The major change between these experimental kernels and Mach was the decision to make a version of the existing 4.2BSD kernel re-implemented on the Accent message-passing concepts. Such a kernel would be binary compatible with existing BSD software, making the system immediately useful for everyday use while still being a useful experimental platform. Additionally, the new kernel would be designed from the start to support multiple processor architectures, even allowing heterogeneous clusters to be constructed. In order to bring the system up as quickly as possible, the system would be implemented by starting with the existing BSD code, and re-implementing it bit by bit as inter-process communication-based (IPC-based) programs. Thus Mach would begin as a monolithic system similar to existing UNIX systems, and evolve more toward the microkernel concept over time.