This section defines the system call interface for your Nachos kernel.
Note on returning errors from system calls : One of the broken things about Nachos is that it does not provide a clean way to return system call errors to a user process. For example, Unix kernels return system call error codes in a designated register, and the system call stubs (e.g., in the standard C library or in start.s ) move them into a program variable, e.g., the global variable errno for C programs. We are not bothering with this in Nachos. What is important is that you detect the error and reject the request with no bad side effects and without crashing the kernel. I recommend that you report errors by returning a 0 or -1 value where possible, instead of returning a value that could be interpreted as a valid result. If there is no clean way to notify the user process of a system call error it is acceptable to simply return from the call and just let the user process struggle forward.