Quiz submission record for quiz6-1-1 at Mon Jul 26 03:13:55 2004:

Your Answer for Question 1:
If only the datapath were pipeplined, the control logic for an instruction that entered the pipeline may change as it goes through the stages.  For example, instruction 1 enters the IF stage.  One cycle later, instruction 2 enters the IF stage with instruction 1 gets to the ID stage.  One more cycle later, instruction 3 is fetched and instruction 1 gets to the EX stage in which the ALU needs the proper control logic to know what operation to perform on the source registers.  However, with unpipelined controls, the processor will not have the correct control values for instruction 1 anymore.

Your Answer for Question 2:
These three possible inputs are from the ID/EX pipeline registers, from the EX/MEM pipeline registers, and from the MEM/WB registers.

ID/EX pipeline registers
add $t0, $t1, $t2
add $t3, $0, $0
add $t4, $0, $0    
While the third instruction executes, $t1 and $t2 are being read from the ID/EX pipeline registers.

EX/MEM pipeline registers
add $t0, $t1, $t2
add $t3, $t0, $t4
add $t5, $0, $0
add $t6, $0, $0
While the fourth instruction executes, $t0 (from instruction 2) are forwarded from the EX/MEM pipeline registers from instruction 1 as inputs to the ALU for the EX stage of instruction 2.

MEM/WB pipeline registers
add $t0, $t1, $t2
add $t3, $0, $0
add $t4, $t0, $0
add $t5, $0, $0
add $t6, $0, $0
While the the fifth instruction executes, $t0 (from instruction 3) is forwarded from the MEM/WB pipeline registers from instruction 1 as inputs to the ALU for the EX stage of instruction 3.

Your Answer for Question 3:
The hazard unit inserts bubbles with a nop instruction or sll $0, $0, 0.  The hazard unit will insert a bubble only when it detects that it needs to insert one.  For example, if a register is read immediately after a lw instruction, then one bubble is used to delay the read instruction to a point in time in which forwarding can solve the problem.  It will not insert bubbles repeatedly because it will not have to.


Your unique submission ID is quiz6-1-1-cs61c-cj-1090836835-1749.