Short Homework 3
			  due Oct. 14

In this homework, you will gain a broad knowledge of xv6, based
on the original UNIX, Sixth Edition (also called Version 6 UNIX).
This was the first versoin of UNIX to be widely released outside
Bell Labs.  It was released in May, 1975, for the DEC PDP-11 minicommputer.
Xv6 is made available by MIT:
    http://pdos.csail.mit.edu/6.828/2014/xv6.html
The contents there, xv6-rev6.pdf, nicely organizes the files according
to purpose.

Many of these pointers are reproduced on our local, hyper-referenced copy:
    http://www.ccs.neu.edu/course/cs5600f15/unix-xv6/
Be sure to also read the
    "HELP on using the doxygen interface to browse the code"
on that page, for tips to efficiently use the search, find the source code,
see who calls a function, etc.

One other pointer that will help you is that it often pays to
read the (relatively short) .h file before reading the corresponding .c file.
The .h file provides the data structures and the names of functions
that manipulate them.

Your job is to read the source code, and answer the following questions.
For those questoins of the form "Where is", you must indicate:
 (a) the filename;
 (b) the line number within that file; and
 (c) the data structure or function name

PROCESSES:

1.  Where is the data structure for the process table?

2.  When there is a context switch from one process to another, where are
    the values of the registers of the old process saved?

3.  What are the possible states of a process?  Also, give a brief phrase
    describing the purpose of each state.

4.  What is the function that does a context switch between two
    processes?

5.  Explain how the context switch function works.

6.  What function calls the context switch function, and explain in detail
    what the calling function does.  (The doxygen hyper-linking
    is not perfect here.  You may have to use 'grep' on
       /course/cs5600f15/unix-xv6/* )

PROCESS STARTUP:

1.  Suppose a new program is going to start.  This requires a call to
    the system call, exec().  On what lines does the operating system
    create the first call frame, to be used by the user process's main()?

2.  The first call frame must have local variables argc and argv.
    Where is the value of argv found in the call frame that is created?
    [ NOTE:  The original version of this question was:
        "Where is the value of argv found in the exec() call?"
        That was a mistake.  Plesse answer the revised question. ]

3.  On what lines does the function create the process table entry
    for the new process?

PROCESS STARTUP (EXTENDED QUESTION):

4.  The exec() function seems to assume that there is already a new
    entry available in the process table.  How was this entry for
    a new process chosen (allocated)?

SYSTEM CALLS:

In class, we discussed how a system call (e.g., open()) is really
a function in the C runtime library, libc.so, and that function
then calls 'syscall()' with the integer for the 'open' system call.
In these questions, we will follow the chain of control from
a user program that calls 'open()' to the code in 'libc' to the
syscall in the kernel, and finally to the function in the kernel
that actually does the work of the 'open()' system call.

1.  The file grep.c makes a call to 'open()'.  The definition of 'open()'
    is inside 'usys.S'.  It makes use of the macro 'SYSCALL'.
    Note that a macro, '$SYS_ ## name', will expand to the concatenation
    of 'SYS_' and the value of the macro parameter, "name".
    The assembly instruction 'int' is the interrupt instruction in
    x86 assembly.  The 'int' assembly instruction takes as an argument
    an integer, 'T_SYSCALL'.
        Give the full details of how a call to 'open()' in grep.c will
    call the function 'sys_open()' in sysfile.c, inside the operating
    system kernel.

FILES AND FILE DESCRIPTORS:

In class, we've talked about files and file descriptors.  We have
not yet discussed i-nodes.  For these questions, you can think of
an i-node as a location on disk that has the "table of contents"
for all information about a file.

In these questions, we will follow the chain of control from
open() to a file descriptor, to a "file handle" (including the
offset into the file", to the i-node.

1.  The function 'sys_open()' returns a file descriptor 'fd'.
    To do this, it opens a new file (new i-node) with 'filealloc()',
    and it allocates a new file descriptor with 'fdalloc()'.
    Where is the file descriptor allocated?  Also, you will see that
    the file descriptor is one entry in an array.  What is the algorithm
    used to choose which entry in the array to use for the new file descriptor?
    [ Comment:  The name 'NOFILE' means "file number".  "No." is sometimes
      used as an abbreviation for the word "number". ]

2.  As you saw above, the file descriptor turned out to be an index
    in an array.  What is the name of the array for which the file
    descriptor is an index?  Also, what is the type of one entry in
    that array.

3.  The type that you saw in the above question is what I was calling
    a "file handle" (with an offset into the file, etc.).
    What is the name of the field that holds the offset into the file?
    We saw it in the function 'sys_open()'.

4.  Remember when we mentioned a call to 'filealloc()' above?
    Since the return value of 'filealloc()' is only a file handle,
    we need to initialize it.  Presumably, we will initialize it with
    a file offset of 0.  What is the line number in 'sys_open()' where
    we initialize the file offset to 0?

5.  The file handle type was initialized to 'FD_INODE'.  What are the
    other types that it could have been initialized to?

6.  Suppose a file handle had been initialized to FD_PIPE.  Find the
    'struct' that hold sthe information about a pipe.  For each field
    in that struct, Explain briefly (just a phrase) the purpose of that
    field.

7.  By examining the function 'sys_dup()', you can discover how a
    system call to 'dup()' will manipulate both a file descriptor
    and a "file handle".  Describe what it does in each of the
    two cases.