針對低功率drowsy BTB設計之預先開啟機制

Abstract

漏電流所造成之電耗已經成為低功率處理器設計上的一項重要考量，特別是針對更先進的製程。BTB是一種常見的動態分支預測器。除了快取記憶體之外，BTB是chip上最大的元件。如果不會被用到的BTB entry能被調整至leakage saving的模式，漏電流所造成之電耗可以被節省。然而，由leakage saving模式調整至正常模式，需要一段的時間延遲。因此，這個研究設計出一套預先開啟的機制，能將下一個即將被存取的BTB entry預先調整至正常模式，以隱藏模式轉換造成之時間延遲，使得我們能藉由更積極的調整BTB至低電耗模式來節省更多在BTB上，因為漏電流所造成之電耗。

Leakage energy consumption is becoming an important design consideration with the scaling of technology. Branch target buffer (BTB) is a popular kind of branch predictor. Besides cache, branch predictors are the largest on-chip array structures. If we put non-accessed branch target buffer entries into leakage saving mode, we gain branch target buffer leakage energy reduction. However, there is a period of time from leakage saving mode from normal mode and it introduces system leakage energy increment. This research proposes an accurate pre-active policy to switch the next accessed branch target buffer entry from leakage saving mode to normal mode in order to hide this latency. And we can put BTB entries into leakage saving mode more aggressively to reduce BTB leakage energy.