Out-of-order execution

In short: A modern processor does not execute instructions strictly in program order but pulls forward instructions whose data is ready, then reorders the results afterwards.

In more detail: If an instruction must wait for slow data from memory, a strict processor would stall. The out-of-order processor handles independent instructions in the meantime and ensures the visible result still matches the program logic.

In Depth

• Reorder buffer: Collects results and commits them in the right order (“retirement”).
• Register renaming: Removes false dependencies between instructions.
• Speculation: At branches the processor guesses the path and computes ahead. If it was wrong, everything is discarded.
• Security: Speculative execution enabled attacks such as Spectre and Meltdown via timing differences in the cache.
• Consequence for code: Predictable branches and predictable memory access run faster.

See also: Memory hierarchy and cache lines