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A short class with Sarah and her students. The other ways are under More.
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Today, we’re going to explore how the CPU uses registers to find memory locations. For instance, if we have register R1 and R5, what can you tell me about how R1 helps find an effective address?
R1 is used to get the value from the memory location that we add to R5.
Exactly! If R1 starts at 1, what would happen when we add it to the content of R5, which is 0?
It means the effective address would point to the first array element.
Correct! Remember, this is a key model called index addressing mode, and I like to use the mnemonic 'RAVE'—Registers Add Value to Effective addresses.
Why do we increment R1?
Great question, every increment allows us to access the next element in a sequence. Let’s summarize: R1+R5 allows us to address memory directly!
Now, let’s shift focus to what happens when an instruction cannot fit in a single word. When might we need multiple memory locations for an instruction?
When the opcode is too long?
Exactly! If we have 16-bit instruction sets, and 8-bit opcodes, we can’t fit everything in one, right?
So, we need two memory locations for the opcode and its operands?
Right again! And every larger instruction size means our program counter behaves complexly, moving non-linearly through memory.
What about examples of these instructions?
Excellent question! Examples include the ADD immediate instruction which spans across memory locations. Let’s recap: multiple word instructions complicate execution because they need multiple fetch cycles.
Let’s dive into some specific addressing modes like immediate, direct, indirect, and displacement. What's an immediate addressing mode?
It uses a constant value directly in the instruction, like an operand.
Correct! How would you explain direct addressing mode?
It refers directly to a memory location for the operand.
Excellent! Now, if we look at indirect addressing mode, it behaves differently. Can anyone highlight that?
It uses a memory address that points to another memory location where the actual operand is held.
Great point! Finally, displacement addressing mode combines an index register with a base address. Recap: addressing modes guide us in locating data, each with its intricacies!
The calculated address in memory of an operand.
Identifies the operation to be performed in an instruction.
Allows access to memory locations via an index.
Necessary when instructions are too large to fit in a single word.
In registers where addresses dwell, an index will direct quite well.
Imagine a librarian using a card catalog. Each card is like an opcode telling where a book—our operand—is found in the library's vast shelves, sometimes needing multiple cards for the big tomes!
Remember 'DICE'—Direct Immediate, Conditional, and Effective addressing modes.
For addressing modes, think 'I-DID'—Immediate, Direct, Indirect, and Displacement.
Term
What is the purpose of the opcode?
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Term
What is multiple word instruction?
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The address in memory where data can be accessed or modified.
A register used to modify operand addresses during program execution.
A part of an instruction that specifies what the instruction actually does.
An addressing mode where the operand value is given explicitly in the instruction.
An addressing mode where the operand's address is directly specified in the instruction.
An addressing mode where the address of the operand is specified indirectly via a pointer.
An addressing mode that combines an index register with a base address.
An instruction that requires more than one word storage due to its length.
This section explores the relationship between memory operations and instruction size, focusing on various addressing modes and their impact on instruction length.
The section delves into how memory locations are calculated using registers and the implications of instruction size on CPU operations. By introducing concepts like effective addresses and multiple word instructions, it illustrates the complexity involved in processing instructions, particularly in diverse addressing modes.
In this section, we examine the foundational concepts surrounding memory and instruction size, specifically addressing how CPUs utilize registers to calculate effective memory addresses. The initial example highlights how registers R1 and R5 are manipulated, demonstrating an index addressing mode where R1 increments to traverse an array's memory locations. As the content of R1 is added to the reset value in R5, the simplicity of the operation is illustrated, marking a fundamental understanding of loading values from memory. The discussion extends into practical implications, such as the constraints of instruction length and the necessity for multiple word instructions in complex scenarios. Here, we observe how opcode, operands, and addressing modes interact. The section concludes with a detailed examination of various addressing modes like immediate, direct, indirect, and displacement addressing, providing a comprehensive understanding of how effectively data is accessed in programming and computation contexts.