A compulsory scan unit was a component of 23rd century computer banks which functioned to redirect all analytical resources toward the solution of any given problem to the exclusion of all else, until ordered to stop. The unit operated through the use of compulsory directives, which served as an order to the computer to work on the problem until a resolution was attained.
In 2267, Lieutenant Commander Spock utilized the Enterprise's compulsory scan unit in an attempt to expel the entity known as "Redjac" from the computers, by issuing a class-A compulsory directive to compute to the last digit the value of pi, a transcendental figure without resolution. Ultimately, the effort was successful, and Redjac was forced to return computer control to the crew. (TOS: "Wolf in the Fold")