Assume you have a machine that has the following resources and executes Tomasulo

Question

Assume you have a machine that has the following resources and executes Tomasulos algorithm. a) Complete the table below listing the clock cycle in which the instruction completes each particular stage (assume R2 is available). For this problem assume that you must write back first before you can start execution if an instruction is waiting for a result. b) What is the maximum number of instructions and their types that could be in progress within the CPU at any one time? c) The following is a list of operations that are performed during the issue stage. Explain why the order of these operations cannot be altered. What would happen if steps 2, 3 were reversed? Check if Reservation station available Lookup source registers and copy either operand into V field or reservation station in to Q field Rename destination register

Explanation / Answer

B) In Tomsulo algorithm,execution is out-of-order,if one instruction is stalled the other instruction can be issued and executed if they don't depend on any other stalled instructions.

The execution rate is one instruction per cycle(when there are no structural hazards).Two insructions can be in issue at one cycle.

c)In tomsulo issue and read function the registers are read for source operands ,If the operands are available they are put in the reservation stations.If no the registers are renamed to eliminate the WAR and WAW hazards.so,2,3 steps happen one in an order and can not be reversed.

INST ISSUE EXECUTE WRITE BACK LD R1,0(R2) 1 4 5 LD R3,#10(R2) 2 5 6 ADD R4,R1,R3 3 10 11 MULT R5,R4,R1 4 21 22 ADD R6,R1,R3 5 15 16

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