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<!-- Created on March, 6 2012 by texi2html 1.66 -->
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Written by: Lionel Cons <Lionel.Cons@cern.ch> (original author)
Karl Berry <karl@freefriends.org>
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Maintained by: Many creative people <dev@texi2html.cvshome.org>
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<HEAD>
<TITLE>Pintos Projects: Project 0--Introducing Pintos</TITLE>
<META NAME="description" CONTENT="Pintos Projects: Project 0--Introducing Pintos">
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<BODY >
<A NAME="SEC15"></A>
<TABLE CELLPADDING=1 CELLSPACING=1 BORDER=0>
<TR><TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="pintos_1.html#SEC1"> << </A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="pintos_3.html#SEC41"> >> </A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="pintos.html#SEC_Top">Top</A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="pintos.html#SEC_Contents">Contents</A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[Index]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="pintos_abt.html#SEC_About"> ? </A>]</TD>
</TR></TABLE>
<HR SIZE=2>
<H1> 2. Project 0: Introducing Pintos </H1>
<!--docid::SEC15::-->
<P>
In this assignment, you will learn about the existing functionality
in Pintos, and add two small features to the system: a more efficient
implementation of <Q><SAMP>sleep</SAMP></Q>, and the ability to pass command line
arguments to user programs.
</P>
<P>
You will be working in the <Q><TT>threads</TT></Q> directory for the first part
of this assignment (with some work in the <Q><TT>devices</TT></Q> directory on
the side), and modify the file <Q><TT>userprog/process.c</TT></Q> in the second part.
</P>
<P>
The tests for Project 0 are executed by changing the working directory
to <Q><TT>intro</TT></Q> and running <Q><SAMP>make</SAMP></Q> followed by <Q><SAMP>make check</SAMP></Q>.
</P>
<P>
Before you read the description of this project, you should read all of
the following sections: <A HREF="pintos_1.html#SEC1">1. Introduction</A>, <A HREF="pintos_6.html#SEC89">B. Coding Standards</A>,
<A HREF="pintos_8.html#SEC96">D. Debugging Tools</A>, and <A HREF="pintos_9.html#SEC109">E. Development Tools</A>. You should at least
skim the material from <A HREF="pintos_5.html#SEC49">A.1 Loading</A> through <A HREF="pintos_5.html#SEC69">A.5 Memory Allocation</A>, especially <A HREF="pintos_5.html#SEC58">A.3 Synchronization</A>.
</P>
<P>
<A NAME="Understanding Threads"></A>
<HR SIZE="6">
<A NAME="SEC16"></A>
<H2> 2.1 Understanding Threads </H2>
<!--docid::SEC16::-->
<P>
The first step is to read and understand the code for the initial thread
system.
Pintos already implements thread creation and thread completion,
a simple scheduler to switch between threads, and synchronization
primitives (semaphores, locks, condition variables, and optimization
barriers).
</P>
<P>
Some of this code might seem slightly mysterious. If
you haven't already compiled and run the base system, as described in
the introduction (see section <A HREF="pintos_1.html#SEC1">1. Introduction</A>), you should do so now. You
can read through parts of the source code to see what's going
on. If you like, you can add calls to <CODE>printf()</CODE> almost
anywhere, then recompile and run to see what happens and in what
order. You can also run the kernel in a debugger and set breakpoints
at interesting spots, single-step through code and examine data, and
so on.
</P>
<P>
When a thread is created, you are creating a new context to be
scheduled. You provide a function to be run in this context as an
argument to <CODE>thread_create()</CODE>. The first time the thread is
scheduled and runs, it starts from the beginning of that function
and executes in that context. When the function returns, the thread
terminates. Each thread, therefore, acts like a mini-program running
inside Pintos, with the function passed to <CODE>thread_create()</CODE>
acting like <CODE>main()</CODE>.
</P>
<P>
At any given time, exactly one thread runs and the rest, if any,
become inactive. The scheduler decides which thread to
run next. (If no thread is ready to run
at any given time, then the special "idle" thread, implemented in
<CODE>idle()</CODE>, runs.)
Synchronization primitives can force context switches when one
thread needs to wait for another thread to do something.
