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@node Project 2--User Programs, Project 3--Virtual Memory, Project 1--Threads, Top
@chapter Project 2: User Programs

Now that you've worked with Pintos and are familiar with its
infrastructure and thread package, it's time to start working on the
parts of the system that will allow users to run programs on top of
your operating system.  The base code already supports loading and
running a single user program at a time with little interactivity
possible.  You will allow multiple programs to be loaded in at once,
and to interact with the OS via system calls.

You will be working out of the @file{userprog} directory for this
assignment.  However, you will also be interacting with almost every
other part of the code for this assignment. We will describe the
relevant parts below. If you are confident in your HW1 code, you can
build on top of it.  However, if you wish you can start with a fresh
copy of the code and re-implement @code{thread_join()}, which is the
only part of project #1 required for this assignment.

Up to now, all of the code you have written for Pintos has been part
of the operating system kernel.  This means, for example, that all the
test code from the last assignment ran as part of the kernel, with
full access to privileged parts of the system.  Once we start running
user programs on top of the operating system, this is no longer true.
This project deals with consequences of the change.

We allow more than one user program to run at a time.  Because user
programs are written and compiled to work under the illusion that they
have the entire machine, when you load into memory and run more than
one process at a time, you must manage things correctly to maintain
this illusion.

FIXME
Before we delve into the details of the new code that you'll be
working with, you should probably undo the test cases from project 1.
All you need to do is make sure the original
@file{threads/pintostest.c} is in place.  This will stop the tests
from being run.

@menu
* Project 2 Code to Hack::      
* Using the File System::       
* How User Programs Work::      
* Global Requirements::         
* Problem 2-1 Argument Passing::  
* Problem 2-2 System Calls::    
* User Programs FAQ::           
* 80x86 Calling Convention::    
* System Calls::                
@end menu

@node Project 2 Code to Hack
@section Code to Hack

The easiest way to get an overview of the programming you will be
doing is to simply go over each part you'll be working with.  In
@file{userprog}, you'll find a small number of files, but here is
where the bulk of your work will be:

@table @file
@item addrspace.c
@itemx addrspace.h
An address space keeps track of all the data necessary to execute a
user program.  Address space data is stored in @code{struct thread},
but manipulated only by @file{addrspace.c}.  Address spaces need to
keep track of things like paging information for the process (so that
it knows which memory the process is using).  Address spaces also
handle loading the program into memory and starting up the process's
execution.

@item syscall.c
@itemx syscall.h
Whenever a user process wants to access some kernel functionality, it
needs to do so via a system call.  This is a skeleton system call
handler.  Currently, it just prints a message and terminates the user
process.  In part 2 of this project you will add code to do everything
else needed by system calls.

@item exception.c
@itemx exception.h
When a user process performs a privileged or prohibited operation, it
traps into the kernel as an ``exception'' or ``fault.''@footnote{We
will treat these terms as synonymous.  There is no standard
distinction between them, although the Intel processor manuals define
them slightly differently on 80@var{x}86.}  These files handle
exceptions.  Currently all exceptions simply print a message and
terminate the process.  @strong{You should not need to modify this
file for project 2.}

@item gdt.c
@itemx gdt.c
The 80@var{x}86 is a segmented architecture.  The Global Descriptor
Table (GDT) is a table that describes the segments in use.  These
files set up the GDT.  @strong{You should not need to modify these
files for any of the projects.}  However, you can read the code if
you're interested in how the GDT works.

@item tss.c
@itemx tss.c
The Task-State Segment (TSS) is used for 80@var{x}86 architectural
task switching.  Pintos uses the TSS only for switching stacks when a
user process enters an interrupt handler, as does Linux.  @strong{You
should not need to modify these files for any of the projects.}
However, you can read the code if you're interested in how the GDT
works.
@end table

Finally, in @file{lib/kernel}, you might want to use
@file{bitmap.[ch]}.  A bitmap is basically an array of bits, each of
which can be true or false.  Bitmaps are typically used to keep track
of the usage of a large array of (identical) resources: if resource
@var{n} is in use, then bit @var{n} of the bitmap is true.  You might
find it useful for tracking memory pages, for example.

