LOAD R1, 32, this means you are directly loading the number 32 into register R1 without needing to refer to memory. Following the standard fetch steps, the processor simply transfers the operand straight from the instruction register to the register in one additional step.
Examples & Analogies
Think of immediate addressing like getting a snack directly from your pocket instead of going to the pantry. If you're hungry and pull out a granola bar you stashed away earlier (the operand), you can eat it right away without having to go through the process of looking for where the snack is stored (like fetching from memory).
Key Concepts
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Immediate Mode: A memory address mode where the operand is given within the instruction itself.
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Direct Mode: A method of accessing operands where the actual address is specified in the instruction.
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Indirect Mode: A process of accessing operands where the instruction points to another address containing the required value.
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Control Signals: Signals that regulate the operation of various components in computer architecture.
Examples & Applications
Immediate Mode Example: LOAD R1, 32 - This loads the value of 32 directly into register R1.
Direct Mode Example: ADD R1, M - Here, R1 takes the value present at memory location M specified in the instruction.
Indirect Mode Example: LOAD R1, (M) - This means that M points to another address which contains the value to load into R1.
Memory Aids
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Rhymes
Four to score, fetch and load, immediate values to decode.
Stories
Imagine a courier (CPU) retrieving a package (operand) directly from a nearby location (instruction), or needing to visit a specific address before delivering.
Memory Tools
F-I-L for Immediate, stand for Fetch, Increment, Load.
Acronyms
D-L-R for Direct Mode
Direct Load from Register.
Flash Cards
Glossary
- Immediate Mode
A type of addressing mode in which the operand's value is specified directly within the instruction.
- Direct Mode
An addressing mode where the instruction specifies the memory address from which to fetch the operand.
- Indirect Mode
An addressing mode where the instruction points to a memory address that contains the effective operand's address.
- Microsteps
Small operational steps that a processor undertakes to execute an instruction.
- Control Signals
Signals generated by the control unit that direct the operations of the computer's components.
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