3 - Scoreboard

3 - Scoreboard - DAP Spr.98 UCB 26 Scoreboard Implications...

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Unformatted text preview: DAP Spr.98 UCB 26 Scoreboard Implications Out-of-order completion => WAR, WAW hazards? Solutions for WAR Queue both the operation and copies of its operands Read registers only during Read Operands stage For WAW, must detect hazard: stall until other write completes Need to have multiple instructions in execution phase => multiple execution units or pipelined execution units Scoreboard keeps track of dependencies, state or operations Scoreboard replaces ID, EX, WB with 4 stages DAP Spr.98 UCB 27 Four Stages of Scoreboard Control 1. Issue decode instructions & check for structural hazards (ID1) If a functional unit for the instruction is free and no other active instruction has the same destination register (WAW), the scoreboard issues the instruction to the functional unit and updates its internal data structure. If a structural or WAW hazard exists, then the instruction issue stalls, and no further instructions will issue until these hazards are cleared. 2. Read operands wait until no data hazards, then read operands (ID2) A source operand is available if no earlier issued active instruction is going to write it, or if the register containing the operand is being written by a currently active functional unit. When the source operands are available, the scoreboard tells the functional unit to proceed to read the operands from the registers and begin execution. The scoreboard resolves RAW hazards dynamically in this step, and instructions may be sent into execution out of order. DAP Spr.98 UCB 28 Four Stages of Scoreboard Control 3. Execution operate on operands (EX) The functional unit begins execution upon receiving operands. When the result is ready, it notifies the scoreboard that it has completed execution. 4. Write result finish execution (WB) Once the scoreboard is aware that the functional unit has completed execution, the scoreboard checks for WAR hazards. If none, it writes results. If WAR, then it stalls the instruction. Example: DIVD F0,F2,F4 ADDD F10,F0, F8 SUBD F8 ,F8,F14 CDC 6600 scoreboard would stall SUBD until ADDD reads operands DAP Spr.98 UCB 29 Three Parts of the Scoreboard 1. Instruction status which of 4 steps the instruction is in 2. Functional unit status Indicates the state of the functional unit (FU). 9 fields for each functional unit Busy Indicates whether the unit is busy or not Op Operation to perform in the unit (e.g., + or ) Fi Destination register Fj, Fk Source-register numbers Qj, Qk Functional units producing source registers Fj, Fk Rj, Rk Flags indicating when Fj, Fk are ready 3. Register result status Indicates which functional unit will write each register, if one exists. Blank when no pending instructions will write that register DAP Spr.98 UCB 30 Detailed Scoreboard Pipeline Control Read operands Execution complete Instruction status Write result Issue Bookkeeping Rj No; Rk No 2200 f(if Qj(f)=FU then Rj(f)...
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3 - Scoreboard - DAP Spr.98 UCB 26 Scoreboard Implications...

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