Control hazards (branches)

• When do you know you have a branch?

  • During ID cycle

• When do you know if the branch is Taken or Not-Taken

  • During EXE cycle (e.g., for the MIPS)

• Easiest solution

  • Stall one cycle after recognizing the branch (haz. det. unit)
  • Fetch the instruction following the branch after EXE cycle
  • Cost 2 cycles or contribution to CPI due to branches =
    2 x Branch freq. ? 0.5

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