</P>
<P>
The mechanics of a context switch are
in <Q><TT>threads/switch.S</TT></Q>, which is 80<VAR>x</VAR>86
assembly code. (You don't have to understand it.) It saves the
state of the currently running thread and restores the state of the
thread we're switching to.
</P>
<P>
Using the GDB debugger, slowly trace through a context
switch to see what happens (see section <A HREF="pintos_8.html#SEC102">D.5 GDB</A>). You can set a
breakpoint on <CODE>schedule()</CODE> to start out, and then
single-step from there.<A NAME="DOCF1" HREF="pintos_fot.html#FOOT1">(1)</A> Be sure
to keep track of each thread's address
and state, and what procedures are on the call stack for each thread.
You will notice that when one thread calls <CODE>switch_threads()</CODE>,
another thread starts running, and the first thing the new thread does
is to return from <CODE>switch_threads()</CODE>. You will understand the thread
system once you understand why and how the <CODE>switch_threads()</CODE> that
gets called is different from the <CODE>switch_threads()</CODE> that returns.
See section <A HREF="pintos_5.html#SEC57">A.2.3 Thread Switching</A>, for more information.
</P>
<P>
<STRONG>Warning</STRONG>: In Pintos, each thread is assigned a small,
fixed-size execution stack just under 4 kB in size. The kernel
tries to detect stack overflow, but it cannot do so perfectly. You
may cause bizarre problems, such as mysterious kernel panics, if you
declare large data structures as non-static local variables,
e.g. <Q><SAMP>int buf[1000];</SAMP></Q>. Alternatives to stack allocation include
the page allocator and the block allocator (see section <A HREF="pintos_5.html#SEC69">A.5 Memory Allocation</A>).
</P>
<P>
<A NAME="Project 1 Source Files"></A>
<HR SIZE="6">
<A NAME="SEC17"></A>
<H3> 2.1.1 Source Files </H3>
<!--docid::SEC17::-->
<P>
Here is a brief overview of the files in the <Q><TT>threads</TT></Q>
directory. You will not need to modify most of this code, but the
hope is that presenting this overview will give you a start on what
code to look at.
</P>
<P>
</P>
<DL COMPACT>
<DT><Q><TT>loader.S</TT></Q>
<DD><DT><Q><TT>loader.h</TT></Q>
<DD>The kernel loader. Assembles to 512 bytes of code and data that the
PC BIOS loads into memory and which in turn finds the kernel on disk,
loads it into memory, and jumps to <CODE>start()</CODE> in <Q><TT>start.S</TT></Q>.
See section <A HREF="pintos_5.html#SEC50">A.1.1 The Loader</A>, for details. You should not need to look at
this code or modify it.
<P>
</P>
<DT><Q><TT>start.S</TT></Q>
<DD>Does basic setup needed for memory protection and 32-bit
operation on 80<VAR>x</VAR>86 CPUs. Unlike the loader, this code is
actually part of the kernel. See section <A HREF="pintos_5.html#SEC51">A.1.2 Low-Level Kernel Initialization</A>,
for details.
<P>
</P>
<DT><Q><TT>kernel.lds.S</TT></Q>
<DD>The linker script used to link the kernel. Sets the load address of
the kernel and arranges for <Q><TT>start.S</TT></Q> to be near the beginning
of the kernel image. See section <A HREF="pintos_5.html#SEC50">A.1.1 The Loader</A>, for details. Again, you
should not need to look at this code
or modify it, but it's here in case you're curious.
<P>
</P>
<DT><Q><TT>init.c</TT></Q>
<DD><DT><Q><TT>init.h</TT></Q>
<DD>Kernel initialization, including <CODE>main()</CODE>, the kernel's "main
program." You should look over <CODE>main()</CODE> at least to see what
gets initialized. You might want to add your own initialization code
here. See section <A HREF="pintos_5.html#SEC52">A.1.3 High-Level Kernel Initialization</A>, for details.
<P>
</P>
<DT><Q><TT>thread.c</TT></Q>
<DD><DT><Q><TT>thread.h</TT></Q>
<DD>Basic thread support. Much of your work will take place in these files.
<Q><TT>thread.h</TT></Q> defines <CODE>struct thread</CODE>, which you are likely to modify
in all three projects. See <A HREF="pintos_5.html#SEC55">A.2.1 <CODE>struct thread</CODE></A> and <A HREF="pintos_5.html#SEC54">A.2 Threads</A> for
more information.
