INSTRUCTION EXECUTION AND RE-EXECUTION WITH IN-LINE BRANCH SEQUENCES

摘要

Instruction re-execution for error recovery is enhanced by function skipping conditional branches in the execution control sequence. Branches continuing the main execution stream control execution and re-execution of basic functions and ancillary verification and saving functions required for error recovery. The latter functions are organized for rapid execution so as not to unduly lessen the execution throughput rate of the system. Ancillary functions include checking of particular basic function result signals and associated ancillary function control signals, 'scratchpad' saving of the particular function result signals in fast access general purpose buffer storage and setting of re-execution branch conditions to signal for skipping of basic functions when re-executing instructions after basic function result signals have been saved. Function skipping branches are taken therefor only in late re-execution; i.e. only when the re-execution branch condition for function skipping has been set prior to occurrence of error in execution. In the function skipping branch previously saved basic function result signals required for continued execution are obtained directly from fast access buffer storage, thereby eliminating original operand signal handling and arithmetic or logic processes of the skipped function. This is especially useful as the associated operand signals may no longer be available at re-execution time. The re-execution branch condition for function skipping is reset at conclusion of each instruction execution control sequence. Foregoing control organization has the advantage that verified and properly saved basic function results need not be reprocessed and recalculated during error recovery. Also by virtue of the standardized organization of the branch the controls may be adapted piecemeal to a variety of different system recovery functions with minimal cost/performance degradation.