1 @node Project 2--User Programs, Project 3--Virtual Memory, Project 1--Threads, Top
2 @chapter Project 2: User Programs
4 Now that you've worked with Pintos and are familiar with its
5 infrastructure and thread package, it's time to start working on the
6 parts of the system that will allow users to run programs on top of
7 your operating system. The base code already supports loading and
8 running a single user program at a time with little interactivity
9 possible. You will allow multiple programs to be loaded in at once,
10 and to interact with the OS via system calls.
12 You will be working out of the @file{userprog} directory for this
13 assignment. However, you will also be interacting with almost every
14 other part of the code for this assignment. We will describe the
15 relevant parts below. If you are confident in your HW1 code, you can
16 build on top of it. However, if you wish you can start with a fresh
17 copy of the code and re-implement @func{thread_join}, which is the
18 only part of project #1 required for this assignment. Your submission
19 should define @code{THREAD_JOIN_IMPLEMENTED} in @file{constants.h}
20 (@pxref{Conditional Compilation}).
22 Up to now, all of the code you have written for Pintos has been part
23 of the operating system kernel. This means, for example, that all the
24 test code from the last assignment ran as part of the kernel, with
25 full access to privileged parts of the system. Once we start running
26 user programs on top of the operating system, this is no longer true.
27 This project deals with consequences of the change.
29 We allow more than one user program to run at a time. Because user
30 programs are written and compiled to work under the illusion that they
31 have the entire machine, when you load into memory and run more than
32 one process at a time, you must manage things correctly to maintain
35 Before we delve into the details of the new code that you'll be
36 working with, you should probably undo the test cases from project 1.
40 * Using the File System::
41 * How User Programs Work::
42 * Virtual Memory Layout::
43 * Global Requirements::
44 * Problem 2-1 Argument Passing::
45 * Problem 2-2 System Calls::
47 * 80x86 Calling Convention::
54 The easiest way to get an overview of the programming you will be
55 doing is to simply go over each part you'll be working with. In
56 @file{userprog}, you'll find a small number of files, but here is
57 where the bulk of your work will be:
62 Loads ELF binaries and starts processes.
66 A simple manager for 80@var{x} page directories and page tables.
67 Although you probably won't want to modify this code for this project,
68 you may want to call some of its functions. In particular,
69 @func{pagedir_get_page} may be helpful for accessing user memory.
73 Whenever a user process wants to access some kernel functionality, it
74 needs to do so via a system call. This is a skeleton system call
75 handler. Currently, it just prints a message and terminates the user
76 process. In part 2 of this project you will add code to do everything
77 else needed by system calls.
81 When a user process performs a privileged or prohibited operation, it
82 traps into the kernel as an ``exception'' or ``fault.''@footnote{We
83 will treat these terms as synonymous. There is no standard
84 distinction between them, although the Intel processor manuals define
85 them slightly differently on 80@var{x}86.} These files handle
86 exceptions. Currently all exceptions simply print a message and
87 terminate the process. Some, but not all, solutions to project 2
88 require modifying @func{page_fault} in this file.
92 The 80@var{x}86 is a segmented architecture. The Global Descriptor
93 Table (GDT) is a table that describes the segments in use. These
94 files set up the GDT. @strong{You should not need to modify these
95 files for any of the projects.} However, you can read the code if
96 you're interested in how the GDT works.
100 The Task-State Segment (TSS) is used for 80@var{x}86 architectural
101 task switching. Pintos uses the TSS only for switching stacks when a
102 user process enters an interrupt handler, as does Linux. @strong{You
103 should not need to modify these files for any of the projects.}
104 However, you can read the code if you're interested in how the TSS
108 Finally, in @file{lib/kernel}, you might want to use
109 @file{bitmap.[ch]}. A bitmap is basically an array of bits, each of
110 which can be true or false. Bitmaps are typically used to keep track
111 of the usage of a large array of (identical) resources: if resource
112 @var{n} is in use, then bit @var{n} of the bitmap is true. You might
113 find it useful for tracking memory pages, for example.
115 @node Using the File System
116 @section Using the File System
118 You will need to use some file system code for this project. First,
119 user programs are loaded from the file system. Second, many of the
120 system calls you must implement deal with the file system. However,
121 the focus of this project is not on the file system code, so we have
122 provided a simple file system in the @file{filesys} directory. You
123 will want to look over the @file{filesys.h} and @file{file.h}
124 interfaces to understand how to use the file system, and especially
125 its many limitations. @strong{You should not modify the file system
126 code for this project}. Proper use of the file system routines now
127 will make life much easier for project 4, when you improve the file
128 system implementation.