@node Using the File System
@section Using the File System

You will need to use some file system code for this project.  First,
user programs are loaded from the file system.  Second, many of the
system calls you must implement deal with the file system.  However,
the focus of this project is not on the file system code, so we have
provided a simple file system in the @file{filesys} directory.  You
will want to look over the @file{filesys.h} and @file{file.h}
interfaces to understand how to use the file system.  @strong{You
should not modify the file system code for this project}.  Proper use
of the file system routines now will make life much easier for project
4, when you improve the file system implementation.

You need to be able to create and format simulated disks.  The
@command{pintos} program provides this functionality with its
@option{make-disk} command.  From the @filesys{build} directory,
execute @code{pintos make-disk fs.dsk 2}.  This command creates a 2 MB
simulated disk named @file{fs.dsk}.  (It does not actually start
Pintos.)  Then format the disk by passing the @option{-f} option to
Pintos on the kernel's command line: @code{pintos run -f}.

You'll need a way to get files in and out of the simulated file
system.  The @code{pintos} @option{put} and @option{get} commands are
designed for this.  To copy @file{@var{file}} into the Pintos file
system, use the command @file{pintos put @var{file}}.  To copy it to
the Pintos file system under the name @file{@var{newname}}, add the
new name to the end of the command: @file{pintos put @var{file}
@var{newname}}.  The commands for copying files out of a VM are
similar, but substitute @option{get} for @option{get}.

Incidentally, these commands work by passing special options
@option{-ci} and @option{-co} on the kernel's command line and copying
to and from a special simulated disk named @file{scratch.dsk}.  If
you're very curious, you can look at the @command{pintos} program as
well as @file{filesys/fsutil.c} to learn the implementation details,
but it's really not relevant for this project.

You can delete a file from the Pintos file system using the @option{-r
@var{file}} kernel option, e.g.@: @code{pintos run -r @var{file}}.
Also, @option{-ls} lists the files in the file system and @option{-p
@var{file}} prints a file's contents to the display.

@node How User Programs Work
@section How User Programs Work

Pintos can run normal C programs.  In fact, it can run any program you
want, provided it's compiled into the proper file format, and uses
only the system calls you implement.  (For example, @code{malloc()}
makes use of functionality that isn't provided by any of the syscalls
we require you to support.)  The only other limitation is that Pintos
can't run programs using floating point operations, since it doesn't
include the necessary kernel functionality to save and restore the
processor's floating-point unit when switching threads.  You can look
in @file{test} directory for some examples.

Pintos loads ELF executables, where ELF is an executable format used
by Linux, Solaris, and many other Unix and Unix-like systems.
Therefore, you can use any compiler and linker that produce
80@var{x}86 ELF executables to produce programs for Pintos.  We
recommend using the tools we provide in the @file{tests} directory.  By
default, the @file{Makefile} in this directory will compile the test
programs we provide.  You can edit the @file{Makefile} to compile your
own test programs as well.

One thing you should realize immediately is that, until you use the
above operation to copy a test program to the emulated disk, Pintos
will be unable to do very much useful work.  You will also find that
you won't be able to do interesting things until you copy a variety of
programs to the disk.  A useful technique is to create a clean
reference disk and copy that over whenever you trash your
@file{fs.dsk} beyond a useful state, which may happen occasionally
while debugging.

@node Global Requirements
@section Global Requirements

For testing and grading purposes, we have some simple requirements for
your output.  The kernel should print out the program's name and exit
status whenever a process exits.  Aside from this, it should print out
no other messages.  You may understand all those debug messages, but
we won't, and it just clutters our ability to see the stuff we care
about.

Additionally, while it may be useful to hard-code which process will
run at startup while debugging, before you submit your code you must
make sure that it takes the start-up process name and arguments from
the @samp{-ex} argument.  For example, running @code{pintos run -ex
"testprogram 1 2 3 4"} will spawn @samp{testprogram 1 2 3 4} as the
first process.

