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
+copy of the code and re-implement @func{thread_join}, which is the
only part of project #1 required for this assignment. Your submission
should define @code{THREAD_JOIN_IMPLEMENTED} in @file{constants.h}
(@pxref{Conditional Compilation}).
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/test.c} is
-in place. This will stop the tests from being run.
@menu
* Project 2 Code::
A simple manager for 80@var{x} page directories and page tables.
Although you probably won't want to modify this code for this project,
you may want to call some of its functions. In particular,
-@code{pagedir_get_page()} may be helpful for accessing user memory.
+@func{pagedir_get_page} may be helpful for accessing user memory.
@item syscall.c
@itemx syscall.h
them slightly differently on 80@var{x}86.} These files handle
exceptions. Currently all exceptions simply print a message and
terminate the process. Some, but not all, solutions to project 2
-require modifying @code{page_fault()} in this file.
+require modifying @func{page_fault} in this file.
@item gdt.c
@itemx gdt.c
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
+However, you can read the code if you're interested in how the TSS
works.
@end table
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 @file{filesys/build} directory,
+@option{make-disk} command. From the @file{userprog/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 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()}
+only the system calls you implement. (For example, @func{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
free to use the entire space of user virtual memory however it
chooses. When the kernel switches from one process to another, it
also switches user virtual address spaces by switching the processor's
-page directory base register (see @code{pagedir_activate() in
+page directory base register (see @func{pagedir_activate in
@file{userprog/pagedir.c}}.
Kernel virtual memory is global. It is always mapped the same way,
User programs can only access user virtual memory. An attempt to
access kernel virtual memory will cause a page fault, handled by
-@code{page_fault()} in @file{userprog/exception.c}, and the process
+@func{page_fault} in @file{userprog/exception.c}, and the process
will be terminated. Kernel threads can access both kernel virtual
memory and, if a user process is running, the user virtual memory of
the running process. However, even in the kernel, an attempt to
@node Problem 2-1 Argument Passing
@section Problem 2-1: Argument Passing
-Currently, @code{process_execute()} does not support passing arguments
+Currently, @func{process_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{process_execute()} so that instead of simply taking a
+extending @func{process_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{process_execute("grep foo *.c")} should
be a legal call. @xref{80x86 Calling Convention}, for information on
@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.
+Stops Pintos by calling @func{power_off} (declared in
+@file{threads/init.h}). 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})
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.
+@itemx bool create (const char *@var{file}, unsigned @var{initial_size})
+Create a new file called @var{file} initially @var{initial_size} bytes
+in size. Returns -1 if failed, 0 if OK.
@item SYS_remove
@itemx bool remove (const char *@var{file})
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. Don't forget
-that @file{process_execute()} also accesses files. @strong{For now, we
+that @func{process_execute} also accesses files. @strong{For now, we
recommend against modifying code in the @file{filesys} directory.}
We have provided you a function for each system call in
@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
+You may find the code for @func{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{All my user programs die with page faults.}
+
+This will generally happen if you haven't implemented problem 2-1
+yet. The reason is that the basic C library for user programs tries
+to read @var{argc} and @var{argv} off the stack. Because the stack
+isn't properly set up yet, this causes a page fault.
+
@item
@b{Is there a way I can disassemble user programs?}
free.
@item
-@b{How do I compile new user programs? How do I make 'echo' compile?}
+@b{How do I compile new user programs?}
You need to modify @file{tests/Makefile}.
@b{What's the difference between @code{tid_t} and @code{pid_t}?}
A @code{tid_t} identifies a kernel thread, which may have a user
-process running in it (if created with @code{process_execute()}) or not
-(if created with @code{thread_create()}). It is a data type used only
+process running in it (if created with @func{process_execute}) or not
+(if created with @func{thread_create}). It is a data type used only
in the kernel.
A @code{pid_t} identifies a user process. It is used by user
The second method is to ``assume and react'': directly dereference
user pointers, after checking that they point below @code{PHYS_BASE}.
Invalid user pointers will then cause a ``page fault'' that you can
-handle by modifying the code for @code{page_fault()} in
+handle by modifying the code for @func{page_fault} in
@file{userprog/exception.cc}. This technique is normally faster
because it takes advantage of the processor's MMU, so it tends to be
used in real kernels (including Linux).
Therefore, for those who want to try the latter technique, we'll
provide a little bit of helpful code:
-@example
+@verbatim
/* Tries to copy a byte from user address USRC to kernel address DST.
Returns true if successful, false if USRC is invalid. */
-static inline bool get_user (uint8_t *dst, const uint8_t *usrc) @{
+static inline bool get_user (uint8_t *dst, const uint8_t *usrc) {
int eax;
asm ("movl $1f, %%eax; movb %2, %%al; movb %%al, %0; 1:"
: "=m" (*dst), "=&a" (eax) : "m" (*usrc));
return eax != 0;
-@}
+}
/* Tries write BYTE to user address UDST.
