Pipeline Control Unit (PCU)

What is it

The pipeline control unit defines global behavior for the pipeline, ensuring that instructions are either bubbled or stalled when necessary.

Events

A boolean value that indicates that a certain hazard condition is imminent

Def-use and non-back-to-back load-use cases are handled using forwarding, so the PCU doesn’t have to get involved.

Remaining situations

• Back-to-back load-use
  • X_out-op == OP_LDUR && ((X_out->dst == src1) || (X_out->dst == src2))
• Mis-predicted conditional branch
  • X_out->op == OP_B_COND && !M_in->cond_holds
• Handing ret
  • D_out->op == OP_RET

Actions

Back-to-back load-use (bubble for stalling)

• (F, D, X, M, W) = (Stall, Stall, Bubble, Normal, Normal)

Mis-predicted conditional branch (bubble for squashing)

• (F, D, X, M, W) = (Normal, Bubble, Bubble, Normal, Normal)

Handling ret (bubble for stalling)

• (F, D, X, M, W) = (Stall, Bubble, Normal, Normal, Normal)