Abstract: A data processing system having a hierarchy of computers including a first computer, a second computer, a third computer, and a fourth computer for displaying data from said third computer. Each of the first and second computers execute independent instruction streams. The first computer includes first circuit locations having redundant connections independently accessible by the second computer under control of the second instruction stream and independent of the first instruction stream. A third computer is provided for executing a third instruction stream. The third instruction stream and the third computer perform support functions which assist and control the operation of the second computer. The second computer includes second circuits having redundant connections independently accessible by the third computer under control of the third instruction stream.