130 You need to be able to create and format simulated disks. The
131 @command{pintos} program provides this functionality with its
132 @option{make-disk} command. From the @file{userprog/build} directory,
133 execute @code{pintos make-disk fs.dsk 2}. This command creates a 2 MB
134 simulated disk named @file{fs.dsk}. (It does not actually start
135 Pintos.) Then format the disk by passing the @option{-f} option to
136 Pintos on the kernel's command line: @code{pintos run -f}.
138 You'll need a way to get files in and out of the simulated file
139 system. The @code{pintos} @option{put} and @option{get} commands are
140 designed for this. To copy @file{@var{file}} into the Pintos file
141 system, use the command @file{pintos put @var{file}}. To copy it to
142 the Pintos file system under the name @file{@var{newname}}, add the
143 new name to the end of the command: @file{pintos put @var{file}
144 @var{newname}}. The commands for copying files out of a VM are
145 similar, but substitute @option{get} for @option{get}.
147 Incidentally, these commands work by passing special options
148 @option{-ci} and @option{-co} on the kernel's command line and copying
149 to and from a special simulated disk named @file{scratch.dsk}. If
150 you're very curious, you can look at the @command{pintos} program as
151 well as @file{filesys/fsutil.c} to learn the implementation details,
152 but it's really not relevant for this project.
154 You can delete a file from the Pintos file system using the @option{-r
155 @var{file}} kernel option, e.g.@: @code{pintos run -r @var{file}}.
156 Also, @option{-ls} lists the files in the file system and @option{-p
157 @var{file}} prints a file's contents to the display.
159 @node How User Programs Work
160 @section How User Programs Work
162 Pintos can run normal C programs. In fact, it can run any program you
163 want, provided it's compiled into the proper file format, and uses
164 only the system calls you implement. (For example, @func{malloc}
165 makes use of functionality that isn't provided by any of the syscalls
166 we require you to support.) The only other limitation is that Pintos
167 can't run programs using floating point operations, since it doesn't
168 include the necessary kernel functionality to save and restore the
169 processor's floating-point unit when switching threads. You can look
170 in @file{tests/userprog} directory for some examples.
172 Pintos loads ELF executables, where ELF is an executable format used
173 by Linux, Solaris, and many other Unix and Unix-like systems.
174 Therefore, you can use any compiler and linker that produce
175 80@var{x}86 ELF executables to produce programs for Pintos. We
176 recommend using the tools we provide in the @file{tests/userprog}
177 directory. By default, the @file{Makefile} in this directory will
178 compile the test programs we provide. You can edit the
179 @file{Makefile} to compile your own test programs as well.
181 One thing you should realize immediately is that, until you use the
182 above operation to copy a test program to the emulated disk, Pintos
183 will be unable to do very much useful work. You will also find that
184 you won't be able to do interesting things until you copy a variety of
185 programs to the disk. A useful technique is to create a clean
186 reference disk and copy that over whenever you trash your
187 @file{fs.dsk} beyond a useful state, which may happen occasionally
190 @node Virtual Memory Layout
191 @section Virtual Memory Layout
193 Virtual memory in Pintos is divided into two regions: user virtual
194 memory and kernel virtual memory. User virtual memory ranges from
195 virtual address 0 up to @code{PHYS_BASE}, which is defined in
196 @file{threads/mmu.h} and defaults to @t{0xc0000000} (3 GB). Kernel
197 virtual memory occupies the rest of the virtual address space, from
198 @code{PHYS_BASE} up to 4 GB.
200 User virtual memory is per-process. Conceptually, each process is
201 free to use the entire space of user virtual memory however it
202 chooses. When the kernel switches from one process to another, it
203 also switches user virtual address spaces by switching the processor's
204 page directory base register (see @func{pagedir_activate in
205 @file{userprog/pagedir.c}}.