<P>
</P>
<DT><Q><TT>switch.S</TT></Q>
<DD><DT><Q><TT>switch.h</TT></Q>
<DD>Assembly language routine for switching threads. Already discussed
above. See section <A HREF="pintos_5.html#SEC56">A.2.2 Thread Functions</A>, for more information.
<P>
</P>
<DT><Q><TT>palloc.c</TT></Q>
<DD><DT><Q><TT>palloc.h</TT></Q>
<DD>Page allocator, which hands out system memory in multiples of 4 kB
pages. See section <A HREF="pintos_5.html#SEC70">A.5.1 Page Allocator</A>, for more information.
<P>
</P>
<DT><Q><TT>malloc.c</TT></Q>
<DD><DT><Q><TT>malloc.h</TT></Q>
<DD>A simple implementation of <CODE>malloc()</CODE> and <CODE>free()</CODE> for
the kernel. See section <A HREF="pintos_5.html#SEC71">A.5.2 Block Allocator</A>, for more information.
<P>
</P>
<DT><Q><TT>interrupt.c</TT></Q>
<DD><DT><Q><TT>interrupt.h</TT></Q>
<DD>Basic interrupt handling and functions for turning interrupts on and
off. See section <A HREF="pintos_5.html#SEC65">A.4 Interrupt Handling</A>, for more information.
<P>
</P>
<DT><Q><TT>intr-stubs.S</TT></Q>
<DD><DT><Q><TT>intr-stubs.h</TT></Q>
<DD>Assembly code for low-level interrupt handling. See section <A HREF="pintos_5.html#SEC66">A.4.1 Interrupt Infrastructure</A>, for more information.
<P>
</P>
<DT><Q><TT>synch.c</TT></Q>
<DD><DT><Q><TT>synch.h</TT></Q>
<DD>Basic synchronization primitives: semaphores, locks, condition
variables, and optimization barriers. You will need to use these for
synchronization in all
four projects. See section <A HREF="pintos_5.html#SEC58">A.3 Synchronization</A>, for more information.
<P>
</P>
<DT><Q><TT>io.h</TT></Q>
<DD>Functions for I/O port access. This is mostly used by source code in
the <Q><TT>devices</TT></Q> directory that you won't have to touch.
<P>
</P>
<DT><Q><TT>vaddr.h</TT></Q>
<DD><DT><Q><TT>pte.h</TT></Q>
<DD>Functions and macros for working with virtual addresses and page table
entries. These will be more important to you in project 2. For now,
you can ignore them.
<P>
</P>
<DT><Q><TT>flags.h</TT></Q>
<DD>Macros that define a few bits in the 80<VAR>x</VAR>86 "flags" register.
Probably of no interest. See [ <A HREF="pintos_10.html#IA32-v1">IA32-v1</A>], section 3.4.3, "EFLAGS
Register," for more information.
</DL>
<P>
<A NAME="devices code"></A>
<HR SIZE="6">
<A NAME="SEC18"></A>
<H4> 2.1.1.1 <Q><TT>devices</TT></Q> code </H4>
<!--docid::SEC18::-->
<P>
The basic threaded kernel also includes these files in the
<Q><TT>devices</TT></Q> directory:
</P>
<P>
</P>
<DL COMPACT>
<DT><Q><TT>timer.c</TT></Q>
<DD><DT><Q><TT>timer.h</TT></Q>
<DD>System timer that ticks, by default, 100 times per second. You will
modify this code in this project.
<P>
</P>
<DT><Q><TT>vga.c</TT></Q>
<DD><DT><Q><TT>vga.h</TT></Q>
<DD>VGA display driver. Responsible for writing text to the screen.
You should have no need to look at this code. <CODE>printf()</CODE>
calls into the VGA display driver for you, so there's little reason to
call this code yourself.
<P>
</P>
<DT><Q><TT>serial.c</TT></Q>
<DD><DT><Q><TT>serial.h</TT></Q>
<DD>Serial port driver. Again, <CODE>printf()</CODE> calls this code for you,
so you don't need to do so yourself.
It handles serial input by passing it to the input layer (see below).
