ARM 3-Stage Pipeline

What is Pipeline?
Pipeline is used to improve the perforamnce of the overall system by allowing multiple instructions(which are in different stage) parallely. So first understand the 3 stages of pipeline :

Fetch--------> Decode -----------> Execute

As per their names, In Fetch, we fetch the instructions, in Decode we decode the instruction and finally execure in Execute stage.

So keeping these 3 stages in mind, Whenever instuction 1 is in Executing stage(as it started first), instruction 2 will be in Decoding stage and instruction 3 will be in Fetching stage.

So if you see this, then there will not be any improvement for a single instruction, as every instruction will be taking the same 3 cycles to execute. But the overall performance will be improved.

Problem with pipeline: 
As like all other good things in the world, pipeline also have it share of cons. Pipeline creates problem when there is a branch instruction, because whenever branch instruction is in executing stage, It has to go to some other instruction instead of the instruction which processor had taken in fetch/decode stage while branch instruction was in fetch/decoding stage. I know it may be confusing, so lets understand this with an example:

[As I dont know assembly language, instruction syntax can be different what you expect]
1. ADD R1,R2,R3 [Add R2, R3 and keep it in R1]
2. ADD R2,R3,R4
3. SUB R3, R4,R5
4. JMP X
5. ADD R2,R3,R4
6. SUB R3, R4,R5
7. X:
8. ADD R1,R2,R3


So you can see when instruction 4(JMP) will be in execution stage, instruction 5 will be in decode stage and instruction 6 in fetch stage. But as isntruction 4 is a jump statement, thee is no use of executing instruction 5 and 6.
           In that case pipeline will be flush, and next 2 cycle will be waste. As for instruction 7 to execute, you will need 2 cycle, and as there is no eligible instruction to execute in pipeline, there will not be any output during this cycle.

So it is not necessary that pipeline will always improve performance.

Branch Prediction:
To overcome this problem caused by pipeline, Architecture came up with branch prediction. It tries to predict which could be executed next, . There are different ways to achieve this, I am not covering those here. you can refer wikipedia or ARM information center.