Abstract: Multiple test cycles may be randomly generated for simultaneous execution on a design under test using a simultaneous random cycles test generator. One form of the test generator is hardware description code run on a simulator. The test generator provides multiple random cycle description generators. A random cycle description generator randomly generates a particular test cycle at runtime using constraints provided by the test generator. A random cycle description generator granted access to a serial common cycle initiator may initiate random test cycles through the common cycle initiator. The common cycle initiator may execute the randomly determined test cycle or define and arm a cycle executor of a plurality of cycle executors to execute the randomly determined test cycle. While one random test cycle is executed, another random cycle description generator is selected to initiate another random test cycle on the common cycle initiator.