In the presence of undocumented and unfamiliar hardware, the process of program comprehension becomes more complex. To perform maintenance activities, programmers must understand the functioning of each element independently, as well as their interactions. In this paper we examine the process taken by the first author during the analysis, porting and re-implementation of a software system that has a heavy reliance on undocumented customized hardware interfaces. This process also demonstrates the use of a two-phase approach when migrating a mission-critical software system. The software was first ported to a new platform running a semi-compatible BASIC interpreter (phase 1) before a complete re-implementation was performed (phase 2). The experiences, strategies used, and lessons learned during the process are reported here.