The Minix kernel provides a message passing facility.

For user mode processes (user processes and server processes), sending or receiving a message is a system call.

It is the only system call that doesn't itself use message passing.

User processes make Minix system calls by sending a message to the appropriate Minix server process.

Server processes can handle some system calls by themselves because the server process has all the information needed in its address space and can execute its code for the system call in user mode.