1. For use with a data processor, apparatus for defining a circular data buffer in a random access data memory comprising: first means for storing the addresses of the topmost location of said buffer, second means for storing the number of locations comprising said buffer, means for applying a first address, means responsive to said means for applying and to said first means for storing for determining if said first address is greater than said address of the topmost location, and means comprising said data processor responsive to said means for determining and to said second means for storing for arithmetically altering said first address.