The Branch Target Buffer is a small CPU predictor table that remembers where indirect branches are likely to jump next. Instead of waiting to resolve every branch in software, the processor uses BTB entries to guess a target early and keep its execution pipeline busy. That speeds up normal code, but it also means the CPU can be influenced by previously observed branch patterns.
In cyber security, the BTB matters because attackers can sometimes train or poison prediction state and steer speculative execution toward unintended code paths. This is a core idea behind Spectre-v2 style attacks, especially when shared or reused code leaves stale predictor entries behind. Defenses aim to limit that reuse with techniques such as IBPB, retpoline, and other branch-history isolation measures. For JIT engines, kernels, and browsers, the practical lesson is that freeing code does not necessarily clear the hardware’s memory of it.