How a CPU Works

Speculative Execution

Let’s suppose that one of this generic instructions is a conditional branching. What will the out-of-order engine do? If the CPU implements a feature called speculative execution (all modern CPUs do), it will execute both branches. Consider the example below:

1. generic instruction
2. generic instruction
3. if a=<b go to instruction 15
4. generic instruction
5. generic instruction
6. generic instruction
7. math instruction
8. generic instruction
9. generic instruction
10. math instruction
…
15. math instruction
16. generic instruction
…

When the out-of-order engine analyses this program, it will pull instruction 15 into one of the FPUs, since it will need one math to fill one of the FPUs that otherwise would be idle. So at a given moment we could have both branches being processed at the same time. If when the CPU finishes processing the third instruction a is greater than b, the CPU will simple discard the processing of instruction 15. You may think this is a waste of time, but in fact it is not. It doesn’t cost anything to the CPU to execute that particular instruction, because the FPU would be otherwise idle anyway. On the other hand, if a=<b the CPU will have a performance boost, since when instruction 3 asks for instruction 15 it will be already processed, going straight to instruction 16 or even further, if instruction 16 also has already been processed by the out-of-order engine.
Of course everything we explained on this tutorial is an over simplification in order to make this very technical subject easier to understand. Read our Inside Pentium 4 Architecture tutorial in order to study the architecture of Pentium 4 processor. We will be also posting an Athlon 64 architecture tutorial very soon.