Delay Evaluation of Issue Queue in Superscalar Processors with Banking Tag RAM and Correct Critical Path Identification

• Published in: IEICE Transactions on Information and Systems Vol. E95.D; no. 9; pp. 2235 - 2246
• Main Authors: YAMAGUCHI, Kyohei, KORA, Yuya, ANDO, Hideki
• Format: Journal Article
• Language: English
• Published: Oxford The Institute of Electronics, Information and Communication Engineers 2012; Oxford University Press
• Subjects: Applied sciences; complexity; delay; Design. Technologies. Operation analysis. Testing; Electronics; Exact sciences and technology; Integrated circuits; Integrated circuits by function (including memories and processors); issue queue; microprocessor; Semiconductor electronics. Microelectronics. Optoelectronics. Solid state devices; superscalar processor; LSI circuit; Random access memory; Superscalar processor; Clock; delay complexity; Non volatile memory; Integrated circuit; issue queue; Critical path; Delay time; Microprocessor; Queue; Execution time
• DOI: 10.1587/transinf.E95.D.2235
• ISSN: 0916-8532

This paper evaluates the delay of the issue queue in a superscalar processor to aid microarchitectural design, where quick quantification of the complexity of the issue queue is needed to consider the tradeoff between clock cycle time and instructions per cycle. Our study covers two aspects. First, we introduce banking tag RAM, which comprises the issue queue, to reduce the delay. Unlike normal RAM, this is not straightforward, because of the uniqueness of the issue queue organization. Second, we explore and identify the correct critical path in the issue queue. In a previous study, the critical path of each component in the issue queue was summed to obtain the issue queue delay, but this does not give the correct delay of the issue queue, because the critical paths of the components are not connected logically. In the evaluation assuming 32-nm LSI technology, we obtained the delays of issue queues with eight to 128 entries. The process of banking tag RAM and identifying the correct critical path reduces the delay by up to 20% and 23% for 4- and 8-issue widths, respectively, compared with not banking tag RAM and simply summing the critical path delay of each component.