207 Kernel virtual memory is global. It is always mapped the same way,
208 regardless of what user process or kernel thread is running. In
209 Pintos, kernel virtual memory is mapped one-to-one to physical
210 memory. That is, virtual address @code{PHYS_ADDR} accesses physical
211 address 0, virtual address @code{PHYS_ADDR} + @t{0x1234} access
212 physical address @t{0x1234}, and so on up to the size of the machine's
215 User programs can only access user virtual memory. An attempt to
216 access kernel virtual memory will cause a page fault, handled by
217 @func{page_fault} in @file{userprog/exception.c}, and the process
218 will be terminated. Kernel threads can access both kernel virtual
219 memory and, if a user process is running, the user virtual memory of
220 the running process. However, even in the kernel, an attempt to
221 access memory at a user virtual address that doesn't have a page
222 mapped into it will cause a page fault.
224 @node Global Requirements
225 @section Global Requirements
227 For testing and grading purposes, we have some simple requirements for
228 your output. The kernel should print out the program's name and exit
229 status whenever a process exits, e.g.@: @code{shell: exit(-1)}. Aside
230 from this, it should print out no other messages. You may understand
231 all those debug messages, but we won't, and it just clutters our
232 ability to see the stuff we care about.
234 Additionally, while it may be useful to hard-code which process will
235 run at startup while debugging, before you submit your code you must
236 make sure that it takes the start-up process name and arguments from
237 the @samp{-ex} argument. For example, running @code{pintos run -ex
238 "testprogram 1 2 3 4"} will spawn @samp{testprogram 1 2 3 4} as the
241 @node Problem 2-1 Argument Passing
242 @section Problem 2-1: Argument Passing
244 Currently, @func{process_execute} does not support passing arguments
245 to new processes. UNIX and other operating systems do allow passing
246 command line arguments to a program, which accesses them via the argc,
247 argv arguments to main. You must implement this functionality by
248 extending @func{process_execute} so that instead of simply taking a
249 program file name, it can take a program name with arguments as a
250 single string. That is, @code{process_execute("grep foo *.c")} should
251 be a legal call. @xref{80x86 Calling Convention}, for information on
252 exactly how this works.
254 @strong{This functionality is extremely important.} Almost all our
255 test cases rely on being able to pass arguments, so if you don't get
256 this right, a lot of things will not appear to work correctly with our
257 tests. If the tests fail, so do you. Fortunately, this part
258 shouldn't be too hard.
260 @node Problem 2-2 System Calls
261 @section Problem 2-2: System Calls
263 Implement the system call handler in @file{userprog/syscall.c} to
264 properly deal with all the system calls described below. Currently,
265 it ``handles'' system calls by terminating the process. You will need
266 to decipher system call arguments and take the appropriate action for
269 You are required to support the following system calls, whose syscall
270 numbers are defined in @file{lib/syscall-nr.h} and whose C functions
271 called by user programs are prototyped in @file{lib/user/syscall.h}:
275 @itemx void halt (void)
276 Stops Pintos and prints out performance statistics. Note that this
277 should be seldom used, since then you lose some information about
278 possible deadlock situations, etc.
281 @itemx void exit (int @var{status})
282 Terminates the current user program, returning @var{status} to the
283 kernel. A @var{status} of 0 indicates a successful exit. Other
284 values may be used to indicate user-defined error conditions.
287 @itemx pid_t exec (const char *@var{file})
288 Run the executable in @var{file} and return the new process's program
289 id (pid). If there is an error loading this program, returns pid -1,
290 which otherwise should not be a valid id number.
293 @itemx int join (pid_t @var{pid})
294 Joins the process @var{pid}, using the join rules from the last
295 assignment, and returns the process's exit status. If the process was
296 terminated by the kernel (i.e.@: killed due to an exception), the exit
297 status should be -1. If the process was not a child of the calling
298 process, the return value is undefined (but kernel operation must not
302 @itemx bool create (const char *@var{file})
303 Create a new file called @var{file}. Returns -1 if failed, 0 if OK.
306 @itemx bool remove (const char *@var{file})
307 Delete the file called @var{file}. Returns -1 if failed, 0 if OK.
310 @itemx int open (const char *@var{file})
311 Open the file called @var{file}. Returns a nonnegative integer handle
312 called a ``file descriptor'' (fd), or -1 if the file could not be
313 opened. File descriptors numbered 0 and 1 are reserved for the
314 console. All open files associated with a process should be closed
315 when the process exits or is terminated.
318 @itemx int filesize (int @var{fd})
319 Returns the size, in bytes, of the file open as @var{fd}, or -1 if the
323 @itemx int read (int @var{fd}, void *@var{buffer}, unsigned @var{size})
324 Read @var{size} bytes from the file open as @var{fd} into
325 @var{buffer}. Returns the number of bytes actually read, or -1 if the
326 file could not be read.
