Register Renaming

In the last tutorial, we tried to understand the drawbacks of out of order execution. Inspite of all the drawbacks to improve the performance of our processors out of order execution is inevitable. Hence, register renaming is very first method we will understand to overcome our road blocks to implement register renaming. 

Modern processor architectures employ the method of register renaming to get around a restriction called register dependencies or register risks. It permits out-of-order instruction execution while maintaining proper program behavior. In order to remove spurious dependencies brought on by instructions that share the same architectural registers, register renaming involves the creation of extra temporary registers, also known as rename registers or physical registers.

Instructions are carried out strictly in accordance with the program's order in a processor without register renaming, and the results are saved in the architectural registers. However, because of dependencies between instructions, this may result in performance reduction. For instance, if the outcome of a prior instruction is necessary for the current instruction, the current instruction must wait for the preceding instruction to finish before continuing. 

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Performance can be slowed down by this dependency, especially when several instructions are awaiting the same register.

The CPU assigns temporary physical registers to instructions when they are issued for execution using register renaming. The values of these physical registers serve as stand-ins for architectural register values. The dependencies can be removed by using these temporary registers instead of the original architectural registers in instructions. Even when instructions are run out of order, the renaming process makes sure they operate on the right data.

Usually, register renaming is handled by the processor's hardware. It entails keeping track of the mapping between the physical registers and architectural registers. A physical register is given to an instruction when it is sent to be executed. The results produced by instructions are stored in the corresponding physical registers, maintaining the correctness of the program.

Register renaming enables the processor to benefit instruction-level parallelism and boost efficiency by enabling out-of-order execution and removing register dependencies. By allowing the execution of separate instructions simultaneously and preventing pipeline pauses brought on by dependencies, it maximizes the use of the execution resources that are available.

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Register renaming is one of the effective methods used in processors today. Further we will learn about branch prediction in the next tutorial.