Dynamic Scheduling Physical Register File ready bits Issue Queue (IQ) ECE 721, Spring 2019 Prof. Eric Rotenberg

Physical Register File ready bits One ready bit per physical register Location in pipeline NOT located with Physical Register File Physically located in the Schedule Stage, close to Issue Queue ECE 721, Spring 2019 Prof. Eric Rotenberg

Physical Register File ready bits If I am the producer of a physical register: Dispatch: Clear ready bit of my physical destination reg. Issue: Set ready bit of my physical destination reg. If I am the consumer of a physical register: Dispatch: Read ready bit of my physical source register to initialize my source operand’s ready bit in Issue Queue If initially “ready”, it means the value is in the PRF (producer has executed) or will be by the time the consumer issues (producer has at least issued) If initially “not-ready”, it means the producer has not yet issued, therefore, consumer must wait in Issue Queue until woken up by producer ECE 721, Spring 2019 Prof. Eric Rotenberg

Issue Queue (IQ) This is the dynamic scheduling unit Contains renamed instructions that have been dispatched but not yet issued Same as “reservation stations” but no data for source operands, just tags. Tags are the physical register names (e.g., p99). ECE 721, Spring 2019 Prof. Eric Rotenberg

IQ: contents and high-level operation IQ CAM (wakeup array) IQ RAM (payload) ready bit ready bit source tag source tag dest. tag misc. instr. information 1 p10 1 p99 … … issue 1 … … p99 p99 Note: also set ready bit of p99 in the PRF ready bit array ECE 721, Spring 2019 Prof. Eric Rotenberg