329 @itemx int write (int @var{fd}, const void *@var{buffer}, unsigned @var{size})
330 Write @var{size} bytes from @var{buffer} to the open file @var{fd}.
331 Returns the number of bytes actually written, or -1 if the file could
335 @itemx void seek (int @var{fd}, unsigned @var{position})
336 Changes the next byte to be read or written in open file @var{fd} to
337 @var{position}, expressed in bytes from the beginning of the file.
338 (Thus, a @var{position} of 0 is the file's start.)
341 @itemx unsigned tell (int @var{fd})
342 Returns the position of the next byte to be read or written in open
343 file @var{fd}, expressed in bytes from the beginning of the file.
346 @itemx void close (int @var{fd})
347 Close file descriptor @var{fd}.
350 The file defines other syscalls. Ignore them for now. You will
351 implement some of them in project 3 and the rest in project 4, so be
352 sure to design your system with extensibility in mind.
354 To implement syscalls, you will need to provide a way of copying data
355 from the user's virtual address space into the kernel and vice versa.
356 This can be a bit tricky: what if the user provides an invalid
357 pointer, a pointer into kernel memory, or points to a block that is
358 partially in one of those regions? You should handle these cases by
359 terminating the user process. You will need this code before you can
360 even obtain the system call number, because the system call number is
361 on the user's stack in the user's virtual address space. We recommend
362 writing and testing this code before implementing any other system
365 You must make sure that system calls are properly synchronized so that
366 any number of user processes can make them at once. In particular, it
367 is not safe to call into the filesystem code provided in the
368 @file{filesys} directory from multiple threads at once. For now, we
369 recommend adding a single lock that controls access to the filesystem
370 code. You should acquire this lock before calling any functions in
371 the @file{filesys} directory, and release it afterward. Don't forget
372 that @func{process_execute} also accesses files. @strong{For now, we
373 recommend against modifying code in the @file{filesys} directory.}
375 We have provided you a function for each system call in
376 @file{lib/user/syscall.c}. These provide a way for user processes to
377 invoke each system call from a C program. Each of them calls an
378 assembly language routine in @file{lib/user/syscall-stub.S}, which in
379 turn invokes the system call interrupt and returns.
381 When you're done with this part, and forevermore, Pintos should be
382 bulletproof. Nothing that a user program can do should ever cause the
383 OS to crash, halt, assert fail, or otherwise stop running. The sole
384 exception is a call to the @code{halt} system call.
386 @xref{System Calls}, for more information on how syscalls work.
388 @node User Programs FAQ
393 @b{Do we need a working project 1 to implement project 2?}
395 You may find the code for @func{thread_join} to be useful in
396 implementing the join syscall, but besides that, you can use
397 the original code provided for project 1.
400 @b{All my user programs die with page faults.}
402 This will generally happen if you haven't implemented problem 2-1
403 yet. The reason is that the basic C library for user programs tries
404 to read @var{argc} and @var{argv} off the stack. Because the stack
405 isn't properly set up yet, this causes a page fault.
408 @b{Is there a way I can disassemble user programs?}
410 The @command{i386-elf-objdump} utility can disassemble entire user
411 programs or object files. Invoke it as @code{i386-elf-objdump -d
412 @var{file}}. You can also use @code{i386-elf-gdb}'s
413 @command{disassemble} command to disassemble individual functions in
414 object files compiled with debug information.
417 @b{Why can't I use many C include files in my Pintos programs?}
419 The C library we provide is very limited. It does not include many of
420 the features that are expected of a real operating system's C library.
421 The C library must be built specifically for the operating system (and
422 architecture), since it must make system calls for I/O and memory
423 allocation. (Not all functions do, of course, but usually the library
424 is compiled as a unit.) If you wish to port libraries to Pintos, feel
428 @b{How do I compile new user programs?}
430 You need to modify @file{tests/Makefile}.
433 @b{What's the difference between @code{tid_t} and @code{pid_t}?}
435 A @code{tid_t} identifies a kernel thread, which may have a user
436 process running in it (if created with @func{process_execute}) or not
437 (if created with @func{thread_create}). It is a data type used only
440 A @code{pid_t} identifies a user process. It is used by user
441 processes and the kernel in the @code{exec} and @code{join} system
444 You can choose whatever suitable types you like for @code{tid_t} and
445 @code{pid_t}. By default, they're both @code{int}. You can make them
446 a one-to-one mapping, so that the same values in both identify the
447 same process, or you can use a more complex mapping. It's up to you.
