Question

Using a 5 stage pipeline architecture (IF-OF-EX-MEM-WB), use a drawing to indicate the stages used by each one of the following instructions: add, sw, lw, j, and lui. (use Y for yes and blank for no)

Answer 1

The stages in a five stage stage architecture are

• IF - Instruction fetch : The instruction is fetched.
• OF - Operand fetch : Operand registers are fetched.
• EX - Execute : ALU operations are performed.
• MEM - Memory reference : Data written into or read from the memory.
• WB - Write back : Data stored into the register.

IF

OF

EX

MEM

WB

add - The MEM stage is not used because, after performing the ALU operation, the result need not be strored into any memory location, but it is stored inside a register itself, this is why, in the last we the WB stage is used in which the result is stored into a register.

Y

Y

Y

-

Y

sw - This instruction will stored the value inside a register to a memory location. WB stage is not used here because the value is not stored inside a register.

Y

Y

Y

Y

-

lw - Load word instruction will load the contents of the specified memory location. All the phases are used here.

Y

Y

Y

Y

Y

j - This is a jump instruction which will jump to the calculated address present inside the PC. In the decode phase itself, it will get to know the destination, so the remaining phases will remain unused.

Y

Y

-

-

-

lui - Load Upper Immediate. In this instruction, the immediate value is left shifted for 16 times and then stored into the register. Here nothing is read or written into the memory therefore the MEM phase is not used.

Y

Y

Y

-

Y