Question

Computer Organization and Arquitecture

Consider the execution sequence "in-order-issue / in-order-completion" shown in the figure. 1) Identify the most probable reason why I2 could not enter the execution stage until the fourth cycle. Could "in-order issue / out-of-order completion" or "out-of-order issue / out-of-order completion" fix this? If so, what? 2) Identify the reason why I6 does not enter the writing stage until the ninth cycle. Could "in-order issue / out-of-order completion" or "out-of-order issue / out-of-order completion" fix this? If so, what?

Decode Execute Write Cycle 1 I1 12 3 13 13 14 15 15 I5 I6

Answer 1

Identifying the reasons:

a. Reasons for why I2 could not enter the execute stage until fourth cycle:

• I1 and I2 are decoded in parallel but in execution state they are not done together because I2 is waiting for I1 to make the resource free if any i.e., likely to have resource conflict. I2 is waiting for I1 to complete the execution process.

• Now “I1” completes its execution process in third cycle and thus, “I2” enters execution stage at fourth clock cycle.

• Therefore these kind of dependencies (True data dependency - RAW) cannot be fixed using out of order issue with out-of-order completion.

Thus, “I2” cannot enter into execution stage till fourth cycle.

b. Reasons for why “I6” could not enter write stage till 9 ^thclock cycle:

• In in-order-issue, in-order-completion instruction “I5” has to completed before “I6” comes to the write stage and instructions “I5 and I6”

• In in-order-issue the instructions “I5 and I6” together should come out from the pipeline stages.

• In case of out-of-order issue it’s not the case where together can come out from the pipeline stages.

• So both the issue policies (in-order-issue out-of-order completion & out-of-order-issue out-of-order completion) will fix this.

Suppose if we consider out-of-order issue, out-of-order completion, then

• There exists true data dependency between instruction “I1 and I2”.

• Instruction “I2” must wait for “I1” to complete its process. So, “i2” stays in instruction window till “I1” comes out from the execution stage.

• Advantage of using “out-of-order issue, out-of-order completion” policy is as soon as resources are available; the instruction from instruction window comes to the execution stage.

• The instruction stays in the window till true data dependency between instructions is resolved.

From the block diagram given,

• Instruction “I2” waits in the window till “I1” is executed.

• “I2” exits from instruction window as “I1” frees up the resources “I2” needs.

• When “I2” completes execution process “I4” enters into execution stage.

• Instruction “I5” proceeds with the execution process without staying in instruction window.

• “I6” has to wait for “I4” to be completed.

Thus, considering out-of-order issue, out-of-order completion “I5” must wait for “I3 and I4” to complete the write stage and after completing the process “I5” enter into write stage in 9^th cycle.