Operand Fetch Cycle (if needed) - 5.1.4.1.3 | Module 5: Control Unit Design | Computer Architecture
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5.1.4.1.3 - Operand Fetch Cycle (if needed)

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Introduction & Overview

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Quick Overview

The Operand Fetch Cycle occurs if an instruction requires operands not already in CPU registers, necessitating their retrieval from main memory. This involves the Control Unit calculating the operand's effective memory address, initiating a memory read, and loading the data into a CPU temporary register or directly to the ALU. ### Medium Summary Following the instruction decode phase, the **Operand Fetch Cycle** is initiated by the Control Unit (CU) when an instruction requires operands (data) that are not immediately available within the CPU's fast internal registers. This cycle primarily involves the CU generating precise control signals to calculate the correct memory address of the operand (potentially using the ALU for complex addressing modes), transferring this address to the Memory Address Register (MAR), sending a read command to the main memory, and finally, loading the retrieved operand from the Memory Data Register (MDR) into an appropriate CPU destination, such as a temporary register or directly to an ALU input. This process is crucial for instructions that operate on data stored in main memory. ### Detailed Summary ### ● Operand Fetch Cycle (if needed): If the instruction requires operands that are not already in internal registers (e.g., operands from memory), the CU generates signals to: * Calculate the operand's effective memory address (if an addressing mode requires it, e.g., base + offset, using the ALU). * Place the effective address into the MAR. * Initiate a memory read. * Load the operand from MDR into a temporary CPU register or directly to an ALU input.

Standard

Following the instruction decode phase, the Operand Fetch Cycle is initiated by the Control Unit (CU) when an instruction requires operands (data) that are not immediately available within the CPU's fast internal registers. This cycle primarily involves the CU generating precise control signals to calculate the correct memory address of the operand (potentially using the ALU for complex addressing modes), transferring this address to the Memory Address Register (MAR), sending a read command to the main memory, and finally, loading the retrieved operand from the Memory Data Register (MDR) into an appropriate CPU destination, such as a temporary register or directly to an ALU input. This process is crucial for instructions that operate on data stored in main memory.

Detailed Summary

● Operand Fetch Cycle (if needed):

If the instruction requires operands that are not already in internal registers (e.g., operands from memory), the CU generates signals to:
* Calculate the operand's effective memory address (if an addressing mode requires it, e.g., base + offset, using the ALU).
* Place the effective address into the MAR.
* Initiate a memory read.
* Load the operand from MDR into a temporary CPU register or directly to an ALU input.

Detailed

● Operand Fetch Cycle (if needed):

If the instruction requires operands that are not already in internal registers (e.g., operands from memory), the CU generates signals to:
* Calculate the operand's effective memory address (if an addressing mode requires it, e.g., base + offset, using the ALU).
* Place the effective address into the MAR.
* Initiate a memory read.
* Load the operand from MDR into a temporary CPU register or directly to an ALU input.

Definitions & Key Concepts

Learn essential terms and foundational ideas that form the basis of the topic.

Key Concepts

  • The Operand Fetch Cycle is conditional, occurring only when operands are in memory, not already in registers.

  • The Control Unit orchestrates this cycle, including address calculation (potentially using the ALU).

  • Data flows from memory -> MDR -> temporary register/ALU input during this cycle, driven by CU-generated control signals.