<P>
</P>
<DT><Q><TT>block.c</TT></Q>
<DD><DT><Q><TT>block.h</TT></Q>
<DD>An abstraction layer for <EM>block devices</EM>, that is, random-access,
disk-like devices that are organized as arrays of fixed-size blocks.
Out of the box, Pintos supports two types of block devices: IDE disks
and partitions.
<P>
</P>
<DT><Q><TT>ide.c</TT></Q>
<DD><DT><Q><TT>ide.h</TT></Q>
<DD>Supports reading and writing sectors on up to 4 IDE disks.
<P>
</P>
<DT><Q><TT>partition.c</TT></Q>
<DD><DT><Q><TT>partition.h</TT></Q>
<DD>Understands the structure of partitions on disks, allowing a single
disk to be carved up into multiple regions (partitions) for
independent use.
<P>
</P>
<DT><Q><TT>kbd.c</TT></Q>
<DD><DT><Q><TT>kbd.h</TT></Q>
<DD>Keyboard driver. Handles keystrokes passing them to the input layer
(see below).
<P>
</P>
<DT><Q><TT>input.c</TT></Q>
<DD><DT><Q><TT>input.h</TT></Q>
<DD>Input layer. Queues input characters passed along by the keyboard or
serial drivers.
<P>
</P>
<DT><Q><TT>intq.c</TT></Q>
<DD><DT><Q><TT>intq.h</TT></Q>
<DD>Interrupt queue, for managing a circular queue that both kernel
threads and interrupt handlers want to access. Used by the keyboard
and serial drivers.
<P>
</P>
<DT><Q><TT>rtc.c</TT></Q>
<DD><DT><Q><TT>rtc.h</TT></Q>
<DD>Real-time clock driver, to enable the kernel to determine the current
date and time. By default, this is only used by <Q><TT>thread/init.c</TT></Q>
to choose an initial seed for the random number generator.
<P>
</P>
<DT><Q><TT>speaker.c</TT></Q>
<DD><DT><Q><TT>speaker.h</TT></Q>
<DD>Driver that can produce tones on the PC speaker.
<P>
</P>
<DT><Q><TT>pit.c</TT></Q>
<DD><DT><Q><TT>pit.h</TT></Q>
<DD>Code to configure the 8254 Programmable Interrupt Timer. This code is
used by both <Q><TT>devices/timer.c</TT></Q> and <Q><TT>devices/speaker.c</TT></Q>
because each device uses one of the PIT's output channel.
</DL>
<P>
<A NAME="lib files"></A>
<HR SIZE="6">
<A NAME="SEC19"></A>
<H4> 2.1.1.2 <Q><TT>lib</TT></Q> files </H4>
<!--docid::SEC19::-->
<P>
Finally, <Q><TT>lib</TT></Q> and <Q><TT>lib/kernel</TT></Q> contain useful library
routines. (<Q><TT>lib/user</TT></Q> will be used by user programs, starting in
project 2, but it is not part of the kernel.) Here's a few more
details:
</P>
<P>
</P>
<DL COMPACT>
<DT><Q><TT>ctype.h</TT></Q>
<DD><DT><Q><TT>inttypes.h</TT></Q>
<DD><DT><Q><TT>limits.h</TT></Q>
<DD><DT><Q><TT>stdarg.h</TT></Q>
<DD><DT><Q><TT>stdbool.h</TT></Q>
<DD><DT><Q><TT>stddef.h</TT></Q>
<DD><DT><Q><TT>stdint.h</TT></Q>
<DD><DT><Q><TT>stdio.c</TT></Q>
<DD><DT><Q><TT>stdio.h</TT></Q>
<DD><DT><Q><TT>stdlib.c</TT></Q>
<DD><DT><Q><TT>stdlib.h</TT></Q>
<DD><DT><Q><TT>string.c</TT></Q>
<DD><DT><Q><TT>string.h</TT></Q>
<DD>A subset of the standard C library. See section <A HREF="pintos_6.html#SEC91">B.2 C99</A>, for
information
on a few recently introduced pieces of the C library that you might
not have encountered before. See section <A HREF="pintos_6.html#SEC92">B.3 Unsafe String Functions</A>, for
information on what's been intentionally left out for safety.
