After walking through OS/161 code with you, I asked the following question
as a "ticket out the door". The goal was to have you discuss what you saw
with peers to come up with a shared understanding of the syscall process:

In 258 (and last week), I asked you to come up with a series of steps 
required to execute a function call. Similarly, consider what we just saw. 
Generate a series of steps required to execute a system call. Start from 
the point where a user invokes stub code and end when the stub returns. Can
you identify the transition points between user and kernel space? What is 
the role of the trapframe?