450 @b{I can't seem to figure out how to read from and write to user
451 memory. What should I do?}
453 The kernel must treat user memory delicately. As part of a system
454 call, the user can pass to the kernel a null pointer, a pointer to
455 unmapped virtual memory, or a pointer to kernel virtual address space
456 (above @code{PHYS_BASE}). All of these types of invalid pointers must
457 be rejected without harm to the kernel or other running processes. At
458 your option, the kernel may handle invalid pointers by terminating the
459 process or returning from the system call with an error.
461 There are at least two reasonable ways to do this correctly. The
462 first method is to ``verify then access'':@footnote{These terms are
463 made up for this document. They are not standard terminology.} verify
464 the validity of a user-provided pointer, then dereference it. If you
465 choose this route, you'll want to look at the functions in
466 @file{userprog/pagedir.c} and in @file{threads/mmu.h}. This is the
467 simplest way to handle user memory access.
469 The second method is to ``assume and react'': directly dereference
470 user pointers, after checking that they point below @code{PHYS_BASE}.
471 Invalid user pointers will then cause a ``page fault'' that you can
472 handle by modifying the code for @func{page_fault} in
473 @file{userprog/exception.cc}. This technique is normally faster
474 because it takes advantage of the processor's MMU, so it tends to be
475 used in real kernels (including Linux).
477 In either case, you need to make sure not to ``leak'' resources. For
478 example, suppose that your system call has acquired a lock or
479 allocated a page of memory. If you encounter an invalid user pointer
480 afterward, you must still be sure to release the lock or free the page
481 of memory. If you choose to ``verify then access,'' then this should
482 be straightforward, but for ``assume and react'' it's more difficult,
483 because there's no way to return an error code from a memory access.
484 Therefore, for those who want to try the latter technique, we'll
485 provide a little bit of helpful code:
488 /* Tries to copy a byte from user address USRC to kernel address DST.
489 Returns true if successful, false if USRC is invalid. */
490 static inline bool get_user (uint8_t *dst, const uint8_t *usrc) {
492 asm ("movl $1f, %%eax; movb %2, %%al; movb %%al, %0; 1:"
493 : "=m" (*dst), "=&a" (eax) : "m" (*usrc));
497 /* Tries write BYTE to user address UDST.
498 Returns true if successful, false if UDST is invalid. */
499 static inline bool put_user (uint8_t *udst, uint8_t byte) {
501 asm ("movl $1f, %%eax; movb %b2, %0; 1:"
502 : "=m" (*udst), "=&a" (eax) : "r" (byte));
507 Each of these functions assumes that the user address has already been
508 verified to be below @code{PHYS_BASE}. They also assume that you've
509 modified @func{page_fault} so that a page fault in the kernel causes
510 @code{eax} to be set to 0 and its former value copied into @code{eip}.
513 @b{I'm also confused about reading from and writing to the stack. Can
518 Only non-@samp{char} values will have issues when writing them to
519 memory. If a digit is in a string, it is considered a character.
520 However, the value of @code{argc} would be a non-char.
523 You will need to write characters and non-characters into main memory.
526 When you add items to the stack, you will be decrementing the stack
527 pointer. You'll need to decrement the stack pointer before writing to
531 Each character is 1 byte.
535 @b{Why doesn't keyboard input work with @option{-v}?}
537 Serial input isn't implemented. Don't use @option{-v} if you want to
538 use the shell or otherwise type at the keyboard.
542 * Problem 2-1 Argument Passing FAQ::
543 * Problem 2-2 System Calls FAQ::
546 @node Problem 2-1 Argument Passing FAQ
547 @subsection Problem 2-1: Argument Passing FAQ
551 @b{What will be the format of command line arguments?}
553 You should assume that command line arguments are delimited by white
557 @b{What is the maximum length of the command line arguments?}
559 You can impose some reasonable maximum as long as you're prepared to
560 defend it in your @file{DESIGNDOC}.
563 @b{How do I parse all these argument strings?}
565 You're welcome to use any technique you please, as long as it works.
566 If you're lost, look at @func{strtok_r}, prototyped in
567 @file{lib/string.h} and implemented with thorough comments in
568 @file{lib/string.c}. You can find more about it by looking at the man
569 page (run @code{man strtok_r} at the prompt).