@node Problem 2-1 Argument Passing
@section Problem 2-1: Argument Passing

Currently, @code{thread_execute()} does not support passing arguments
to new processes.  UNIX and other operating systems do allow passing
command line arguments to a program, which accesses them via the argc,
argv arguments to main.  You must implement this functionality by
extending @code{thread_execute()} so that instead of simply taking a
program file name, it can take a program name with arguments as a
single string.  That is, @code{thread_execute("grep foo *.c")} should
be a legal call.  @xref{80x86 Calling Convention}, for information on
exactly how this works.

@strong{This functionality is extremely important.}  Almost all our
test cases rely on being able to pass arguments, so if you don't get
this right, a lot of things will not appear to work correctly with our
tests.  If the tests fail, so do you.  Fortunately, this part
shouldn't be too hard.

@node Problem 2-2 System Calls
@section Problem 2-2: System Calls

Implement the system call handler in @file{userprog/syscall.c} to
properly deal with all the system calls described below.  Currently,
it ``handles'' system calls by terminating the process.  You will need
to decipher system call arguments and take the appropriate action for
each.

In addition, implement system calls and system call handling.  You are
required to support the following system calls, whose syscall numbers
are defined in @file{lib/syscall-nr.h} and whose C functions called by
user programs are prototyped in @file{lib/user/syscall.h}:

@table @code
@item SYS_halt
@itemx void halt (void)
Stops Pintos and prints out performance statistics.  Note that this
should be seldom used, since then you lose some information about
possible deadlock situations, etc.

@item SYS_exit
@itemx void exit (int @var{status})
Terminates the current user program, returning @var{status} to the
kernel.  A @var{status} of 0 indicates a successful exit.  Other
values may be used to indicate user-defined error conditions.

@item SYS_exec
@itemx pid_t exec (const char *@var{file})
Run the executable in @var{file} and return the new process's program
id (pid).  If there is an error loading this program, returns pid -1,
which otherwise should not be a valid id number.

@item SYS_join
@itemx int join (pid_t @var{pid})
Joins the process @var{pid}, using the join rules from the last
assignment, and returns the process's exit status.  If the process was
terminated by the kernel (i.e.@: killed due to an exception), the exit
status should be -1.  If the process was not a child process, the
return value is undefined (but kernel operation must not be
disrupted).

@item SYS_create
@itemx bool create (const char *@var{file})
Create a new file called @var{file}.  Returns -1 if failed, 0 if OK.

@item SYS_remove
@itemx bool remove (const char *@var{file})
Delete the file called @var{file}.  Returns -1 if failed, 0 if OK.

@item SYS_open
@itemx int open (const char *@var{file})
Open the file called @var{file}.  Returns a nonnegative integer handle
called a ``file descriptor'' (fd), or -1 if the file could not be
opened.  File descriptors numbered 0 and 1 are reserved for the
console.  All open files associated with a process should be closed
when the process exits or is terminated.

@item SYS_filesize
@itemx int filesize (int @var{fd})
Returns the size, in bytes, of the file open as @var{fd}, or -1 if the
file is invalid.

@item SYS_read
@itemx int read (int @var{fd}, void *@var{buffer}, unsigned @var{size})
Read @var{size} bytes from the file open as @var{fd} into
@var{buffer}.  Returns the number of bytes actually read, or -1 if the
file could not be read.

@item SYS_write
@itemx int write (int @var{fd}, const void *@var{buffer}, unsigned @var{size})
Write @var{size} bytes from @var{buffer} to the open file @var{fd}.
Returns the number of bytes actually written, or -1 if the file could
not be written.

@item SYS_close
@itemx void close (int @var{fd})
Close file descriptor @var{fd}.
@end table

The file defines other syscalls.  Ignore them for now.  You will
implement some of them in project 3 and the rest in project 4, so be
sure to design your system with extensibility in mind.

