Spring 2003CSE P5481 Precise Interrupts Precise interrupts preserve the model that instructions execute in program-generated order, one at a time If an interrupt occurs, the processor can recover from it What happens on a precise interrupt: identify the instruction that caused the interrupt let the instructions before faulting instruction finish disable writes for faulting & subsequent instructions force trap instruction into pipeline trap routine save the state of the executing program correct the cause of the interrupt restore program state restart faulting & subsequent instructions

Spring 2003CSE P5482 How Pipelines Complicate Interrupt Handling It’s fairly simple to maintain precise interrupts in the R3000 integer pipeline one instruction fetched and executed each cycle instructions executed in fetch order Still, there are issues…. Pipelines execute several instructions at once how do you determine which one caused the exception?

Spring 2003CSE P5483 How Pipelines Complicate Interrupt Handling Simultaneous interrupts 2 stages cause an interrupt at the same time a solution: handle them in program order still precise

Spring 2003CSE P5484 A 5-Stage Pipeline

Spring 2003CSE P5485 How Pipelines Complicate Interrupt Handling Interrupts out of order wrt sequential instruction execution subsequent instruction causes an interrupt before a previous instruction interrupts still must be handled in program order for precise interrupts A solution: interrupt handled before the write stage interrupt recorded in a per-instruction bit vector which flows with it down the pipeline interrupts for instruction in write stage are handled before it changes any state restart all instructions in the pipeline

Spring 2003CSE P5486 How Pipelines Complicate Interrupt Handling Multicycle operations in separate pipelines some types floating point operations integer multiply & divide can cause imprecise interrupts because operations don’t necessarily complete in program-generated order example: cannot restart interrupting & subsequent instructions because some have completed a completed instruction may have overwritten one its source operands divf FP exception multf not done add completed

Spring 2003CSE P5487

Spring 2003CSE P5488 How Pipelines Complicate Interrupt Handling Dealing with imprecise interrupts ignore the imprecise interrupt (IBM 360) precise mode/imprecise mode (Alphas?) trade-off: correctness vs. performance stall until no previous instruction will fault (Pentium Pro) buffer writes until instructions complete future file or shadow register; today called register renaming history file (VAX) costs: additional registers & bypass logic to FUs

Spring 2003CSE P5489 How Multicycle Operations Complicate Pipelining Current structural hazards: multiple writes in the same cycle when instructions finish in a different order than their fetch order track use of the write port & stall instructions in the decode stage if a conflict set the bit in a shift register that is the number of cycles until the register file will be written shift left each cycle if this instruction’s bit is already set, stall adv: check for all structural hazards in 1 place disadv: shift register & write conflict logic detect the hazard just before it occurs stall before the common final stages adv: easier to prioritize among instructions involved in the hazard disadv: more complicated pipeline control because might need to check for structural hazards in multiple places

Spring 2003CSE P54810 How Multicycle Operations Complicate Pipelining

Spring 2003CSE P54811 ISA & Pipelines How instruction set architectures complicate pipelines: variable cycle instructions cause: more complicated hazard detection more forwarding logic more stalling harder to implement precise interrupts complex addressing modes cause: WAR hazards from autoincrement same problems as above from multiple memory accesses self-modifying code causes pipeline flush on address match between instruction being written & instruction later in the pipeline