A Dynamically Reconfigurable Mixed In-Order/Out-of-Order Issue Queue for Power-Aware Microprocessors

In this work we focus on power-aware solutions for the issue queue in an out-of-order superscalar processor. We propose two different schemes. Our first approach partitions the issue queue into FIFOs such that only the instructions at the head of each FIFO may request to issue. We then dynamically monitor the FIFO usage and disable FI-FOs that are not being efficiently used. In our second approach we also use a FIFO scheme, but dynamically vary the number and size of each FIFO simultaneously while at the same time keeping the total number of issue queue entries constant. We analyze both approaches and compare them in terms of the performance and power reduction. We find that although the first scheme of completely disabling issue queue entries is more straight-forward to implement, it may not be the best option, particularly for floating point applications. Our best experimental result shows an average power saving of 27.3 % in the issue queue with a performance degradation of only 2.7%. 1