[Computer Architecture] Pipeling

Previously in CS211

  • Iron law of performance:
    • time/program = insts/program * cycles/inst * time/cycle
  • Classic 5-stage RISC pipeline
  • Structural, data, and control hazards
  • Structural hazards handled with interlock or more hardware
  • Data hazards include RAW, WAR, WAW
    • Handle data hazards with interlock, bypass, or speculation
  • Control hazards (branches, interrupts) most difficult as change which is next instruction
    • Branch prediction commonly used
  • Precise traps: stop cleanly on one instruction, all previous instructions completed, no following instructions have changed architectural state
    ## Trap

An external or internal event that needs to be processed by another (system) program. The event is usually unexpected or rare from program’s point of view.


  • Save \(EPC\) before enabling interrupts to allow nested interrupts
    • need instruction to move \(EPC\) into GPR
    • need a way to mask further interrupts.
  • Needs to read a status register that indicates the cause of the trap.
  • jump inst \(ERET\)

Synchronous Trap

  • A synchronous trap is caused by an exception on a
  • In general, the instruction cannot be completed and needs to be restarted after the exception has been handled
  • requires undoing the effect of one or more partially executed instructions
  • In the case of a system call trap, the instruction is considered to have been completed
  • a special jump instruction involving a change to a privileged mode

Speculating on Exceptions • Prediction mechanism

  • Exceptions are rare, so simply predicting no exceptions is very accurate!
  • Check prediction mechanism
  • Exceptions detected at end of instruction execution pipeline, special
    hardware for various exception types • Recovery mechanism
  • Only write architectural state at commit point, so can throw away partially executed instructions after exception
  • Launch exception handler after flushing pipeline
  • Bypassing allows use of uncommitted instruction results by following instructions