572 @b{Why is the top of the stack at @t{0xc0000000}? Isn't that off the
573 top of user virtual memory? Shouldn't it be @t{0xbfffffff}?}
575 When the processor pushes data on the stack, it decrements the stack
576 pointer first. Thus, the first (4-byte) value pushed on the stack
577 will be at address @t{0xbffffffc}.
579 Also, the stack should always be aligned to a 4-byte boundary, but
580 @t{0xbfffffff} isn't.
583 @b{Is @code{PHYS_BASE} fixed?}
585 No. You should be able to support @code{PHYS_BASE} values that are
586 any multiple of @t{0x10000000} from @t{0x80000000} to @t{0xc0000000},
587 simply via recompilation.
590 @node Problem 2-2 System Calls FAQ
591 @subsection Problem 2-2: System Calls FAQ
595 @b{What should I do with the parameter passed to @func{exit}?}
597 This value, the exit status of the process, must be returned to the
598 thread's parent when @func{join} is called.
601 @b{Can I just cast a pointer to a @struct{file} object to get a
602 unique file descriptor? Can I just cast a @code{struct thread *} to a
603 @code{pid_t}? It's so much simpler that way!}
605 This is a design decision you will have to make for yourself.
606 However, note that most operating systems do distinguish between file
607 descriptors (or pids) and the addresses of their kernel data
608 structures. You might want to give some thought as to why they do so
609 before committing yourself.
612 @b{Can I set a maximum number of open files per process?}
614 From a design standpoint, it would be better not to set an arbitrary
615 maximum. That said, if your design calls for it, you may impose a
616 limit of 128 open files per process (as the Solaris machines here do).
619 @anchor{Removing an Open File}
620 @b{What happens when two (or more) processes have a file open and one of
623 You should copy the standard Unix semantics for files. That is, when
624 a file is removed an process which has a file descriptor for that file
625 may continue to do operations on that descriptor. This means that
626 they can read and write from the file. The file will not have a name,
627 and no other processes will be able to open it, but it will continue
628 to exist until all file descriptors referring to the file are closed
629 or the machine shuts down.
632 @b{What happens if a system call is passed an invalid argument, such
633 as Open being called with an invalid filename?}
635 Pintos should not crash. Acceptable options include returning an
636 error value (for those calls that return a value), returning an
637 undefined value, or terminating the process.
640 @b{I've discovered that some of my user programs need more than one 4
641 kB page of stack space. What should I do?}
643 You may modify the stack setup code to allocate more than one page of
644 stack space for each process.
647 @b{What do I need to print on thread completion?}
649 You should print the complete thread name (as specified in the
650 @code{SYS_exec} call) followed by the exit status code,
651 e.g.@: @samp{example 1 2 3 4: 0}.
654 @node 80x86 Calling Convention
655 @section 80@var{x}86 Calling Convention
657 What follows is a quick and dirty discussion of the 80@var{x}86
658 calling convention. Some of the basics should be familiar from CS
659 107, and if you've already taken CS 143 or EE 182, then you should
660 have seen even more of it. I've omitted some of the complexity, since
661 this isn't a class in how function calls work, so don't expect this to
662 be exactly correct in full, gory detail. If you do want all the
663 details, you can refer to @bibref{SysV-i386}.
665 Whenever a function call happens, you need to put the arguments on the
666 call stack for that function, before the code for that function
667 executes, so that the callee has access to those values. The caller
668 has to be responsible for this (be sure you understand why).
669 Therefore, when you compile a program, the assembly code emitted will
670 have in it, before every function call, a bunch of instructions that
671 prepares for the call in whatever manner is conventional for the
672 machine you're working on. This includes saving registers as needed,
673 putting stuff on the stack, saving the location to return to somewhere
674 (so that when the callee finishes, it knows where the caller code is),
675 and some other bookkeeping stuff. Then you do the jump to the
676 callee's code, and it goes along, assuming that the stack and
677 registers are prepared in the appropriate manner. When the callee is
678 done, it looks at the return location as saved earlier, and jumps back
679 to that location. The caller may then have to do some cleanup:
680 clearing arguments and the return value off the stack, restoring
681 registers that were saved before the call, and so on.
