chapter3-m5-ziavras

For renaming the rob only to track when instrs can

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Unformatted text preview: ) between the time they issue & the time they begin execution This function is still provided by RSs. Since every function is still provided by RSs Since every instr. has a position in the ROB until it commits, we tag a result using the ROB entry # rather than using the RS using the RS # This tagging requires that the ROB assigned for an instr. must be tracked in the RS Later: another implementation that uses extra regs. for renaming & the ROB only to track when instrs. can commit S. Ziavras Steps/Stages in Instruction Execution 1. Issue (also: dispatch stage) – Get instr. from instr. queue – Issue if there’s available RS & empty slot in ROB – Send operands to RS if they’re available in either regs. or ROB – Update control entries to indicate buffers are control entries to indicate buffers are in use – Send # of ROB allocated for result to RS (can be used to tag result when it’s placed on CDB) – If either all RSs full or ROB full stall instr. issue issue S. Ziavras Steps/Stages in Instruction Execution (2) 2. Execute – If 1 or more operands not yet available, monitor CDB while waiting for reg. to be computed – Check for RAW hazards – When both operands are available at RS, both operands are available at RS execute • Loads require 2 steps • Stores need only base reg. (only EA calculation) S. Ziavras Steps/Stages in Instruction Execution (3) 3. Write result – When result is available • Write it on CDB (with ROB tag sent when instr. issued) • Write it from CDB to ROB • Write it to any RSs waiting for result • Mark RS as available – For store • If value available, write it into Value field of ROB entry for store • If not available, CDB is monitored for broadcasts in order to update Value field of ROB entry for store (here we assume that it ROB entry for store (here, we assume that it’s done during the Write Results stage of store) S. Ziavras Steps/Stages in Instruction Execution (4) 4. Commit (also: completion, graduation sta...
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This document was uploaded on 02/09/2014.

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