<P>
</P>
<DT><Q><TT>debug.c</TT></Q>
<DD><DT><Q><TT>debug.h</TT></Q>
<DD>Functions and macros to aid debugging. See section <A HREF="pintos_8.html#SEC96">D. Debugging Tools</A>, for
more information.
<P>
</P>
<DT><Q><TT>random.c</TT></Q>
<DD><DT><Q><TT>random.h</TT></Q>
<DD>Pseudo-random number generator. The actual sequence of random values
will not vary from one Pintos run to another, unless you do one of
three things: specify a new random seed value on the <Q><SAMP>-rs</SAMP></Q>
kernel command-line option on each run, or use a simulator other than
Bochs, or specify the <Q><SAMP>-r</SAMP></Q> option to <CODE>pintos</CODE>.
<P>
</P>
<DT><Q><TT>round.h</TT></Q>
<DD>Macros for rounding.
<P>
</P>
<DT><Q><TT>syscall-nr.h</TT></Q>
<DD>System call numbers. Not used until project 2.
<P>
</P>
<DT><Q><TT>kernel/list.c</TT></Q>
<DD><DT><Q><TT>kernel/list.h</TT></Q>
<DD>Doubly linked list implementation. Used all over the Pintos code, and
you'll probably want to use it a few places yourself.
<P>
</P>
<DT><Q><TT>kernel/bitmap.c</TT></Q>
<DD><DT><Q><TT>kernel/bitmap.h</TT></Q>
<DD>Bitmap implementation. You can use this in your code if you like, but
you probably won't have any need for it in project 0 and project 1.
<P>
</P>
<DT><Q><TT>kernel/hash.c</TT></Q>
<DD><DT><Q><TT>kernel/hash.h</TT></Q>
<DD>Hash table implementation.
<P>
</P>
<DT><Q><TT>kernel/console.c</TT></Q>
<DD><DT><Q><TT>kernel/console.h</TT></Q>
<DD><DT><Q><TT>kernel/stdio.h</TT></Q>
<DD>Implements <CODE>printf()</CODE> and a few other functions.
</DL>
<P>
<A NAME="Project 1 Synchronization"></A>
<HR SIZE="6">
<A NAME="SEC20"></A>
<H3> 2.1.2 Synchronization </H3>
<!--docid::SEC20::-->
<P>
Proper synchronization is an important part of the solutions to these
problems. Any synchronization problem can be easily solved by turning
interrupts off: while interrupts are off, there is no concurrency, so
there's no possibility for race conditions. Therefore, it's tempting to
solve all synchronization problems this way, but <STRONG>don't</STRONG>.
Instead, use semaphores, locks, and condition variables to solve the
bulk of your synchronization problems. Read the tour section on
synchronization (see section <A HREF="pintos_5.html#SEC58">A.3 Synchronization</A>) or the comments in
<Q><TT>threads/synch.c</TT></Q> if you're unsure what synchronization primitives
may be used in what situations.
</P>
<P>
In the Pintos projects, the only class of problem best solved by
disabling interrupts is coordinating data shared between a kernel thread
and an interrupt handler. Because interrupt handlers can't sleep, they
can't acquire locks. This means that data shared between kernel threads
and an interrupt handler must be protected within a kernel thread by
turning off interrupts.
</P>
<P>
This project only requires accessing a little bit of thread state from
interrupt handlers. For the alarm clock, the timer interrupt needs to
wake up sleeping threads. In the advanced scheduler, the timer
interrupt needs to access a few global and per-thread variables. When
you access these variables from kernel threads, you will need to disable
interrupts to prevent the timer interrupt from interfering.
</P>
<P>
When you do turn off interrupts, take care to do so for the least amount
of code possible, or you can end up losing important things such as
timer ticks or input events. Turning off interrupts also increases the
interrupt handling latency, which can make a machine feel sluggish if
taken too far.
</P>
<P>
The synchronization primitives themselves in <Q><TT>synch.c</TT></Q> are
implemented by disabling interrupts. You may need to increase the
amount of code that runs with interrupts disabled here, but you should
still try to keep it to a minimum.
</P>
<P>
Disabling interrupts can be useful for debugging, if you want to make
sure that a section of code is not interrupted. You should remove
debugging code before turning in your project. (Don't just comment it
out, because that can make the code difficult to read.)
