Reorder Buffer

A reorder buffer (ROB) is a hardware structure in an out-of-order processor that tracks instructions in their original program order, ensuring that results are committed to the architectural state in the sequence the programmer intended. Despite the processor's aggressive internal reordering — instructions may execute in any data-dependency order — the reorder buffer guarantees that exceptions, interrupts, and state updates appear sequential to software. It is the architectural contract-keeper: it permits the hardware to lie about timing while enforcing truth about ordering.

Each entry in the reorder buffer holds the instruction's original program counter, its destination register, and a flag indicating whether execution has completed. When an instruction reaches the head of the buffer and all prior instructions have committed, the result is written to the architectural register file or memory. If a prior instruction triggers an exception, the reorder buffer discards all subsequent speculative results, preserving precise exception semantics. The buffer is typically implemented as a circular queue with head and tail pointers, sized to match the processor's instruction window.

The reorder buffer is the boundary between the dataflow interior of a superscalar processor and the sequential exterior that the compiler and operating system depend on. Without it, out-of-order execution would be unusable for general-purpose computing, because the hardware could not guarantee the precise interrupt semantics that operating systems require to manage processes. The ROB is not merely an optimization structure. It is the mechanism by which the processor maintains its contract with the world of software.

The reorder buffer is where computer architecture becomes philosophy: it is the assertion that the order of events matters more than the speed of their execution. A system that executes faster but loses track of what happened when has not optimized; it has destroyed information.