To implement syscalls, you will need to provide a way of copying data
from the user's virtual address space into the kernel and vice versa.
This can be a bit tricky: what if the user provides an invalid
pointer, a pointer into kernel memory, or points to a block that is
partially in one of those regions?  You should handle these cases by
terminating the user process.  You will need this code before you can
even obtain the system call number, because the system call number is
on the user's stack in the user's virtual address space.  We recommend
writing and testing this code before implementing any other system
call functionality.

You must make sure that system calls are properly synchronized so that
any number of user processes can make them at once.  In particular, it
is not safe to call into the filesystem code provided in the
@file{filesys} directory from multiple threads at once.  For now, we
recommend adding a single lock that controls access to the filesystem
code.  You should acquire this lock before calling any functions in
the @file{filesys} directory, and release it afterward.  Because it
calls into @file{filesys} functions, you will have to modify
@file{addrspace_load()} in the same way.  @strong{For now, we
recommend against modifying code in the @file{filesys} directory.}

We have provided you a function for each system call in
@file{lib/user/syscall.c}.  These provide a way for user processes to
invoke each system call from a C program.  Each of them calls an
assembly language routine in @file{lib/user/syscall-stub.S}, which in
turn invokes the system call interrupt and returns.

When you're done with this part, and forevermore, Pintos should be
bulletproof.  Nothing that a user program can do should ever cause the
OS to crash, halt, assert fail, or otherwise stop running.  The sole
exception is a call to the @code{halt} system call.

@xref{System Calls}, for more information on how syscalls work.

@node User Programs FAQ
@section FAQ

@enumerate 1
@item General FAQs

@enumerate 1
@item
@b{Do we need a working project 1 to implement project 2?}

You may find the code for @code{thread_join()} to be useful in
implementing the join syscall, but besides that, you can use
the original code provided for project 1.

@item
@b{Is there a way I can disassemble user programs?}

The @command{i386-elf-objdump} utility can disassemble entire user
programs or object files.  Invoke it as @code{i386-elf-objdump -d
@var{file}}.  You can also use @code{i386-elf-gdb}'s
@command{disassemble} command to disassemble individual functions in
object files compiled with debug information.

@item
@b{Why can't I use many C include files in my Pintos programs?}

The C library we provide is very limited.  It does not include many of
the features that are expected of a real operating system's C library.
The C library must be built specifically for the operating system (and
architecture), since it must make system calls for I/O and memory
allocation.  (Not all functions do, of course, but usually the library
is compiled as a unit.)  If you wish to port libraries to Pintos, feel
free.

@item
@b{How do I compile new user programs? How do I make 'echo' compile?}

You need to modify @file{tests/Makefile}.

@item
@b{Help, Solaris only allows 128 open files at once!}

Solaris limits the number of file descriptors a process may keep open
at any given time.  The default limit is 128 open file descriptors.

To see the current limit for all new processes type @samp{limit} at
the shell prompt and look at the line titled ``descriptors''. To
increase this limit to the maximum allowed type @code{ulimit
descriptors} in a @command{csh} derived shell or @code{unlimit
descriptors} in a @command{sh} derived shell.  This will increase the
number of open file descriptors your Pintos process can use, but it
will still be limited.

Refer to the @command{limit(1)} man page for more information.

@item
@b{I can't seem to figure out how to read from and write to
memory. What should I do?}

Here are some pointers:

FIXME

@item
@b{I'm also confused about reading from and writing to the stack. Can
you help?}

FIXME: relevant?

@itemize @bullet
@item
Only non-@samp{char} values will have issues when writing them to
memory.  If a digit is in a string, it is considered a character.
However, the value of @code{argc} would be a non-char.

@item
You will need to write characters and non-characters into main memory.

@item
When you add items to the stack, you will be decrementing the stack
pointer.  You'll need to decrement the stack pointer before writing to
the location.

