This paper describes analysis and simulation for understanding of large Ada-83 real-time software. Ada real-time software is frequently very large in size and very complex. The objective is to describe: (1) discovery of the architecture of the software by decomposing the software into a hierarchical structure, (2) generation of state machines that preserve the real-time properties of the hierarchically organized units, and (3) simulation of the software represented by the state machines. Because of the severe limitation of space, these capabilities are only briefly described, without an example. The first two items above have been exercised in processing over 100,000 lines of Ada-83 code. The third item is still being developed.