Computer Architecture: ALU Control Logic

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Questions and Answers

What is a necessary step when adding a new instruction to a datapath?

Check if existing registers can be reused.

Remove all existing instruction components.

Analyze the instruction's opcode only.

Determine the control signals required for the new instruction. (correct)

What does ImmGen typically output in the context of the 'lw' instruction?

The opcode of the instruction.

The address of the data to load.

The immediate value to be used in operations.

A constant value used in address calculation. (correct)

When adding a new component to a datapath, what should be considered?

The interconnections to the current datapath. (correct)

The color and size of the components.

The efficiency of existing components.

The cost and availability of components.

Why might it not matter what value goes to Read register 2 during the 'lw' operation?

Read register 2 is ignored for memory reads. Signup and view all the answers

What is an essential consideration when determining if a new multiplexor is needed for an added instruction?

Whether existing multiplexors can handle the new instruction adequately. Signup and view all the answers

What is the ALU control code for the AND operation in R-type instructions?

0000 Signup and view all the answers

Which operation does the ALU control code 0110 correspond to?

Subtract Signup and view all the answers

For lw/sw instructions, what is the ALU operation performed?

Add Signup and view all the answers

What is the 2-bit ALUOp derived from the opcode for a beq instruction?

01 Signup and view all the answers

Which ALU function is defined for the R-type instruction with funct7 as 0100000?

Subtract Signup and view all the answers

What does the funct field determine in the R-type ALU operations?

The specific ALU operation Signup and view all the answers

Which ALU control code represents the operation for 'OR' in R-type instructions?

0001 Signup and view all the answers

When the opcode is for a load word (lw) or store word (sw), what is the operation performed by the ALU?

Add Signup and view all the answers

What is stored in Reg[rd] when executing the JAL instruction?

PC + 4 Signup and view all the answers

What is the maximum range for the jump target in the JAL instruction?

±218 32-bit instructions Signup and view all the answers

What is the total delay for an R-type instruction in a single-cycle datapath?

400 ps Signup and view all the answers

What is the clock cycle time determined by in a single-clock implementation?

The longest instruction's time Signup and view all the answers

How much time does a Load instruction take in a single-cycle datapath?

600 ps Signup and view all the answers

Which instruction class has the highest percentage of usage in the provided instruction mix?

R-type Signup and view all the answers

What is the average time per instruction using the variable clock method?

455 ps Signup and view all the answers

By how much is the variable clock cycle faster compared to the fixed clock cycle?

In what fraction of all cycles is the data memory used considering the instruction mix?

25% Signup and view all the answers

What can be concluded about the fixed clock cycle with respect to performance?

It is determined by the longest instruction, limiting performance. Signup and view all the answers

Study Notes

ALU Control Logic

R-type instruction requires ALU function selection based on funct field
ALU control is generated by the ALUOp field derived from the opcode
- The ALUOp field is used to decide the operation to perform based on funct3 and funct7 fields.
Load/Store instructions have ALUOp set to 00 and perform addition, requiring funct3 for specific operation
beq instruction uses ALUOp 01 and performs subtraction

Datapath in Operation

Datapath operation for R-type instructions involves fetching instructions, performing operations, and updating registers, as defined by the funct3 and funct7 fields.
Datapath operation for lw instruction involves fetching instructions, performing addition to calculate the address and loading data from memory.
Datapath operation for beq instructions involves fetching instructions, performing subtraction and checking the ALU output for branch implementation.
Datapath operation for sw instruction involves fetching instructions, performing addition to calculate the address and storing data into memory.

Implementing jal Instruction

jal instruction saves the program counter address in the register rd, providing the return address.
jal instruction updates the program counter based on the immediate value, using the PC- relative jump technique.
jal instruction supports branches within a range of ±219 locations, 2 bytes apart, covering a total of ±218 32-bit instructions.

Performance Evaluation

Instruction mix is considered for performance evaluation, with 25% loads, 10% stores, 45% R-type instructions, and 20% branches.
The functional units like memory, ALU, and register file have different execution times of 200 ps, 100 ps, and 50 ps, respectively.
Fixed cycle time implementation uses the maximum time needed by any instruction for all instructions, resulting in a fixed clock cycle.
Variable cycle time implementation determines the time needed for each instruction, causing variable cycle times depending on the instruction.
With a variable clock cycle for different instructions, the average performance is significantly faster compared to a fixed clock cycle time.

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Description

This quiz delves into the functionality of ALU control logic in computer architecture. It covers aspects of R-type instructions, the use of ALUOp fields, and the operations of different instruction types like load/store and beq. Test your understanding of how these elements interact within the datapath operations.