We have developed a COBOL symbolic execution system for generating unit test data from source code. One of the main feature of the system is, based on a bit-vector logic, to deal with symbolic values for byte data in memory. This makes it possible to generate test data for programs that manipulate bit and byte data directly. The resulting symbolic execution system is useful for unit testing, specially in legacy migration projects, where many problems are often caused by the difference in byte data representation and bit/byte manipulation between existing legacy system and new systems.