Pipelined Processor II CPSC 321 Andreas Klappenecker

Basic Idea

Non-Pipelined vs. Pipelined Execution

Pipelining - Ideal Preconditions Instruction set design ideally, instructions are of equal length few instruction formats memory operands only in load and stores aligned data: only one memory access per load Sources of problems instructions with variable lengths (memory needs to be accessed multiple times) unaligned data (multiple memory accesses to obtain single data item)

Single Cycle Datapath

Pipelined Version

Obstacles to Pipelining Structural Hazards hardware cannot support the combination of instructions in the same clock cycle Control Hazards need to make decision based on results of one instruction while other is still executing Data Hazards instruction depends on results of instruction still in pipeline

Control Hazards Solutions: stalling: delay by one cycle to wait for result prediction: predict the result delayed decision: rearrange and perform operations first that do not harm

Stall on Branch Assume that all branch computations are done in stage 2 Delay by one cycle to wait for the result

Branch Prediction Predict branch result For example, predict always that branch is not taken (e.g. reasonable for while instructions) if choice is correct, then pipeline runs at full speed if choice is incorrect, then pipeline stalls

Branch Prediction

Delayed Branch

Problems Write back data to register file may create data hazard Selection of the next PC may create a control hazard Solutions: have a separate data path for each instruction (not really, you got to be kidding!) chop data path into small pieces keep everything that belongs together into one piece introduce registers to separate the stages

Pipelined Version Pipeline separation registers, width varies

Store Word Instruction fetch: as before decode & register file read: as before execute and address calculation: as before forwarding of register contents to EX/MEM pipelining register Memory access send data and address to memory Last step: nothing happens

Summary Some instructions do not require the complete datapath No information transfer from one pipeline stage to another is possible, except through pipeline registers Everything that happened in any previous stage will be overwritten Correction for store required: Where is the information on the write register?

Pipeline Control Data is traveling along the pipeline stages All data belonging to one instruction must be kept within that stage Information transfer only through pipeline registers Control information must travel with the instruction

Outlook Control for pipelined architecture Effect of various hazards Superscalar architectures issue multiple instructions per cycle Dynamic pipeline scheduling rescheduling of instructions by hardware to avoid pipeline stalls Intel architectures