Back-End Bound metric represents a Pipeline Slots fraction where no uOps are being delivered due to a lack of required resources for accepting new uOps in the Back-End. Back-End is a portion of the processor core where an out-of-order scheduler dispatches ready uOps into their respective execution units, and, once completed, these uOps get retired according to program order. For example, stalls due to data-cache misses or stalls due to the divider unit being overloaded are both categorized as Back-End Bound. Back-End Bound is further divided into two main categories: Memory Bound and Core Bound.
A significant proportion of pipeline slots are remaining empty. When operations take too long in the back-end, they introduce bubbles in the pipeline that ultimately cause fewer pipeline slots containing useful work to be retired per cycle than the machine is capable of supporting. This opportunity cost results in slower execution. Long-latency operations like divides and memory operations can cause this, as can too many operations being directed to a single execution port (for example, more multiply operations arriving in the back-end per cycle than the execution unit can support).