Method and system for increased instruction synchronization efficiency in a superscalar processsor system utilizing partial data dependency interlocking

摘要

A method and system for increased instruction synchronization efficiency in a superscalar processor system which includes instructions having multiple source and destination operands. Simultaneous dispatching of multiple instructions creates a source-to-destination data dependency problem in that the results of one instruction may be necessary to accomplish execution of a second instruction. Data dependency hazards may be eliminated by prohibiting each instruction from dispatching until all possible data dependencies have been eliminated by the completion of preceding instructions; however, instruction dispatch efficiency is substantially decreased utilizing this technique. Data dependency interlock circuitry may be utilized to clear possible data dependency hazards; however, the complexity of such circuitry increases dramatically as the number of interlocked sources and destinations increases. The method and system of the present invention utilizes data dependency interlock circuitry capable of interlocking two source operands by two destination operands for each instruction. Instructions having three or more source operands are interlocked at the dispatch stage for the first two source operands utilizing existing data dependency interlock circuitry. Thereafter, the instruction is dispatched only after data dependency hazards are cleared for the first two source operands, utilizing the data dependency interlock circuitry, and all instructions preceding the instruction have been completed, eliminating possible data dependency hazards for the third source operand. In this manner, instructions which include three source operands may be synchronized without requiring a substantial increase in data dependency interlock circuitry and with only a slight degradation in system efficiency.