Out-of-order execution

Executing ready instructions before older instructions whose operands or resources are unavailable, while preserving the specified architectural behavior. Reservation stations, register renaming and a reorder buffer support different parts of this organization.

An age-ordered structure holding issued instructions' results and completion or exception status until instruction retirement. It permits execution to complete out of order while commitment follows program order.

Committing a completed instruction's architectural effects in program order. An instruction with no register result still needs completion status before instruction retirement can pass it.

A shared path carrying tagged results or completion information to waiting reservation stations, the register file or a reorder buffer. A tag identifies the particular producer, not merely the destination's logical name.

Mapping logical processor registers to versions identified by producer tags or physical registers. A conditional destination must still deliver the old logical value when its predicate is false.

A hardware value-storage location distinguished from an architectural processor register name. Several live versions of one logical register can occupy different physical registers.

Storage for an issued instruction and its operands or producer tags until execution can proceed. In a scheme using station identifiers as persistent result tags, the identifier cannot be reused before the old result has completed writeback.