Dependency table for reducing dependency checking hardware

Abstract

A dependency table stores a reorder buffer tag for each register. The stored reorder buffer tag corresponds to the last of the instructions within the reorder buffer (in program order) to update the register. Otherwise, the dependency table indicates that the value stored in the register is valid. When operand fetch is performed for a set of concurrently decoded instructions, dependency checking is performed including checking for dependencies between the set of concurrently decoded instructions as well as accessing the dependency table to select the reorder buffer tag stored therein. Either the reorder buffer tag of one of the concurrently decoded instructions, the reorder buffer tag stored in the dependency table, the instruction result corresponding to the stored reorder buffer tag, or the value from the register itself is forwarded as the source operand for the instruction. Information from the comparators and the information stored in the dependency table is sufficient to select which value is forwarded. Additionally, the dependency table stores the width of the register being updated. Prior to forwarding the reorder buffer tag stored within the dependency table, the width stored therein is compared to the width of the source operand being requested. If a narrow-to-wide dependency is detected the instruction is stalled until the instruction indicated in the dependency table retires. Still further, the dependency table recovers from branch mispredictions and exceptions by redispatching the instructions into the dependency table.