Initial Blocks and Simulation Time

An initial block is a piece of testbench code that runs once, starting at time zero, from top to bottom. It is where you write your stimulus, the sequence of values you want to feed the design. Unlike hardware, which runs continuously, an initial block executes its statements a single time and is then finished. It is the most basic way to script what happens during a simulation, and it is where your control over time begins.

Without any delays, every statement in an initial block would happen at time zero, one after another, with no time passing between them. To spread stimulus out across time, you insert a delay. A delay tells the simulator to advance simulated time by a set number of units before running the next statement. Delays are the mechanism by which you say exactly when each thing happens on the timeline.

The time inside a simulation is not the wall clock time on your computer. A run covering a thousand units of simulated time might finish in a fraction of a real second, or take much longer, depending on how much the design has to compute. Simulated time is a model of how the hardware behaves over time, kept entirely inside the simulator. Whenever this page speaks of time, it means simulated time, the units you control with delays, not seconds on a clock.

Here is the shift in thinking. In a testbench, time does not simply happen to you; you author it. Every delay you write places an event at a precise moment on the simulation timeline, and the order and spacing of those events is entirely your design. Get the timing wrong and the design sees the wrong inputs at the wrong moments, producing wrong behaviour that is often blamed on the design when the fault is in the testbench. Controlling simulated time deliberately is a core testbench skill, and this lab makes you place events at exact times.

On the previous page, you wrote your first testbench and used a single small delay for propagation. Here you extend that idea: instead of one delay, you build a whole timed schedule where every event lands at a precise moment.