Sections 3.2 and 3.3 Dynamic Scheduling – Tomasulo's Algorithm
EEF011 Computer Architecture 計算機結構 Sections 3.2 and 3.3 Dynamic Scheduling – Tomasulo’s Algorithm 吳俊興 高雄大學資訊工程學系 October 2004 A Dynamic Algorithm: Tomasulo’s Algorithm • For IBM 360/91 (before caches!) – 3 years after CDC • Goal: High Performance without special compilers • Small number of floating point registers (4 in 360) prevented interesting compiler scheduling of operations – This led Tomasulo to try to figure out how to get more effective registers — renaming in hardware! • Why Study 1966 Computer? • The descendants of this have flourished! – Alpha 21264, HP 8000, MIPS 10000, Pentium III, PowerPC 604, … Example to eleminate WAR and WAW by register renaming • Original DIV.D F0, F2, F4 ADD.D F6, F0, F8 S.D F6, 0(R1) SUB.D F8, F10, F14 MUL.D F6, F10, F8 WAR between ADD.D and SUB.D, WAW between ADD.D and MUL.D (Due to that DIV.D needs to take much longer cycles to get F0) • Register renaming DIV.D F0, F2, F4 ADD.D S, F0, F8 S.D S, 0(R1) SUB.D T, F10, F14 MUL.D F6, F10, T Tomasulo Algorithm • Register renaming provided – by reservation stations, which buffer the operands of instructions waiting to issue – by the issue logic • Basic idea: – a reservation station fetches and buffers an operand as soon as it is available, eliminating the need to get the operand from a register (WAR) – pending instructions designate the reservation station that will provide their input (RAW) – when successive writes to a register overlap in execution, only the last one is actually used to update the register (WAW) As instructions are issued, the register specifiers for pending operands are renamed to the names of the reservation station, which provides register renaming • more reservation stations than real registers Properties of Tomasulo Algorithm 1.
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