683 If you think about it, some of these things should remind you of
686 As an aside, in general, function calls are not cheap. You have to do
687 a bunch of memory writes to prepare the stack, you need to save and
688 restore registers before and after a function call, you need to write
689 the stack pointer, you have a couple of jumps which probably wrecks
690 some of your caches. This is why inlining code can be much faster.
693 * Argument Passing to main::
696 @node Argument Passing to main
697 @subsection Argument Passing to @code{main()}
699 In @func{main}'s case, there is no caller to prepare the stack
700 before it runs. Therefore, the kernel needs to do it. Fortunately,
701 since there's no caller, there are no registers to save, no return
702 address to deal with, etc. The only difficult detail to take care of,
703 after loading the code, is putting the arguments to @func{main} on
706 (The above is a small lie: most compilers will emit code where main
707 isn't strictly speaking the first function. This isn't an important
708 detail. If you want to look into it more, try disassembling a program
709 and looking around a bit. However, you can just act as if
710 @func{main} is the very first function called.)
712 Pintos is written for the 80@var{x}86 architecture. Therefore, we
713 need to adhere to the 80@var{x}86 calling convention. Basically, you
714 put all the arguments on the stack and move the stack pointer
715 appropriately. You also need to insert space for the function's
716 ``return address'': even though the initial function doesn't really
717 have a caller, its stack frame must have the same layout as any other
718 function's. The program will assume that the stack has been laid out
719 this way when it begins running.
721 So, what are the arguments to @func{main}? Just two: an @samp{int}
722 (@code{argc}) and a @samp{char **} (@code{argv}). @code{argv} is an
723 array of strings, and @code{argc} is the number of strings in that
724 array. However, the hard part isn't these two things. The hard part
725 is getting all the individual strings in the right place. As we go
726 through the procedure, let us consider the following example command:
727 @samp{/bin/ls -l *.h *.c}.
729 The first thing to do is to break the command line into individual
730 strings: @samp{/bin/ls}, @samp{-l}, @samp{*.h}, and @samp{*.c}. These
731 constitute the arguments of the command, including the program name
732 itself (which belongs in @code{argv[0]}).
734 These individual, null-terminated strings should be placed on the user
735 stack. They may be placed in any order, as you'll see shortly,
736 without affecting how main works, but for simplicity let's assume they
737 are in reverse order (keeping in mind that the stack grows downward on
738 an 80@var{x}86 machine). As we copy the strings onto the stack, we
739 record their (virtual) stack addresses. These addresses will become
740 important when we write the argument vector (two paragraphs down).
742 After we push all of the strings onto the stack, we adjust the stack
743 pointer so that it is word-aligned: that is, we move it down to the
744 next 4-byte boundary. This is required because we will next be
745 placing several words of data on the stack, and they must be aligned
746 in order to be read correctly. In our example, as you'll see below,
747 the strings start at address @t{0xffed}. One word below that would be
748 at @t{0xffe9}, so we could in theory put the next word on the stack
749 there. However, since the stack pointer should always be
750 word-aligned, we instead leave the stack pointer at @t{0xffe8}.
752 Once we align the stack pointer, we then push the elements of the
753 argument vector, that is, a null pointer, then the addresses of the
754 strings @samp{/bin/ls}, @samp{-l}, @samp{*.h}, and @samp{*.c}) onto
755 the stack. This must be done in reverse order, such that
756 @code{argv[0]} is at the lowest virtual address, again because the
757 stack is growing downward. (The null pointer pushed first is because
758 @code{argv[argc]} must be a null pointer.) This is because we are now
759 writing the actual array of strings; if we write them in the wrong
760 order, then the strings will be in the wrong order in the array. This
761 is also why, strictly speaking, it doesn't matter what order the
762 strings themselves are placed on the stack: as long as the pointers
763 are in the right order, the strings themselves can really be anywhere.
764 After we finish, we note the stack address of the first element of the
765 argument vector, which is @code{argv} itself.
767 Then we push @code{argv} (that is, the address of the first element of
768 the @code{argv} array) onto the stack, along with the length of the
769 argument vector (@code{argc}, 4 in this example). This must also be
770 done in this order, since @code{argc} is the first argument to
771 @func{main} and therefore is on first (smaller address) on the
772 stack. Finally, we push a fake ``return address'' and leave the stack
773 pointer to point to its location.