</P>
<P>
There should be no busy waiting in your submission. A tight loop that
calls <CODE>thread_yield()</CODE> is one form of busy waiting.
</P>
<P>
<A NAME="Development Suggestions"></A>
<HR SIZE="6">
<A NAME="SEC21"></A>
<H3> 2.1.3 Development Suggestions </H3>
<!--docid::SEC21::-->
<P>
In the past, many groups divided the assignment into pieces, then each
group member worked on his or her piece until just before the
deadline, at which time the group reconvened to combine their code and
submit. <STRONG>This is a bad idea. We do not recommend this
approach.</STRONG> Groups that do this often find that two changes conflict
with each other, requiring lots of last-minute debugging. Some groups
who have done this have turned in code that did not even compile or
boot, much less pass any tests.
</P>
<P>
Instead, we recommend integrating your team's changes early and often,
using the source code control system git.
This is less likely to produce surprises, because everyone can see
everyone else's code as it is written, instead of just when it is
finished. These systems also make it possible to review changes and,
when a change introduces a bug, drop back to working versions of code.
</P>
<P>
You should expect to run into bugs that you simply don't understand
while working on this and subsequent projects. When you do,
reread the appendix on debugging tools, which is filled with
useful debugging tips that should help you to get back up to speed
(see section <A HREF="pintos_8.html#SEC96">D. Debugging Tools</A>). Be sure to read the section on backtraces
(see section <A HREF="pintos_8.html#SEC100">D.4 Backtraces</A>), which will help you to get the most out of every
kernel panic or assertion failure.
</P>
<P>
<A NAME="Understanding User Programs"></A>
<HR SIZE="6">
<A NAME="SEC22"></A>
<H2> 2.2 Understanding User Programs </H2>
<!--docid::SEC22::-->
<P>
The tests for both the alarm clock assignment in Project 0, and the
priority scheduler in Project 1, run as part of the operating system
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.
</P>
<P>
We allow more than one process to run at a time. Each process has one
thread (multithreaded processes are not supported). User programs are
written under the illusion that they have the entire machine. This
means that when you load and run multiple processes at a time, you must
manage memory, scheduling, and other state correctly to maintain this
illusion.
</P>
<P>
In Project 2, we will test your operating system by running
user programs. This gives you much greater freedom. You must make sure
that the user program interface meets the specifications described here,
but given that constraint you are free to restructure or rewrite kernel
code however you wish.
</P>
<P>
<A NAME="User Program Source Files"></A>
<HR SIZE="6">
<A NAME="SEC23"></A>
<H3> 2.2.1 Source Files </H3>
<!--docid::SEC23::-->
<P>
</P>
<DL COMPACT>
<DT><Q><TT>process.c</TT></Q>
<DD><DT><Q><TT>process.h</TT></Q>
<DD>Loads ELF binaries and starts processes.
<P>
</P>
<DT><Q><TT>pagedir.c</TT></Q>
<DD><DT><Q><TT>pagedir.h</TT></Q>
<DD>A simple manager for 80<VAR>x</VAR>86 hardware page tables.
Although you probably won't want to modify this code for this project,
you may want to call some of its functions.
See <A HREF="pintos_4.html#Page Tables">Page Tables</A>, for more information.
<P>
</P>
<DT><Q><TT>syscall.c</TT></Q>
<DD><DT><Q><TT>syscall.h</TT></Q>
<DD>Whenever a user process wants to access some kernel functionality, it
invokes a system call.
<P>
</P>
<DT><Q><TT>exception.c</TT></Q>
<DD><DT><Q><TT>exception.h</TT></Q>
<DD>When a user process performs a privileged or prohibited operation, it
traps into the kernel as an "exception" or "fault."<A NAME="DOCF2" HREF="pintos_fot.html#FOOT2">(2)</A> These files handle
exceptions. In project 2, you will need to modify the page fault
handler to support lazy page loading and stack growth.
<P>
</P>
<DT><Q><TT>gdt.c</TT></Q>
<DD><DT><Q><TT>gdt.h</TT></Q>
<DD>The 80<VAR>x</VAR>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. You should not need to modify these
files for any of the projects. You can read the code if
you're interested in how the GDT works.
