13.2.2 ALU Instructions

Summary

This script outlines the process of executing ALU instructions with two register operands in a computer's datapath logic. It details the five steps: Fetch, Decode, Read, Execute, and Write-back, emphasizing the role of the program counter, register file, and ALU. The script explains how the system's clock dictates instruction execution frequency and introduces the concept of sign-extension for constant operands. It highlights the simplicity of a reduced-instruction set computer architecture, showing the straightforward datapath logic required for most Beta ISA instructions.

Takeaways

• 📚 The script discusses the datapath logic for executing ALU instructions with two register operands in a computer system.
• 🔍 The process involves five main steps: Fetch, Decode, Read, Execute, and Write-back, which are fundamental to instruction execution.
• 💾 During Fetch, the instruction is read from the main memory location specified by the program counter (PC).
• 🔑 In Decode, the opcode field of the instruction is used to determine the values for the datapath control signals.
• 📝 The Read step involves reading the contents of the registers specified by the RA and RB fields of the instruction.
• 🔧 Execute is where the actual operation, as determined by the ALU, is performed on the two operand values.
• 🔄 The next value for the PC is computed to prepare for the next instruction cycle.
• ⏱ The system's clock signal synchronizes the register file and the PC register, writing new values at the rising edge of the clock.
• 🔢 The clock frequency indicates the rate of instruction execution; for example, a 10ns clock period corresponds to a 100 MHz frequency and 100 MIPS.
• 🛠️ The datapath includes components like an adder for PC+4 computation, a MUX for selecting the initial PC value, and a ROM for control logic based on opcodes.
• 🔄 ALU instructions can use either two register operands or a register and a sign-extended constant, which is selected by a BSEL control signal.
• 📉 The script highlights the simplicity of the datapath logic in a reduced-instruction set computer (RISC) architecture, which is efficient for executing most instructions.

Q & A

What are the main steps involved in executing ALU instructions with two register operands?

The main steps are Fetch, Decode, Read, Execute, and Write-back. Fetch involves reading the instruction from main memory. Decode determines the control signals using the opcode field. Read gets the register contents specified by RA and RB. Execute performs the operation on the operands. Write-back writes the result to the register file.

What is the role of the program counter (PC) in the Fetch step?

The program counter (PC) specifies the location in main memory from where the 32-bit encoded instruction is read during the Fetch step.

How does the Decode step determine the values for the datapath control signals?

In the Decode step, the opcode field (instruction bits [31:26]) is used to determine the values for the datapath control signals.

What is the purpose of the Read step in the execution of ALU instructions?

The Read step involves reading the contents of the registers specified by the RA and RB fields from the register file to provide the operands for the Execute step.

What is the significance of the Execute step in the datapath logic?

The Execute step is where the actual operation, as determined by the control signals, is performed on the two operand values obtained from the register file.

What does the Write-back step entail?

The Write-back step involves writing the result of the operation, which is computed during the Execute phase, to the register file in the register specified by the RC field.

How does the system's clock signal influence the execution of instructions?

The system's clock signal is connected to the register file and the PC register. At the rising edge of the clock, the new values computed during the Execute phase are written to these registers, marking the end of execution for the current instruction and the beginning of the next.

What is the relationship between the clock frequency and the rate of instruction execution?

The clock frequency indicates the rate at which instructions are executed. For example, if the clock period is 10ns, the frequency would be 100 MHz, and the system would execute 100 million instructions per second (100 MIPS).

How is the next value for the PC determined for ALU instructions?

For ALU instructions, the address of the next instruction is simply the address of the current instruction plus 4. This computation is performed by a dedicated adder, and the result is used as the next value of the PC.

What is the purpose of the MUX in the context of the RESET signal?

The MUX is used to select the initial value for the PC when the RESET signal is 1, allowing the system to start execution from a predefined location.

How is the ALU function determined for executing ALU instructions?

The ALU function is determined by the control logic, which in this case is implemented using a read-only memory (ROM). The opcode bits are used as the ROM's address, and the ROM's outputs are the control signals that tell the ALU what operation to perform.

What is the role of the WERF (write-enable register file) control signal?

The WERF control signal determines whether the write operation to the register file is enabled. It should have the value 1 when the result of the ALU operation needs to be written into the RC register.

How is the sign-extension of a 16-bit constant performed in the datapath?

Sign-extension is performed by replicating the high-order bit (ID[15]) of the 16-bit constant sixteen times to form a 32-bit operand. This is achieved through wiring without the need for additional logic gates.

What is the significance of the BSEL control signal in the datapath?

The BSEL control signal selects the second operand for the ALU. When BSEL is 0, the output of the register file is selected, and when BSEL is 1, the sign-extended constant is selected as the operand.