@item
Each character is 1 byte.
@end itemize
@end enumerate

@item Argument Passing FAQs

@enumerate 1
@item
@b{What will be the format of command line arguments?}

You should assume that command line arguments are delimited by white
space.

@item
@b{How do I parse all these argument strings?}

We recommend you look at @code{strtok_r()}, prototyped in
@file{lib/string.h} and implemented with thorough comments in
@file{lib/string.c}.  You can find more about it by looking at the man
page (run @code{man strtok_r} at the prompt).

@item
@b{Why is the top of the stack at @t{0xc0000000}?  Isn't that off the
top of user virtual memory?  Shouldn't it be @t{0xbfffffff}?}

When the processor pushes data on the stack, it decrements the stack
pointer first.  Thus, the first (4-byte) value pushed on the stack
will be at address @t{0xbffffffc}.

Also, the stack should always be aligned to a 4-byte boundary, but
@t{0xbfffffff} isn't.
@end enumerate

@item System Calls FAQs

@enumerate 1
@item
@b{What should I do with the parameter passed to @code{exit()}?}

This value, the exit status of the process, must be returned to the
thread's parent when @code{join()} is called.

@item
@b{Can I just cast a pointer to a @code{struct file} object to get a
unique file descriptor?  Can I just cast a @code{struct thread *} to a
@code{pid_t}?  It's so much simpler that way!}

This is a design decision you will have to make for yourself.
However, note that most operating systems do distinguish between file
descriptors (or pids) and the addresses of their kernel data
structures.  You might want to give some thought as to why they do so
before committing yourself.

@item
@b{Can I set a maximum number of open files per process?}

From a design standpoint, it would be better not to set an arbitrary
maximum.  That said, if your design calls for it, you may impose a
limit of 128 open files per process (as the Solaris machines here do).

@item
@b{What happens when two (or more) processes have a file open and one of
them removes it?}

You should copy the standard Unix semantics for files.  That is, when
a file is removed an process which has a file descriptor for that file
may continue to do operations on that descriptor.  This means that
they can read and write from the file.  The file will not have a name,
and no other processes will be able to open it, but it will continue
to exist until all file descriptors referring to the file are closed
or the machine shuts down.

@item
@b{What happens if a system call is passed an invalid argument, such
as Open being called with an invalid filename?}

Pintos should not crash.  You should have your system calls check for
invalid arguments and return error codes.

@item
@b{I've discovered that some of my user programs need more than one 4
kB page of stack space.  What should I do?}

You may modify the stack setup code to allocate more than one page of
stack space for each process.

@item
@b{What do I need to print on thread completion?}

You should print the complete thread name (as specified in the
@code{SYS_exec} call) followed by the exit status code,
e.g.@: @samp{example 1 2 3 4: 0}.
@end enumerate
@end enumerate

@node 80x86 Calling Convention
@section 80@var{x}86 Calling Convention

What follows is a quick and dirty discussion of the 80@var{x}86
calling convention.  Some of the basics should be familiar from CS
107, and if you've already taken CS 143 or EE 182, then you should
have seen even more of it.  I've omitted some of the complexity, since
this isn't a class in how function calls work, so don't expect this to
be exactly correct in full, gory detail.  If you do want all the
details, you can refer to @cite{[SysV-i386]}.

Whenever a function call happens, you need to put the arguments on the
call stack for that function, before the code for that function
executes, so that the callee has access to those values.  The caller
has to be responsible for this (be sure you understand why).
Therefore, when you compile a program, the assembly code emitted will
have in it, before every function call, a bunch of instructions that
prepares for the call in whatever manner is conventional for the
machine you're working on.  This includes saving registers as needed,
putting stuff on the stack, saving the location to return to somewhere
(so that when the callee finishes, it knows where the caller code is),
and some other bookkeeping stuff.  Then you do the jump to the
callee's code, and it goes along, assuming that the stack and
registers are prepared in the appropriate manner.  When the callee is
done, it looks at the return location as saved earlier, and jumps back
to that location.  The caller may then have to do some cleanup:
clearing arguments and the return value off the stack, restoring
registers that were saved before the call, and so on.