Returns true if successful, false if UDST is invalid. */
-static inline bool put_user (uint8_t *udst, uint8_t byte) @{
+static inline bool put_user (uint8_t *udst, uint8_t byte) {
int eax;
asm ("movl $1f, %%eax; movb %b2, %0; 1:"
: "=m" (*udst), "=&a" (eax) : "r" (byte));
return eax != 0;
-@}
-@end example
+}
+@end verbatim
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 @code{page_fault()} so that a page fault in the kernel causes
+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}.
@item
@end itemize
@item
-@b{Why doesn't keyboard input work with @option{-nv}?}
+@b{Why doesn't keyboard input work with @option{-v}?}
-Serial input isn't implemented. Don't use @option{-nv} if you want to
+Serial input isn't implemented. Don't use @option{-v} if you want to
use the shell or otherwise type at the keyboard.
@end enumerate
-@item Argument Passing FAQs
+@menu
+* Problem 2-1 Argument Passing FAQ::
+* Problem 2-2 System Calls FAQ::
+@end menu
+
+@node Problem 2-1 Argument Passing FAQ
+@subsection Problem 2-1: Argument Passing FAQ
@enumerate 1
@item
@b{How do I parse all these argument strings?}
You're welcome to use any technique you please, as long as it works.
-If you're lost, look at @code{strtok_r()}, prototyped in
+If you're lost, look at @func{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).
simply via recompilation.
@end enumerate
-@item System Calls FAQs
+@node Problem 2-2 System Calls FAQ
+@subsection Problem 2-2: System Calls FAQ
@enumerate 1
@item
-@b{What should I do with the parameter passed to @code{exit()}?}
+@b{What should I do with the parameter passed to @func{exit}?}
This value, the exit status of the process, must be returned to the
-thread's parent when @code{join()} is called.
+thread's parent when @func{join} is called.
@item
-@b{Can I just cast a pointer to a @code{struct file} object to get a
+@b{Can I just cast a pointer to a @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!}
limit of 128 open files per process (as the Solaris machines here do).
@item
+@anchor{Removing an Open File}
@b{What happens when two (or more) processes have a file open and one of
them removes it?}
@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
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]}.
+details, you can refer to @bibref{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
@node Argument Passing to main
@subsection Argument Passing to @code{main()}
-In @code{main()}'s case, there is no caller to prepare the stack
+In @func{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
+after loading the code, is putting the arguments to @func{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.)
+@func{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. Basically, you
function's. The program will assume that the stack has been laid out
this way when it begins running.
-So, what are the arguments to @code{main()}? Just two: an @samp{int}
+So, what are the arguments to @func{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
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
-@code{main()} and therefore is on first (smaller address) on the
+@func{main} and therefore is on first (smaller address) on the
stack. Finally, we push a fake ``return address'' and leave the stack
pointer to point to its location.
@item @t{0xbfffffe0} @tab @code{argv[2]} @tab @t{0xbffffff8}
@item @t{0xbfffffdc} @tab @code{argv[1]} @tab @t{0xbffffff5}
@item @t{0xbfffffd8} @tab @code{argv[0]} @tab @t{0xbfffffed}
-@item @t{0xbfffffd4} @tab @code{argv} @tab @t{0xbffffffd8}
+@item @t{0xbfffffd4} @tab @code{argv} @tab @t{0xbfffffd8}
@item @t{0xbfffffd0} @tab @code{argc} @tab 4
@item @t{0xbfffffcc} @tab ``return address'' @tab 0
@end multitable
the user virtual address space, in the page just below virtual address
@code{PHYS_BASE} (defined in @file{threads/mmu.h}).
-You may find the non-standard @code{hex_dump()} function, declared in
+You may find the non-standard @func{hex_dump} function, declared in
@file{<stdio.h>}, useful for debugging your argument passing code.
Here's what it would show in the above example, given that
@code{PHYS_BASE} is @t{0xc0000000}:
-@example
+@verbatim
bfffffc0 00 00 00 00 | ....|
bfffffd0 04 00 00 00 d8 ff ff bf-ed ff ff bf f5 ff ff bf |................|
bfffffe0 f8 ff ff bf fc ff ff bf-00 00 00 00 00 2f 62 69 |............./bi|
bffffff0 6e 2f 6c 73 00 2d 6c 00-2a 2e 68 00 2a 2e 63 00 |n/ls.-l.*.h.*.c.|
-@end example
+@end verbatim
@node System Calls
@section System Calls
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
+system call handler @func{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
+@func{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
@html
<CENTER>
@end html
-@multitable {Address} {Value}
+@multitable {@t{0xbffffe7c}} {Value}
@item Address @tab Value
@item @t{0xbffffe7c} @tab 3
@item @t{0xbffffe78} @tab 2
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}.
+so by modifying the @samp{eax} member of @struct{intr_frame}.
projects / pintos-anon / blobdiff