A logic simulator is a computer program that allows designers and experimenters to conduct virtual tests of complex digital circuitry before working with any hardware. The user can interact with the program to find a component arrangement that will perform a desired task. Once a suitable design has been found, the logic simulator makes it easy to optimize, debug, and modify the circuitry. All digital systems comprise multiple logic gates, often in vast numbers. Some large or sophisticated systems also contain smaller, self-contained digital devices such flip flops, multiplexers, oscillators, integrators, differentiators, and counters. Each smaller device plays a unique and vital role in the complete system. Before the advent of logic simulators, engineers had to design digital devices and systems by going through a tedious combination of trial-and-error hardware manipulation and educated guesswork. Logic simulators vary from vendor to vendor, but all offer intuitive GUIs (graphical user interfaces) including toolbars, drag and drop, color coding, and online help. Some programs also offer animation, signal tracing, and alternative logic-gate interconnection options. On the downside, logic simulators run far more slowly than the actual systems do (in some cases millions of times more slowly). Logic simulators can also place significant demands on computer resources because of the vast number of parallel hardware processes that take place in any nontrivial digital system