Abstract
To reduce the processor energy consumption under low workload and low clock frequency executions, a possible solution is to use ALU cascading while keeping the supply voltage unchanged. This cascading scheme uses a single cycle to execute multiple ALU instructions which have a data dependence relationship between them and thus saves clock cycles for the whole execution. Since the processor energy consumption is the product result of both power and execution time, ALU cascading is expected to help energy optimization for microprocessors operating under low frequency status. To implement ALU cascading in a current superscalar processor, a specific instruction scheduler is required to wakeup a pair of cascadable instructions simultaneously despite there being a data dependence relationship between them. Furthermore, ALU cascading is only applied under low clock frequency execution mode so that the instruction scheduler must support standard scheduling for the normal clock frequency execution. In this paper, we propose an instruction scheduling method that enables the additional wakeup features for the utilization of ALU cascading without large hardware extensions. With this scheduler, the average IPC improvement becomes 3.7% in SPECint2000 and 6.4% in Mediabench, as compared to the baseline execution. The delay of additional hardware required for the ALU cascading purpose is also evaluated to study the complexity of ALU cascading.
