regardless of what user process or kernel thread is running. In
Pintos, kernel virtual memory is mapped one-to-one to physical
memory, starting at @code{PHYS_BASE}. That is, virtual address
-@code{PHYS_ADDR} accesses physical
-address 0, virtual address @code{PHYS_ADDR} + @t{0x1234} access
+@code{PHYS_BASE} accesses physical
+address 0, virtual address @code{PHYS_BASE} + @t{0x1234} access
physical address @t{0x1234}, and so on up to the size of the machine's
physical memory.
Each of these functions assumes that the user address has already been
verified to be below @code{PHYS_BASE}. They also assume that you've
-modified @func{page_fault} so that a page fault in the kernel causes
-@code{eax} to be set to 0 and its former value copied into @code{eip}.
+modified @func{page_fault} so that a page fault in the kernel merely sets
+@code{eax} to 0 and copies its former value into @code{eip}.
@node Project 2 Suggested Order of Implementation
@section Suggested Order of Implementation
@itemize
@item
-Argument passing (@pxref{Argument Passing}). Every user programs will
+Argument passing (@pxref{Argument Passing}). Every user program will
page fault immediately until argument passing is implemented.
For now, you may simply wish to change
examine its arguments, although its name will be printed as
@code{(null)}.
+Until you implement argument passing, you should only run programs
+without passing command-line arguments. Attempting to pass arguments to
+a program will include those arguments in the name of the program, which
+will probably fail.
+
@item
User memory access (@pxref{Accessing User Memory}). All system calls
need to read user memory. Few system calls need to write to user
@deftypefn {System Call} int open (const char *@var{file})
Opens the file called @var{file}. Returns a nonnegative integer handle
called a ``file descriptor'' (fd), or -1 if the file could not be
-opened. All open files associated with a process should be closed
-when the process exits or is terminated.
+opened.
File descriptors numbered 0 and 1 are reserved for the console: fd 0
is standard input (@code{stdin}), fd 1 is standard output
-(@code{stdout}). These special file descriptors are valid as system
-call arguments only as explicitly described below.
+(@code{stdout}). The @code{open} system call will never return either
+of these file descriptors, which are valid as system call arguments only
+as explicitly described below.
+
+Each process has an independent set of file descriptors. File
+descriptors are not inherited by child processes.
Consider implementing this function in terms of @func{filesys_open}.
@end deftypefn
@end deftypefn
@deftypefn {System Call} void close (int @var{fd})
-Closes file descriptor @var{fd}.
+Closes file descriptor @var{fd}.
+Exiting or terminating a process implicitly closes all its open file
+descriptors, as if by calling this function for each one.
Consider implementing this function in terms of @func{file_close}.
@end deftypefn