6. Single-Cycle Processor Design

Question 1

What are the main functions of the processor?

Answer 1

• fetch instruction from instruction memory
• read the register operands
• use the ALU for computation
• access data memory for load/store
• store the result of computation or loaded data into the destination register
• update the PC.

Question 2

What occurs in the instruction fetch?

Answer 2

The program counter is read, and is incremented by 4.

Question 3

After the instruction fetch, how is an R-format instruction executed?

Answer 3

The instruction is decoded, and the data is passed to the ALU to operate on the data. The result is then written to a register.

Question 4

After the instruction fetch, how is a load instruction executed?

Answer 4

The instruction is decoded, and the load address is calculated using 16-bit offset.

Read the memory, and update the destination register.

Question 5

After the instruction fetch, how is a write instruction executed?

Answer 5

The instruction is decoded, and the write address is calculated using a 16-bit offset.

Write the memory.

Question 6

After the instruction fetch, how is a branch instruction executed?

Answer 6

The instruction is decoded, registers are read and passed to the ALU to subtract and compare to zero.

Sign-extend the immediate, shift-left to word align and add to pc+4.

Question 7

In a R-format instruction, which control lines are set?

• RegDst
• ALUSrc
• MemtoReg
• RegWrite
• MemRead
• MemWrite
• Branch
• ALUOp

Answer 7

• RegDst: 1
• ALUSrc: 0
• MemtoReg: 0
• RegWrite: 1
• MemRead: 0
• MemWrite: 0
• Branch: 0
• ALUOp: 01 (Depends)

Question 8

In a lw instruction, which control lines are set?

• RegDst
• ALUSrc
• MemtoReg
• RegWrite
• MemRead
• MemWrite
• Branch
• ALUOp

Answer 8

• RegDst: 0
• ALUSrc: 1
• MemtoReg: 1
• RegWrite: 1
• MemRead: 1
• MemWrite: 0
• Branch: 0
• ALUOp: 00 (add)

Question 9

In a sw instruction, which control lines are set?

• RegDst
• ALUSrc
• MemtoReg
• RegWrite
• MemRead
• MemWrite
• Branch
• ALUOp

Answer 9

• RegDst: X
• ALUSrc: 1
• MemtoReg: X
• RegWrite: 0
• MemRead: 0
• MemWrite: 1
• Branch: 0
• ALUOp: 00 (add)

Question 10

In a branch instruction, which control lines are set?

• RegDst
• ALUSrc
• MemtoReg
• RegWrite
• MemRead
• MemWrite
• Branch
• ALUOp

Answer 10

• RegDst: X
• ALUSrc: 0
• MemtoReg: X
• RegWrite: 0
• MemRead: 0
• MemWrite: 0
• Branch: 1
• ALUOp: 01 (sub)