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How delayed branches affect pipelining in a processor?
The effect is to execute one or more instructions following the conditional branch before the branch is taken. This avoids stalling the pipeline while the branch condition is evaluated, thus keeping the pipeline full and minimizing the effect of conditional branches on processor performance.
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.
How do you delay stalls?
Stall delay can occur on airfoils subject to a high angle of attack and a three-dimensional flow. When the angle of attack on an airfoil is increasing rapidly, the flow will remain substantially attached to the airfoil to a significantly higher angle of attack than can be achieved in steady-state conditions.
What’s the point of the delay slot in Assembly?
The point of the delay slot specifically is to execute an instruction that has already made it through part of the pipeline and is now in a slot that would otherwise just have to be thrown away. An optimizer could take the first instruction at the branch target and move it to the delay slot, getting it executed “for free”.
Why are branch delay slots important in RISC?
Branch Delay Slots are one of the awkward features of RISC architectures. RISC CPUs are pipelined by definition, so while the current instruction is in execution, the following instruction (s) will be in the pipeline already.
Is it safe to use a branch delay slot?
Abusing the load delay or branch delay on MIPS-I (and probably later chips too) is only safe if there is no way an interrupt can occur. When an interrupt occurs in a branch delay slot, the cpu subtracts 4 from the PC. However if you have a branch in a branch delay slot, then the CPU will subtract 4 from the address the second branch pointed at.
Is the delay instruction independent of the branch?
The first three instructions are a compare/branch sequence. If the branch is taken, execution will continue at 0x834, otherwise at 0x82c, after the delay slot. The delay instruction is independent of the branch, nothing special here yet.