Processes

Refer closely to notes and examples from classes 8, 9, and 10.
You must create a program that replaces the trivial os2 from
the examples. It will be a very small and low-functioning
operating system.

Stages:

1. Act as a shell: wait for the user to type a command, then
   obey that command, and repeat. Add only trivial commands
   like exit, echo, what-time-is-it, and make sure it works
   before continuing.

2. Add a run command: the user types a file name, assume it
   is the name of a .exe file in your regular unix directory.
   Read it (using TAPE_LOAD and TAPE_READ operations) into
   the user area of the virtual address space, give it a very
   small stack, and run it.
   At this stage, do not expect the operating system to regain
   control when the user program exits, just let the whole thing
   stop.

3. Trap and deal with page faults. The user stack should be 
   allowed to grow (within reason) as needed, but other page
   faults should be treated as fatal errors.

4. Add a trivial syscall, just to make sure you can.
   Then add at least one real syscall: exit(). The exit
   syscall should allow the operating system to regain control
   and begin accepting commands again.

5. Switch to user mode when running the user program.
   The "io" import will give you trouble. What must you do?