Dynamic Scheduling

1. Dynamic Scheduling • Physical Register File ready bits • Issue Queue (IQ) Prof. Eric Rotenberg

2. 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 Prof. Eric Rotenberg

3. 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 Prof. Eric Rotenberg

4. 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). Prof. Eric Rotenberg

5. issue 1 … 1 … p99 … 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 0 p99 … … p99 Note: also set ready bit of p99 in the PRF ready bit array Prof. Eric Rotenberg