What is a branch target buffer and how does it contribute to a reduction of the branch penalty?

Question

What will be an ideal response?

Answer 1

The fundamental problem with a branch is that if it is taken, instructions already in the pipeline have to be
flushed. Consequently, you want to detect a branch instruction as soon as possible. Then you can begin execution
at the target address.
Branch target prediction operates by detecting the branch, guessing its outcome and fetching instructions from
the next or target address as soon as possible.
The branch target buffer, BTB, is a form of memory cache that caches the addresses of branch instructions. The
program counter searches the BTB. If the current instruction address corresponds to a branch, the cache can be
accessed and the predicted outcome of the branch read (This is true only of BTBs that have a prediction bit. In
general, it is assumed that every cached branch will be taken). The BTB contains the address of the target of the
branch. This means that instructions can be loaded from that address immediately (without having to read the
branch instruction and compute the target address). If you also cache the instruction at the target address you
can get the instruction almost immediately. The BTB lets you resolve the branch much earlier in the pipeline and
therefore reduce the branch penalty.