ilp2 - Dynamic Scheduling (or (or out-of-order execution)...

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ynamic Scheduling Dynamic Scheduling Or yDanicm ceShuldngi CSE 240A Dean Tullsen Dynamic Scheduling r out f rder execution) (or out-of-order execution) CDC 6600 scoreboard – Instruction storage added to each functional execution unit – Instructions issue to FU when no structural hazards, begin execution when dependences satisfied. Thus, instructions issued to different FUs can execute out of order. – “scoreboard” tracks RAW, WAR, WAW hazards, tells each instruction when to proceed. – No forwarding – No register renaming omasulo BM 360/91 Tomasulo (IBM 360/91 Instruction Queue (MIPS R10000, Alpha 21264, …) CSE 240A Dean Tullsen Tomasulo Algorithm For IBM 360/91 about 3 years after CDC 6600 Goal: High Performance without special compilers ifferences between IBM 360 & CDC 6600 ISA Differences between IBM 360 & CDC 6600 ISA – IBM has only 2 register specifiers/instr vs. 3 in CDC 6600 – IBM has 4 FP registers vs. 8 in CDC 6600 – Implications? CSE 240A Dean Tullsen Differences between Tomasulo lgorithm & Scoreboard Algorithm & Scoreboard ontrol & buffers distributed with Function Units vs centralized in Control & buffers distributed with Function Units vs. centralized in scoreboard; called “reservation stations” => instrs schedule themselves Registers in instructions replaced by pointers to reservation station buffer gp y p scoreboard => registers primary operand storage Tomasulo => reservation stations as operand storage HW renaming of registers to avoid WAR, WAW hazards Scoreboard => both source registers read together Tomasulo => each register read as soon as available. Common Data Bus broadcasts results to all FUs RS’s (FU’s), registers, etc. responsible for collecting own data off CDB Load and Store Queues treated as FUs as well CSE 240A Dean Tullsen
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Tomasulo Organization CSE 240A Dean Tullsen Reservation Station Components ti t f i th it ( + Op—Operation to perform in the unit (e.g., + or –) Qj, Qk—Reservation stations producing source registers j Vk alue of Source operands Vj, Vk—Value of Source operands
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ilp2 - Dynamic Scheduling (or (or out-of-order execution)...

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