Controlling Stalls -- the DRR

• The DRR will stall the FETCH unit if the CRS did not accept all previously issued instructions or if the ROB is full.

  • If the CRS did not accept ALL of the previously issued instructions, then REFETCH the operands of rejected instructions; change the accepted instructions to NOPS; then issue these instructions to the CRS.
  • If the ROB is full, issue all NOPS to the CRS and stall the FETCH Unit.

• The ‘inc_pc’ line from the DRR is used to stall the fetch unit.

  • if ‘0’, then the fetch unit does not increment the PC and simply refetches the last EUMAX instructions.

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