Register Cache System Not for Latency Reduction Purpose

A register cache has been proposed to solve the problems of the huge register files of recent super scalar processors. The register cache reduces the effective access latency of the register file for IPC improvement, simplifies the bypass network, and reduces the ports of the main register file. Though the primary purpose of the previous works is to improve IPC, the misses on the register cache may degrade the IPC. We propose Non-Latency-Oriented Register Cache System (NORCS). Though the effects of NORCS are the same as the conventional systems, it is free from register cache miss penalties that the conventional systems suffer from. In NORCS, the register cache itself is not different from that of the conventional systems. The difference is that the instruction pipeline has stages to read the main register file, which all instructions go through regardless of register cache hit / miss. Therefore, the instruction pipeline of NORCS is not immediately disturbed by the register cache misses. For a realistic 4-way super scalar processor, NORCS can simplify the bypass network to the same complexity as a 1-cycle-latency register file, and reduce the ports of the main register file from 12 to 4. CACTI simulation shows that the area and power consumption are reduced to 24.9% and 31.9% compared to the baseline model without register cache. Though these results are not different from the conventional systems, IPCs differ greatly. IPC of the conventional system decreases to 83.1% because of the cache miss penalties, while that of NORCS is retained at 98.0%.