hardware based speculation

Hardware speculation

·Hardware-based speculation combines three key ideas: dynamic branch prediction to choose which instructions to execute, speculation to allow the execution of instructions before the control dependences are resolved and dynamic scheduling to deal with the scheduling of different combinations of basic blocks.

·Hardware-based speculation follows the predicted flow of data values to choose when to execute instructions. This method of executing programs is essentially a data-flow execution: operations execute as soon as their operands are available.

·The approach is implemented in a number of processors (PowerPC 603/604/G3/G4, MIPS R10000/R12000, Intel Pentium II/III/ 4, Alpha 21264, and AMD K5/K6/Athlon), is to implement speculative execution based on Tomasulo’s algorithm.

·The key idea behind implementing speculation is to allow instructions to execute out of order but to force them to commit in order and to prevent any irrevocable action until an instruction commits.

·In the simple single-issue five-stage pipeline we could ensure that instructions committed in order, and only after any exceptions for that instruction had been detected, simply by moving writes to the end of the pipeline.