<P>
</P>
<DT><Q><TT>tss.c</TT></Q>
<DD><DT><Q><TT>tss.h</TT></Q>
<DD>The Task-State Segment (TSS) is used for 80<VAR>x</VAR>86 architectural
task switching. Pintos uses the TSS only for switching stacks when a
user process enters an interrupt handler, as does Linux. You
should not need to modify these files for any of the projects.
You can read the code if you're interested in how the TSS
works.
</DL>
<P>
<A NAME="Using the File System"></A>
<HR SIZE="6">
<A NAME="SEC24"></A>
<H3> 2.2.2 Using the File System </H3>
<!--docid::SEC24::-->
<P>
You will need to interface to the file system code, because
user programs are loaded from the file system and most of the
system calls you must implement deal with the file system.
You will want to look over the <Q><TT>filesys.h</TT></Q> and <Q><TT>file.h</TT></Q>
interfaces to understand how to use the file system, and especially
its many limitations.
</P>
<P>
There is no need to modify the file system code in this course, and so
we recommend that you do not. Working on the file system is likely to
distract you from the project's foci.
</P>
<P>
You will have to tolerate the following limitations, however:
</P>
<P>
<UL>
<LI>
No internal synchronization. Concurrent accesses will interfere with one
another. You should use synchronization to ensure that only one process at a
time is executing file system code.
<P>
</P>
<LI>
File size is fixed at creation time. The root directory is
represented as a file, so the number of files that may be created is also
limited.
<P>
</P>
<LI>
File data is allocated as a single extent, that is, data in a single
file must occupy a contiguous range of sectors on disk. External
fragmentation can therefore become a serious problem as a file system is
used over time.
<P>
</P>
<LI>
No subdirectories.
<P>
</P>
<LI>
File names are limited to 14 characters.
<P>
</P>
<LI>
A system crash mid-operation may corrupt the disk in a way
that cannot be repaired automatically. There is no file system repair
tool anyway.
</UL>
<P>
One important feature is included:
</P>
<P>
<UL>
<LI>
Unix-like semantics for <CODE>filesys_remove()</CODE> are implemented.
That is, if a file is open when it is removed, its blocks
are not deallocated and it may still be accessed by any
threads that have it open, until the last one closes it. See <A HREF="pintos_2.html#Removing an Open File">Removing an Open File</A>, for more information.
</UL>
<P>
You need to be able to create a simulated disk with a file system
partition. The <CODE>pintos-mkdisk</CODE> program provides this
functionality. From the <Q><TT>userprog/build</TT></Q> directory, execute
<CODE>pintos-mkdisk filesys.dsk --filesys-size=2</CODE>. This command
creates a simulated disk named <Q><TT>filesys.dsk</TT></Q> that contains a 2
MB Pintos file system partition. Then format the file system
partition by passing <Q><SAMP>-f -q</SAMP></Q> on the kernel's command line:
<CODE>pintos -f -q</CODE>. The <Q><SAMP>-f</SAMP></Q> option causes the file system to
be formatted, and <Q><SAMP>-q</SAMP></Q> causes Pintos to exit as soon as the
format is done.
</P>
<P>
You'll need a way to copy files in and out of the simulated file system.
The <CODE>pintos</CODE> <Q><SAMP>-p</SAMP></Q> ("put") and <Q><SAMP>-g</SAMP></Q> ("get")
options do this. To copy <Q><TT><VAR>file</VAR></TT></Q> into the
Pintos file system, use the command <Q><TT>pintos -p <VAR>file</VAR> -- -q</TT></Q>.
(The <Q><SAMP>--</SAMP></Q> is needed because <Q><SAMP>-p</SAMP></Q> is for the <CODE>pintos</CODE>
script, not for the simulated kernel.) To copy it to the Pintos file
system under the name <Q><TT><VAR>newname</VAR></TT></Q>, add <Q><SAMP>-a
<VAR>newname</VAR></SAMP></Q>: <Q><TT>pintos -p <VAR>file</VAR> -a <VAR>newname</VAR> -- -q</TT></Q>. The
commands for copying files out of a VM are similar, but substitute
<Q><SAMP>-g</SAMP></Q> for <Q><SAMP>-p</SAMP></Q>.
</P>
<P>
Incidentally, these commands work by passing special commands
<CODE>extract</CODE> and <CODE>append</CODE> on the kernel's command line and copying
to and from a special simulated "scratch" partition. If you're very
curious, you can look at the <CODE>pintos</CODE> script as well as
<Q><TT>filesys/fsutil.c</TT></Q> to learn the implementation details.
