The host issues one of the four instructions listed in Table 1 to the SPCU through the instruction bus. Before invoking the SPCU, the host has to first supply each link in the input graph along with the instruction code ‘00’. Subsequently, it places the index of a source node on the data bus and tasks the SPCU to perform the shortest-path computation by issuing the instruction code ‘01’. After this instruction, the host releases the data bus (by placing its data outputs in highimpedance state) although it continues to monitor (read) the bus’ contents. The SPCU signals the completion of the shortest-path computation by outputting a special value on the data bus. The host can then read the result of the computation by repeatedly issuing the instruction code ‘10’. The last instruction code ‘11’ indicates that the host has released its control of the bi-directional data bus for the internal use of the SPCU. The host also uses this instruction code as a separator between any two instructions.