In real-time systems, the use of databases is increasing. If the application is safety-critical, one must guarantee in advance, by suitable verification methods, that all deadlines hold in all possible cases of use. Predictability of the database's concurrency control protocol is one of the most important prerequisites for this. However, most real-time concurrency control protocols are influenced by traditional database requirements. For hard real-time systems, they are usually unsuitable, because they are not predictable and they noticeably interfere with task scheduling. We present a new semantic concurrency control protocol called PRED-DF (PREDeclaration and Data Flow analysis) for main-memory real-time database systems. PRED-DF uses pre-declaration, is locking-based and generates serializable schedules. It uses additional knowledge gained from advance analysis of the application's data flow to minimize blocking times. PRED-DF's behavior is predictable and so the verification of real-time requirements is possible.