If you're not already familiar with UNIX fork system call, here is it's function description and its entry on Wikipedia.

Basically, in sys_fork, we need to do the follow things:

1. Copy parent's trap frame, and pass it to child thread
2. Copy parent's address space
3. Create child thread (using thread_fork)
4. Copy parent's file table into child
5. Parent returns with child's pid immediately
6. Child returns with 0

So, let's get started.

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