Minix uses mechanism I, but only to implement message passing.
int 33 ; 33 identifies the kernel sys_call function
Minix operating system code is structured into separate processes:
- kernel process
- clock process
- process manager process
- file system processes
- driver manager process
To make an operating system call, an ordinary Minix user process must send a message to the appropriate Minix operating system process.
The user level library routine to send the message contains a trap operation in order to implement actually sending the message passing. The trap switches to code in the Minix kernel process which handles transferring the message to its the intended operating system process.