Divide the pipeline into three parts:

1. Instruction fetch, decode and issue unit
2. Execute units
3. Commit unit

Once all operands are ready and the functional unit is free, the operation is performed.

The commit unit determines when it is safe to write results to the register file or memory.

Two choices for commit:

1. In-order completion
2. Out-of-order completion

In-order completion is where instructions are committed in program execution order. If an exception occurs, only those instructions before the exception will have written results.
Out-of-order completion does not have such precise interrupts. There may be a need to undo some instructions.

To make things even more complicated, consider dynamic branch prediction. This leads to speculative execution, since you do not want to keep your functional units idle while you wait for a branch condition to be determined. Again you will need to undo instructions.