VHDL uses the following simulation cycle to model the stimulus and response nature of digital hardware
The VHDL timing model controls the stimulus and response sequence of signals in a VHDL model. At the start of a simulation, signals with default values are assigned those values. In the first execution of the simulation cycle, all processes are executed until they reach their first wait statement. These process executions will include signal assignment statements that assign new signal values after prescribed delays.
After all the processes are suspended at their respective wait statements, the simulator will advance simulation time just enough so that the first pending signal assignments can be made (e.g. 1 ns, 3 ns, 1 delta cycle).
After the relevant signals assume their new values, all processes examine their wait conditions to determine if they can proceed. Processes that can proceed will then execute concurrently again until they all reach their respective subsequent wait conditions.
This cycle continues until the simulation termination conditions are met or until all processes are suspended indefinitely because no new signal assignments are scheduled to unsuspend any waiting processes.