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Learn it your way
A short class with Sarah and her students. The other ways are under More.
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Today, we're discussing multiple word instructions. Can anyone tell me why we might need to split instructions over different memory locations?
Is it because some instructions are too long to fit into a single memory word?
Exactly! Since our CPU might have an 8-bit data bus, it limits how much data can be handled at once. So, we may require multiple memory locations to store an instruction fully.
How does the program counter know where to go next with multi-word instructions?
Great question! The program counter keeps track of where to retrieve the next opcode or operand. If an instruction occupies two words, it jumps two addresses ahead.
So, if it takes two words, do we need to handle more information each time?
That's right! More complex instructions necessitate precise handling of address calculation.
To summarize: We need multiple word instructions to handle more data than a single word can accommodate, and the program counter adapts its jumps based on the instruction size.
Can anyone name some types of addressing modes we've covered?
There's immediate addressing and direct addressing!
Correct! Immediate addressing brings the data with the instruction, while direct addressing points to a memory location. Can someone explain how indirect addressing works?
I think it points to a memory location that holds the actual address of the data?
Exactly! This flexibility is key when we have larger instructions. What role does displacement addressing play?
It helps allow offsets based on registers, right?
Correct! Displacement addressing uses an index register to compute an effective address. Summarizing, many addressing modes assist in effective data handling.
Now, let's discuss how the increase in instruction length and memory addresses affects CPU operations. Why is having larger memory addresses a challenge?
Because it can require more than one address to retrieve the necessary data?
Right! For example, if a memory address requires 32 bits, it might mean multiple memory fetches. Can this create a bottleneck in performance?
Yes it can, since fetching additional data takes time.
Exactly, and it can lead to increased complexity in managing the addresses for CPU instructions. So, summarizing, larger memory addresses require careful handling to maintain performance.
Let’s dive into some actual examples! What could be a practical example of immediate addressing?
How about using a value like ADD 75 in an instruction?
Correct! That's immediate addressing since the data is part of the instruction. Can we set an example for direct addressing?
It could be like ADD 0075, which points directly to memory.
Exactly! Now, what about indirect addressing? How would that work?
It would reference a memory address that gives us the actual operand, like ADD @8000.
Perfect! Through these practical examples, we can see the importance of understanding how different addressing modes work. To recap, immediate addressing gives us data right away while direct and indirect involve looking up the data.
Instructions that span more than one memory word due to size constraints.
The methodology by which addresses are determined, especially in the context of complex instructions.
The different techniques, such as immediate, direct, indirect, and displacement, used to reference the data in instructions.
ADD 75 (immediate addressing) adds 75 directly to the accumulator.
ADD 0075 (direct addressing) adds the value at memory location 0075 to the accumulator.
ADD @8000 (indirect addressing) indicates that the actual operand's address is located at memory address 8000.
ADDX R4, 8050 uses displacement addressing, where R4 provides an index offset.
To fetch data, both near and far, look for the opcode to know what you are.
Imagine a postman delivering letters to different houses. Each letter has an address, where the opcode is the 'to' action, and the addresses are the house numbers showing where they go.
Direct is at the door, Indirect needs a map to explore, Immediate means 'right inside', Displacement walks with pride.
D-I-D-I: Direct, Indirect, Displacement, Immediate - all ways of addressing.
Term
What does opcode stand for?
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Term
What are operand values?
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Term
Explain displacement addressing.
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A code that specifies the operation to be performed in an instruction.
The values or addresses involved in the operation described by the opcode.
A register that contains the address of the next instruction to be executed.
A method used to specify the operands of an instruction.
An addressing mode that combines an address with an offset from a register.
An addressing mode where the operand's address is stored in a register or memory location.
An addressing mode where the operand's value is specified directly within the instruction.
An addressing mode where the address of the operand is explicitly given in the instruction.
This section discusses how larger instruction sizes in computer architecture can complicate operations and addresses mechanisms for handling them.
In this section, the concept of larger instruction sizes in computer systems is explored. It examines how instructions can span multiple memory words and the implications on program counter behavior. Various addressing modes are illustrated, showcasing how effective addresses are calculated through various techniques like direct, indirect, and displacement addressing.
The section focuses on the challenges associated with larger instruction sizes in computer architecture. It explains that in typical scenarios, a CPU instruction would require more than one word due to limited width memory, leading to the concept of multiple word instructions. It elaborates on how the program counter behaves non-linearly when dealing with instructions that do not fit into a single word.
The instruction processing model thus unfolds as the need for properly managing multi-word instructions and recognizing how addressing modes impact CPU operations.