If you think about it, some of these things should remind you of
context switching.

As an aside, in general, function calls are not cheap.  You have to do
a bunch of memory writes to prepare the stack, you need to save and
restore registers before and after a function call, you need to write
the stack pointer, you have a couple of jumps which probably wrecks
some of your caches.  This is why inlining code can be much faster.

@menu
* Argument Passing to main::    
@end menu

@node Argument Passing to main
@subsection Argument Passing to @code{main()}

In @code{main()}'s case, there is no caller to prepare the stack
before it runs.  Therefore, the kernel needs to do it.  Fortunately,
since there's no caller, there are no registers to save, no return
address to deal with, etc.  The only difficult detail to take care of,
after loading the code, is putting the arguments to @code{main()} on
the stack.

(The above is a small lie: most compilers will emit code where main
isn't strictly speaking the first function.  This isn't an important
detail.  If you want to look into it more, try disassembling a program
and looking around a bit.  However, you can just act as if
@code{main()} is the very first function called.)

Pintos is written for the 80@var{x}86 architecture.  Therefore, we
need to adhere to the 80@var{x}86 calling convention, which is
detailed in the FAQ.  Basically, you put all the arguments on the
stack and move the stack pointer appropriately.  The program will
assume that this has been done when it begins running.

So, what are the arguments to @code{main()}? Just two: an @samp{int}
(@code{argc}) and a @samp{char **} (@code{argv}).  @code{argv} is an
array of strings, and @code{argc} is the number of strings in that
array.  However, the hard part isn't these two things.  The hard part is
getting all the individual strings in the right place.  As we go
through the procedure, let us consider the following example command:
@samp{/bin/ls -l *.h *.c}.

The first thing to do is to break the command line into individual
strings: @samp{/bin/ls}, @samp{-l}, @samp{*.h}, and @samp{*.c}.  These
constitute the arguments of the command, including the program name
itself (which belongs in @code{argv[0]}).

These individual, null-terminated strings should be placed on the user
stack.  They may be placed in any order, as you'll see shortly,
without affecting how main works, but for simplicity let's assume they
are in reverse order (keeping in mind that the stack grows downward on
an 80@var{x}86 machine).  As we copy the strings onto the stack, we
record their (virtual) stack addresses.  These addresses will become
important when we write the argument vector (two paragraphs down).

After we push all of the strings onto the stack, we adjust the stack
pointer so that it is word-aligned: that is, we move it down to the
next 4-byte boundary.  This is required because we will next be
placing several words of data on the stack, and they must be aligned
in order to be read correctly.  In our example, as you'll see below,
the strings start at address @t{0xffed}.  One word below that would be
at @t{0xffe9}, so we could in theory put the next word on the stack
there.  However, since the stack pointer should always be
word-aligned, we instead leave the stack pointer at @t{0xffe8}.

Once we align the stack pointer, we then push the elements of the
argument vector (that is, the addresses of the strings @samp{/bin/ls},
@samp{-l}, @samp{*.h}, and @samp{*.c}) onto the stack.  This must be
done in reverse order, such that @code{argv[0]} is at the lowest
virtual address (again, because the stack is growing downward).  This
is because we are now writing the actual array of strings; if we write
them in the wrong order, then the strings will be in the wrong order
in the array.  This is also why, strictly speaking, it doesn't matter
what order the strings themselves are placed on the stack: as long as
the pointers are in the right order, the strings themselves can really
be anywhere.  After we finish, we note the stack address of the first
element of the argument vector, which is @code{argv} itself.

Finally, we push @code{argv} (that is, the address of the first
element of the @code{argv} array) onto the stack, along with the
length of the argument vector (@code{argc}, 4 in this example).  This
must also be done in this order, since @code{argc} is the first
argument to main and therefore is on first (smaller address) on the
stack.  We leave the stack pointer to point to the location where
@code{argc} is, because it is at the top of the stack, the location
directly below @code{argc}.

