08-Lecture - 1 The College of Computing Georgia Institute...

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Unformatted text preview: 1 The College of Computing Georgia Institute of Technology 1 CS 4290/6290 Lecture 08 Case Studies CS 4290/6290 Lecture 08 Case Studies (Lectures based on the work of Jay Brockman, Sharon Hu, Randy Katz, Peter Kogge, Bill Leahy, Ken MacKenzie, Richard Murphy, Michael Niemier, and Milos Pruvlovic) 2 The College of Computing Georgia Institute of Technology 2 CS 4290/6290 Lecture 08 Case Studies Speculation: The Picture 3 The College of Computing Georgia Institute of Technology 3 CS 4290/6290 Lecture 08 Case Studies ROB vs. Register Renaming How many ports do we need for the ROB? Lots! Look at a single-issue processor: Issue: read two entries and write one Write Result: write one entry Commit: read and write one entry ROB has a dual role Keeps results (names) Keeps order Lets split the two roles 4 The College of Computing Georgia Institute of Technology 4 CS 4290/6290 Lecture 08 Case Studies ROB vs. Register Renaming Keeping results: physical registers Have a large physical register file Keep architected-to-physical mapping in a table Physical registers hold all values (names) Keeping order: simplified ROB Only keeps info needed to commit instructions Reservation stations also simplified No need to keep values Called instruction window instead of RS 5 The College of Computing Georgia Institute of Technology 5 CS 4290/6290 Lecture 08 Case Studies How does it work? Rename Find in the rename RAT (Register Allocation Table) which physical registers are sources Get a free physical register for destination and change rename RAT Dispatch Wait in window until all source registers have values, then Read source values from registers Write Result Send result to destination register Send destination register number to window 6 The College of Computing Georgia Institute of Technology 6 CS 4290/6290 Lecture 08 Case Studies Committing Wait until oldest instruction done Change commit RAT Before it said Rn is in Pj Now change it so Rn is in Pk (the destination) Free physical register Pj Everything that wants Pj is already committed All future uses of Rn should use Pk 7 The College of Computing Georgia Institute of Technology 7 CS 4290/6290 Lecture 08 Case Studies Recovering Precise State To get precise state after instruction X, we Wait until X commits The commit RAT is the precise state E.g. recovery from branch misprediction Wait until X commits Rename map = commit map Flush window & ROB, restart fetch 8 The College of Computing Georgia Institute of Technology 8 CS 4290/6290 Lecture 08 Case Studies Reading Assignment J. E. Smith and A. R. Pleszkun, Implementing Precise Interrupts in Pipelined Processors", IEEE Transactions on Computers, 37(5), pages 562-573, May, 1988....
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08-Lecture - 1 The College of Computing Georgia Institute...

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