Problem Set Solutions/Interrupts Exceptions/Problem 3

Using Tomasulo’s algorithm, for each instruction in the following sequence determine when (in which cycle, counting from the start) it issues, begins execution, and writes its result to the CDB. Assume that the result of an instruction can be written in the cycle after it finishes its execution, and that a dependent instruction can (if selected) begin its execution in the cycle after that. The execution time of all instructions is two cycles, except for multiplication (which takes 4 cycles) and division (which takes 8 cycles). The processor has one multiply/divide unit and one add/subtract unit. The multiply/divide unit has two reservation stations and the add/subtract unit has four reservation stations. None of the execution units is pipelined – each can only be executing one instruction at a time. If a conflict for the use of an execution unit occurs when selecting which instruction should start to execute, the older instruction (the one that appears earlier in program order) has priority. If a conflict for use of the CBD occurs, the result of the add/subtract unit has priority over the result of the multiply/divide unit. Assume that at start all instructions are already in the instruction queue, but none has yet been issued to any reservation stations. The processor can issue only one instruction per cycle, and there is only one CDB for writing results.

instructionSceh.png

Explain in one sentence why it is not possible, using the processor described above, to get the correct printout from the exception handler, regardless of which instructions we let finish and which ones we delete in the cycle after the exception is detected.

Solution:

By the time the exception is detected, I6 and I10 already overwrote the value of R4, so the value that was supposed to be printed for R4 no longer exists in the processor.