Page "Mach (kernel)" Paragraph 18

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 known as copy-on-write.