Chinese Journal of Computers   Full Text
Title	Active-Cycle Based Register File Design for Reduced Ports and Energy
Authors	ZHAO Yu-LaiLI Xian-FengTONG DongSUN Han-XinCHEN JieCHENG Xu
Address	(School of Electronics Engineering and Computer Science， Peking University， Beijing 100871)
Year	2008
Issue	No.2(299—308)
Abstract &   Background	Abstract Multi-ported register file helps exploiting instruction-level and thread-level parallelism but brings area, energy and access time pressure. Oriented for superscalar and SMT processor, this paper give a method that is to manage a small auxiliary active value file (AVF) and selectively store physical register values in active cycle (during the time between production and last use). The AVF structure can share the register file access pressure and reduce the number of register ports, is simple to implement, and can filter some writes. It achieves significant energy savings with no impact on frequency and only small IPC loss. keywords physical register file； register renaming； register life time； out-of-order execution； SMT background Power and energy consumption has become the major constraint as the instruction window and issue width scale to achieve more ILP and TLP.Physical register file (RF) is power-critical due to its large size and number of ports.It is prevalent to replicate the contents of RF in dual copies to reduce the number of ports in recent microprocessor such as Alpha21464.However, it is still not energy-efficient since each produced value must be written twice.Although previous studies have been focused on reducing RF ports or size, they paid less attention to SMT processors where multiple contexts share and compete for datapath resources.The authors aim at reducing the number of RF ports and energy on wide-issue superscalar microprocessors which support SMT execution.To achieve this goal, it is important to characterize the behavior of both single- and multi-threaded workloads.Based on the observation that the active cycles of registers are quite short, the authors propose to manage the active register values in a small active value file (AVF) if can keep tracking of its usage.Compared with previous techniques utilizing auxiliary structure for energy reduction, AVF can circumvent more read requests on the main RF, reduce the number of writes on the auxiliary structure and therefore save more energy.Meanwhile, it achieves the goal with small IPC loss, no frequency impact and little additional design complexity.