Single-Program Multiple Data (SPMD): Keeping track of multiple streams of instructions can be very difficult for a programmer. If each instruction stream comes from independent source code, the software can quickly become unmanageable. There are a number of solutions to this problem. One is to have a single program (SP) that is used for all of the streams of instructions. An process/thread ID (or rank) is defined for each instance of the program and this can be used to index into multiple data sets (MD) or branch into different sub-sets of instructions.