13.2.2 ALU Instructions | Summary and Q&A

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July 12, 2019
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13.2.2 ALU Instructions

TL;DR

The provided content explains the steps involved in executing ALU instructions with two register operands, including fetching the instruction, decoding the opcode, reading the register values, performing the operation, updating the PC, and writing back the result.

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Questions & Answers

Q: What are the five processing steps involved in executing ALU instructions with two register operands?

The five processing steps involved in executing ALU instructions with two register operands are Fetch, Decode, Read, Execute, and Write-back. In the Fetch step, the instruction is retrieved from main memory. In the Decode step, the opcode of the instruction is used to determine the values for the datapath control signals. In the Read step, the contents of the specified registers are read from the register file. In the Execute step, the requested operation is performed on the two operand values. Finally, in the Write-back step, the result of the operation is written back to the register file.

Q: How is the clock signal used in the execution of instructions?

The clock signal is used to synchronize the execution of instructions. At the rising edge of the clock, the new values computed during the Execute phase are written to the register file and the PC register. This marks the end of execution for the current instruction and the beginning of execution for the next instruction. The frequency of the clock determines the rate at which instructions are executed.

Q: How is the ALU function determined for each instruction?

The ALU function is determined by the control logic, which uses the opcode field of the instruction. In the provided content, it is assumed that the control logic is implemented using a read-only memory (ROM). The opcode bits serve as the ROM's address, and the ROM's outputs are the control signals. The ALU function is selected based on the control signals generated by the control logic.

Q: How is sign-extension performed in the execution of ALU instructions with a constant operand?

Sign-extension is performed by replicating the high-order (sign) bit of the 16-bit two's complement constant as many times as necessary to form a 32-bit operand. This is done using wiring and does not require any additional logic gates. The extended constant is selected as the second operand using a MUX controlled by the BSEL control signal.

Summary & Key Takeaways

  • ALU instructions with two register operands require five processing steps: Fetch, Decode, Read, Execute, and Write-back.

  • The clock signal is used to synchronize the execution of instructions, with each instruction executed in one clock cycle.

  • The datapath logic for executing ALU instructions with two register operands is simple and straightforward.

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