775 All this may sound very confusing, so here's a picture which will
776 hopefully clarify what's going on. This represents the state of the
777 stack and the relevant registers right before the beginning of the
778 user program (assuming for this example that the stack bottom is
784 @multitable {@t{0xbfffffff}} {``return address''} {@t{/bin/ls\0}}
785 @item Address @tab Name @tab Data
786 @item @t{0xbffffffc} @tab @code{*argv[3]} @tab @samp{*.c\0}
787 @item @t{0xbffffff8} @tab @code{*argv[2]} @tab @samp{*.h\0}
788 @item @t{0xbffffff5} @tab @code{*argv[1]} @tab @samp{-l\0}
789 @item @t{0xbfffffed} @tab @code{*argv[0]} @tab @samp{/bin/ls\0}
790 @item @t{0xbfffffec} @tab word-align @tab @samp{\0}
791 @item @t{0xbfffffe8} @tab @code{argv[4]} @tab @t{0}
792 @item @t{0xbfffffe4} @tab @code{argv[3]} @tab @t{0xbffffffc}
793 @item @t{0xbfffffe0} @tab @code{argv[2]} @tab @t{0xbffffff8}
794 @item @t{0xbfffffdc} @tab @code{argv[1]} @tab @t{0xbffffff5}
795 @item @t{0xbfffffd8} @tab @code{argv[0]} @tab @t{0xbfffffed}
796 @item @t{0xbfffffd4} @tab @code{argv} @tab @t{0xbfffffd8}
797 @item @t{0xbfffffd0} @tab @code{argc} @tab 4
798 @item @t{0xbfffffcc} @tab ``return address'' @tab 0
804 In this example, the stack pointer would be initialized to
807 As shown above, your code should start the stack at the very top of
808 the user virtual address space, in the page just below virtual address
809 @code{PHYS_BASE} (defined in @file{threads/mmu.h}).
811 You may find the non-standard @func{hex_dump} function, declared in
812 @file{<stdio.h>}, useful for debugging your argument passing code.
813 Here's what it would show in the above example, given that
814 @code{PHYS_BASE} is @t{0xc0000000}:
817 bfffffc0 00 00 00 00 | ....|
818 bfffffd0 04 00 00 00 d8 ff ff bf-ed ff ff bf f5 ff ff bf |................|
819 bfffffe0 f8 ff ff bf fc ff ff bf-00 00 00 00 00 2f 62 69 |............./bi|
820 bffffff0 6e 2f 6c 73 00 2d 6c 00-2a 2e 68 00 2a 2e 63 00 |n/ls.-l.*.h.*.c.|
824 @section System Calls
826 We have already been dealing with one way that the operating system
827 can regain control from a user program: interrupts from timers and I/O
828 devices. These are ``external'' interrupts, because they are caused
829 by entities outside the CPU.
831 The operating system is also called to deal with software exceptions,
832 which are events generated in response to the code. These can be
833 errors such as a page fault or division by zero. However, exceptions
834 are also the means by which a user program can request services
835 (``system calls'') from the operating system.
837 In the 80@var{x}86 architecture, the @samp{int} instruction is the
838 most commonly used means for invoking system calls. This instruction
839 is handled in the same way as other software exceptions. In Pintos,
840 user programs invoke @samp{int $0x30} to make a system call. The
841 system call number and any additional arguments are expected to be
842 pushed on the stack in the normal fashion before invoking the
845 The normal calling convention pushes function arguments on the stack
846 from right to left and the stack grows downward. Thus, when the
847 system call handler @func{syscall_handler} gets control, the system
848 call number is in the 32-bit word at the caller's stack pointer, the
849 first argument is in the 32-bit word at the next higher address, and
850 so on. The caller's stack pointer is accessible to
851 @func{syscall_handler} as the @samp{esp} member of the @code{struct
852 intr_frame} passed to it.
854 Here's an example stack frame for calling a system call numbered 10
855 with three arguments passed as 1, 2, and 3. The stack addresses are
861 @multitable {@t{0xbffffe7c}} {Value}
862 @item Address @tab Value
863 @item @t{0xbffffe7c} @tab 3
864 @item @t{0xbffffe78} @tab 2
865 @item @t{0xbffffe74} @tab 1
866 @item @t{0xbffffe70} @tab 10
872 In this example, the caller's stack pointer would be at
875 The 80@var{x}86 convention for function return values is to place them
876 in the @samp{EAX} register. System calls that return a value can do
877 so by modifying the @samp{eax} member of @struct{intr_frame}.