While running a simulation for any length of time, you can pause a simulation using the Break command, which leaves the simulation session open.
To pause a running simulation, select Simulation > Break or click the Break button 
.
The simulator stops at the next executable HDL line. The line at which the simulation stopped is displayed in the text editor.
Note: This behavior applies to designs that are compiled with the
-debug <kind> switch.Resume the simulation any time using the Run All, Run, or Step commands. See Stepping Through a Simulation for more information.