All of which may sound very confusing, so here's a picture which will
hopefully clarify what's going on. This represents the state of the
stack and the relevant registers right before the beginning of the
user program (assuming for this example a 16-bit virtual address space
with addresses from @t{0x0000} to @t{0xffff}):

@html
<CENTER>
@end html
@multitable {@t{0xffff}} {word-align} {@t{/bin/ls\0}}
@item Address @tab Name @tab Data
@item @t{0xfffc} @tab @code{*argv[3]} @tab @samp{*.c\0}
@item @t{0xfff8} @tab @code{*argv[2]} @tab @samp{*.h\0}
@item @t{0xfff5} @tab @code{*argv[1]} @tab @samp{-l\0}
@item @t{0xffed} @tab @code{*argv[0]} @tab @samp{/bin/ls\0}
@item @t{0xffec} @tab word-align @tab @samp{\0}
@item @t{0xffe8} @tab @code{argv[3]} @tab @t{0xfffc}
@item @t{0xffe4} @tab @code{argv[2]} @tab @t{0xfff8}
@item @t{0xffe0} @tab @code{argv[1]} @tab @t{0xfff5}
@item @t{0xffdc} @tab @code{argv[0]} @tab @t{0xffed}
@item @t{0xffd8} @tab @code{argv} @tab @t{0xffdc}
@item @t{0xffd4} @tab @code{argc} @tab 4
@end multitable
@html
</CENTER>
@end html

In this example, the stack pointer would be initialized to @t{0xffd4}.

Your code should start the stack at the very top of the user virtual
address space, in the page just below virtual address @code{PHYS_BASE}
(defined in @file{threads/mmu.h}).

@node System Calls
@section System Calls

We have already been dealing with one way that the operating system
can regain control from a user program: interrupts from timers and I/O
devices.  These are ``external'' interrupts, because they are caused
by entities outside the CPU.

The operating system is also called to deal with software exceptions,
which are events generated in response to the code.  These can be
errors such as a page fault or division by zero.  However, exceptions
are also the means by which a user program can request services
(``system calls'') from the operating system.

Some exceptions are ``restartable'': the condition that caused the
exception can be fixed and the instruction retried. For example, page
faults call the operating system, but the user code should re-start on
the load or store that caused the exception (not the next one) so that
the memory access actually occurs.  On the 80@var{x}86, restartable
exceptions are called ``faults,'' whereas most non-restartable
exceptions are classed as ``traps.''  Other architectures may define
these terms differently.

In the 80@var{x}86 architecture, the @samp{int} instruction is the
most commonly used means for invoking system calls.  This instruction
is handled in the same way that other software exceptions.  In Pintos,
user program invoke @samp{int $0x30} to make a system call.  The
system call number and any additional arguments are expected to be
pushed on the stack in the normal fashion before invoking the
interrupt.

The normal calling convention pushes function arguments on the stack
from right to left and the stack grows downward.  Thus, when the
system call handler @code{syscall_handler()} gets control, the system
call number is in the 32-bit word at the caller's stack pointer, the
first argument is in the 32-bit word at the next higher address, and
so on.  The caller's stack pointer is accessible to
@code{syscall_handler()} as the @samp{esp} member of the @code{struct
intr_frame} passed to it.

Here's an example stack frame for calling a system call numbered 10
with three arguments passed as 1, 2, and 3.  The stack addresses are
arbitrary:

@html
<CENTER>
@end html
@multitable {Address} {Value}
@item Address @tab Value
@item @t{0xfe7c} @tab 3
@item @t{0xfe78} @tab 2
@item @t{0xfe74} @tab 1
@item @t{0xfe70} @tab 10
@end multitable
@html
</CENTER>
@end html

In this example, the caller's stack pointer would be at @t{0xfe70}.

The 80@var{x}86 convention for function return values is to place them
in the @samp{EAX} register.  System calls that return a value can do
so by modifying the @samp{eax} member of @code{struct intr_frame}.