Consider the following loop in assembly:
loop: ADDI R1, R1, #-1 ;subtract 1 from the value stored in R1
BNEZ R1, loop ; if the value of R1 is not 0, go to the code at label "loop"
SUBI R2, R2, #2 ; completely unrelated to the loop
This code will continuously subtract 1 from the value stored in R1, until the value in R1 is 0. Pretty simple, if not practical. Equivalent C code might read
while(a != 0)
According to Wikipedia:
In computer architecture, a branch predictor is a digital circuit that tries to guess which way a branch (e.g. an if-then-else structure) will go before this is known for sure.
In this example, the branch predictor will try to guess if the value of R1 is, in fact, 0, by various methods covered in the Wikipedia article. All the branch predictor does is determine "Yes, this branch is going to be taken", or "No, this branch will not be taken."
On the other hand, a Branch Target Predictor will take the results of the Branch Predictor, and give the address that the program is going to jump to.
Using the example above, if the branch predictor says that the
BNEZ instruction is going to take the branch, the branch target predictor will give the Address of the next instruction to execute after the
ADDI instruction. This might be the
ADDI instruction, at the label
loop, or it might be the
SUBI instruction after the the branch instruction.
The Branch Predictor predicts the result of a comparison. The Branch Target Predictor give where the program is going because of a branch.
Branches (and jumps, for that matter), are Program Counter (PC) relative. The Branch Target Predictor will add the offset (given by the branch instruction), and add it to the current program counter. This gives the address of the instruction to execute after the branch.
Branch Predictors give a Yes/No answer to the question "Am I going to branch". The Branch Target Predictor needs that Yes/No answer to determine the answer to "WHERE am I going after the branch".