Execute: Control Signals for ALU Operation and Register Writes - 5.2.4 | Module 5: Control Unit Design | Computer Architecture
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5.2.4 - Execute: Control Signals for ALU Operation and Register Writes

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Introduction & Overview

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Quick Overview

The **Execute Cycle** is where the actual computation occurs. The Control Unit (CU) sends specific signals to the ALU to select its operation and enable its execution. The ALU's result, along with status flags (Zero, Negative, Carry, Overflow), are generated and then written back to a designated destination register, orchestrated entirely by CU control signals. ### Medium Summary The **Execute Cycle** is the core computational stage of instruction execution, directly following operand fetching. During this phase, the Control Unit (CU) plays a critical role by sending precise multi-bit control signals to the Arithmetic Logic Unit (ALU), dictating the exact arithmetic or logical operation to be performed on the previously prepared input operands. As the ALU computes the result, it also generates and updates **Status Register flags** (Zero, Negative, Carry, Overflow), which reflect characteristics of the outcome. The CU then orchestrates the **write-back** of the ALU's computed result, enabling its transfer from an internal result bus to the specified destination general-purpose register, ensuring the program's data is updated as intended. ### Detailed Summary ### ● Execute: Control Signals for ALU Operation and Register Writes. This is the core computational phase where the actual operation specified by the instruction takes place. ○ **ALU Operation Selection:** The Control Unit sends a specific multi-bit signal to the ALU's control input. This signal acts as a selector, telling the ALU precisely which arithmetic or logical operation to perform on its inputs (which have already been set up in the previous operand fetch stage). ■ Example Control Signals: A set of 3-4 bits, say `ALU_FN_SELECT`, where 000 = ADD, 001 = SUB, 010 = AND, 011 = OR, etc. The CU asserts the specific binary pattern for the desired operation. ○ **Result Generation and Status Flags:** As the ALU computes the result, it also generates status information about that result. This includes: ■ **Zero Flag (Z):** Set to 1 if the result is 0; otherwise 0. ■ **Negative Flag (N):** Set to 1 if the result is negative (MSB is 1); otherwise 0. ■ **Carry Flag (C):** Set to 1 if an unsigned overflow/carry-out occurs; otherwise 0. ■ **Overflow Flag (V):** Set to 1 if a signed overflow occurs; otherwise 0. The CU asserts `STATUS_REG_LOAD_ENABLE` to capture these flags from the ALU into the CPU's dedicated Status Register. These flags are critical for subsequent conditional branching. ○ **Result Write Back:** The ALU's computed result is placed on an internal result bus. The CU then enables the input of the designated destination register (e.g., R1 in ADD R1, R2, R3) to latch this result. ■ Example Control Signal: `ALU_RESULT_OUT_R1_LOAD`.

Standard

The Execute Cycle is the core computational stage of instruction execution, directly following operand fetching. During this phase, the Control Unit (CU) plays a critical role by sending precise multi-bit control signals to the Arithmetic Logic Unit (ALU), dictating the exact arithmetic or logical operation to be performed on the previously prepared input operands. As the ALU computes the result, it also generates and updates Status Register flags (Zero, Negative, Carry, Overflow), which reflect characteristics of the outcome. The CU then orchestrates the write-back of the ALU's computed result, enabling its transfer from an internal result bus to the specified destination general-purpose register, ensuring the program's data is updated as intended.

Detailed Summary

● Execute: Control Signals for ALU Operation and Register Writes.

This is the core computational phase where the actual operation specified by the instruction takes place.
○ ALU Operation Selection: The Control Unit sends a specific multi-bit signal to the ALU's control input. This signal acts as a selector, telling the ALU precisely which arithmetic or logical operation to perform on its inputs (which have already been set up in the previous operand fetch stage).
■ Example Control Signals: A set of 3-4 bits, say ALU_FN_SELECT, where 000 = ADD, 001 = SUB, 010 = AND, 011 = OR, etc. The CU asserts the specific binary pattern for the desired operation.
○ Result Generation and Status Flags: As the ALU computes the result, it also generates status information about that result. This includes:
■ Zero Flag (Z): Set to 1 if the result is 0; otherwise 0.
■ Negative Flag (N): Set to 1 if the result is negative (MSB is 1); otherwise 0.
■ Carry Flag (C): Set to 1 if an unsigned overflow/carry-out occurs; otherwise 0.
■ Overflow Flag (V): Set to 1 if a signed overflow occurs; otherwise 0.
The CU asserts STATUS_REG_LOAD_ENABLE to capture these flags from the ALU into the CPU's dedicated Status Register. These flags are critical for subsequent conditional branching.
○ Result Write Back: The ALU's computed result is placed on an internal result bus. The CU then enables the input of the designated destination register (e.g., R1 in ADD R1, R2, R3) to latch this result.
■ Example Control Signal: ALU_RESULT_OUT_R1_LOAD.

Detailed

● Execute: Control Signals for ALU Operation and Register Writes.

This is the core computational phase where the actual operation specified by the instruction takes place.
○ ALU Operation Selection: The Control Unit sends a specific multi-bit signal to the ALU's control input. This signal acts as a selector, telling the ALU precisely which arithmetic or logical operation to perform on its inputs (which have already been set up in the previous operand fetch stage).
■ Example Control Signals: A set of 3-4 bits, say ALU_FN_SELECT, where 000 = ADD, 001 = SUB, 010 = AND, 011 = OR, etc. The CU asserts the specific binary pattern for the desired operation.
○ Result Generation and Status Flags: As the ALU computes the result, it also generates status information about that result. This includes:
■ Zero Flag (Z): Set to 1 if the result is 0; otherwise 0.
■ Negative Flag (N): Set to 1 if the result is negative (MSB is 1); otherwise 0.
■ Carry Flag (C): Set to 1 if an unsigned overflow/carry-out occurs; otherwise 0.
■ Overflow Flag (V): Set to 1 if a signed overflow occurs; otherwise 0.
The CU asserts STATUS_REG_LOAD_ENABLE to capture these flags from the ALU into the CPU's dedicated Status Register. These flags are critical for subsequent conditional branching.
○ Result Write Back: The ALU's computed result is placed on an internal result bus. The CU then enables the input of the designated destination register (e.g., R1 in ADD R1, R2, R3) to latch this result.
■ Example Control Signal: ALU_RESULT_OUT_R1_LOAD.

Definitions & Key Concepts

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

Key Concepts

  • The Execute Cycle is the core of an instruction's processing, involving the ALU and its operations.

  • The Control Unit precisely directs the ALU's function using control signals.

  • Beyond the main result, the ALU generates Status Flags (Z, N, C, V) that are crucial for conditional program flow.

  • The CU orchestrates the write-back of the ALU's result to the appropriate destination register, updating the CPU's state.