The IPC system initially had performance problems, so a few strategies were developed to minimize the impact. Like its predecessor, Accent, Mach used a single shared-memory mechanism for physically passing the message from one program to another. Physically copying the message would be too slow, so Mach relies on the machine's memory management unit (MMU) to quickly map the data from one program to another. Only if the data is written to would it have to be physically copied, a process called "copy-on-write".