How does a branch delay slot work?
The idea of the branch shadow or delay slot is to recover one of those clocks. If you declare that the instruction after a branch is always executed then when a branch is taken the instruction in the decode slot also gets executed, the instruction in the fetch slot is discarded and you have one hole of time not two.
What do you mean by delayed branching?
The delayed branch means that the instruction following the branch is always executed before the PC is modified to perform the branch.
What is a delay slot in MIPS?
On the MIPS architecture, jump and branch instructions have a “delay slot”. This means that the instruction after the jump or branch instruction is executed before the jump or branch is executed. The MIPS processors execute the jump or branch instruction and the delay slot instruction as an indivisible unit.
What is the drawback of delayed branching?
Disadvantage of Delayed Branching Interrupt processing becomes more difficult. This is because interrupt requests caused by instructions in the delay slot have to be processed differently from those arising from ‘normal’ instructions.
Why is branch prediction algorithm needed?
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 definitively. The purpose of the branch predictor is to improve the flow in the instruction pipeline.
What is dynamic branch prediction?
This says whether the branch was recently taken or not. Based on this, the processor fetches the next instruction from the target address / sequential address. If the prediction is wrong, flush the pipeline and also flip prediction. So, every time a wrong prediction is made, the prediction bit is flipped.
What is delay slot in pipelining?
The branch delay slot is a side effect of pipelined architectures due to the branch hazard, i.e. the fact that the branch would not be resolved until the instruction has worked its way through the pipeline. Each cycle where a stall is inserted is considered one branch delay slot.
Why is branch prediction so important?
The purpose of the branch predictor is to improve the flow in the instruction pipeline. Branch predictors play a critical role in achieving high effective performance in many modern pipelined microprocessor architectures such as x86.
What is a branch target buffer?
A branch target buffer is a buffer that is index by the branch instruction address with a tag for remaining bits. The info stored can be branch taken history and/or target address so it doesnt have to be recomputed.
What is meant by pipeline stall?
In the design of pipelined computer processors, a pipeline stall is a delay in execution of an instruction in order to resolve a hazard.
What is meaning of delayed branch and branch prediction?
Delayed branch and branch prediction are two different ways of mitigating the effects of a long execution pipeline. Delayed branch simply means that some number of instructions that appear after the branch in the instruction stream will be executed regardless of which way the branch ultimately goes.
How accurate is branch prediction?
On the SPEC’89 benchmarks, very large bimodal predictors saturate at 93.5% correct, once every branch maps to a unique counter. The predictor table is indexed with the instruction address bits, so that the processor can fetch a prediction for every instruction before the instruction is decoded.
What happens to the delay slot in a delayed branch?
In a delayed branch, the hardware always executes (does not cancel) the delay slot instructions after the branch (two instructions in the example). By always executing the delay slot instructions, the pipeline simplified. The compiler’s job is to try to fill these delay slots with useful instructions.
How does branch delay slot work in MIPS?
On classic MIPS this next instruction is fetched, decoded, and executed, and meanwhile the branch may or may not modify the PC to the branch target, so the branch-delay slot instruction will get executed every time.
How is the fetch delayed on every branch?
On every branch, fetch is stalled until the branch outcome is known. Every branch is predicted not-taken and the mis-fetched instructions are squashed if the branch is taken. The processor has two delay slots and the two instructions following the branch are always fetched and executed, and
How does the compiler fill a delay slot?
The compiler can fill a delay slot with an instruction from the taken or not taken path, but such an instruction cannot be one that overwrites state used by the other path (or after the paths join) since delay slot instructions are not canceled (unlike with prediction).