Instruction dependency scoreboard with a hierarchical structure

Abstract

One embodiment of the present invention provides a system that selects instructions to be executed in a computer system that supports out-of-order execution of program instructions. The system receives dependency information for a first instruction. This dependency information identifies preceding instructions in the execution stream of a program that need to complete before the first instruction can be executed. The system divides this dependency information into a recent set and a less recent set. The recent set includes dependency information for a block of instructions immediately preceding the first instruction that need to complete before the first instruction can be executed. The less recent set includes dependency information for instructions not in the block of instructions immediately preceding the first instruction that need to complete before the first instruction can be executed. The system stores the recent set of dependency information in a first store, and stores the less recent set of dependency information in a second store. The first store is smaller and faster than the second store so that an update to dependency information takes less time to propagate through the first store than the second store. In one embodiment of the present invention, the system receives the dependency information for the first instruction from the first store and the second store, and determines from the dependency information if the first instruction is available to be executed by determining whether all preceding dependencies related to the first instruction have been satisfied. In one embodiment of the present invention, the system selects a second instruction from instructions that are available to be executed, and executes the second instruction.