</P>
<P>
Here's a summary of how to create a disk with a file system partition,
format the file system, copy the <CODE>echo</CODE> program into the new
disk, and then run <CODE>echo</CODE>, passing argument <CODE>x</CODE>.
(Argument passing won't work until you implemented it.) It assumes
that you've already built the examples in <Q><TT>examples</TT></Q> and that the
current directory is <Q><TT>userprog/build</TT></Q>:
</P>
<P>
<TABLE><tr><td> </td><td class=example><pre>pintos-mkdisk filesys.dsk --filesys-size=2
pintos -f -q
pintos -p ../../examples/echo -a echo -- -q
pintos -q run 'echo x'
</pre></td></tr></table><P>
The three final steps can actually be combined into a single command:
</P>
<P>
<TABLE><tr><td> </td><td class=example><pre>pintos-mkdisk filesys.dsk --filesys-size=2
pintos -p ../../examples/echo -a echo -- -f -q run 'echo x'
</pre></td></tr></table><P>
If you don't want to keep the file system disk around for later use or
inspection, you can even combine all four steps into a single command.
The <CODE>--filesys-size=<VAR>n</VAR></CODE> option creates a temporary file
system partition
approximately <VAR>n</VAR> megabytes in size just for the duration of the
<CODE>pintos</CODE> run. The Pintos automatic test suite makes extensive
use of this syntax:
</P>
<P>
<TABLE><tr><td> </td><td class=example><pre>pintos --filesys-size=2 -p ../../examples/echo -a echo -- -f -q run 'echo x'
</pre></td></tr></table><P>
You can delete a file from the Pintos file system using the <CODE>rm
<VAR>file</VAR></CODE> kernel action, e.g. <CODE>pintos -q rm <VAR>file</VAR></CODE>. Also,
<CODE>ls</CODE> lists the files in the file system and <CODE>cat
<VAR>file</VAR></CODE> prints a file's contents to the display.
</P>
<P>
<A NAME="How User Programs Work"></A>
<HR SIZE="6">
<A NAME="SEC25"></A>
<H3> 2.2.3 How User Programs Work </H3>
<!--docid::SEC25::-->
<P>
Pintos can run normal C programs, as long as they fit into memory and use
only the system calls you implement. Notably, <CODE>malloc()</CODE> cannot be
implemented because none of the system calls required for this project
allow for memory allocation. Pintos also can't run programs that use
floating point operations, since the kernel doesn't save and restore the
processor's floating-point unit when switching threads.
</P>
<P>
The <Q><TT>src/examples</TT></Q> directory contains a few sample user
programs. The <Q><TT>Makefile</TT></Q> in this directory
compiles the provided examples, and you can edit it
compile your own programs as well. Some of the example programs will
not work with the current implementation of Pintos.
</P>
<P>
Pintos can load <EM>ELF</EM> executables with the loader provided for you
in <Q><TT>userprog/process.c</TT></Q>. ELF is a file format used by Linux,
Solaris, and many other operating systems for object files,
shared libraries, and executables. You can actually use any compiler
and linker that output 80<VAR>x</VAR>86 ELF executables to produce programs
for Pintos. (We've provided compilers and linkers that should do just
fine.)
</P>
<P>
You should realize immediately that, until you copy a
test program to the simulated file system, Pintos will be unable to do
useful work. You won't be able to do
interesting things until you copy a variety of programs to the file system.
You might want to create a clean reference file system disk and copy that
over whenever you trash your <Q><TT>filesys.dsk</TT></Q> beyond a useful state,
which may happen occasionally while debugging.
</P>
<P>
<A NAME="Virtual Memory Layout"></A>
<HR SIZE="6">
<A NAME="SEC26"></A>
<H3> 2.2.4 Virtual Memory Layout </H3>
<!--docid::SEC26::-->
<P>
Virtual memory in Pintos is divided into two regions: user virtual
memory and kernel virtual memory. User virtual memory ranges from
virtual address 0 up to <CODE>PHYS_BASE</CODE>, which is defined in
<Q><TT>threads/vaddr.h</TT></Q> and defaults to <TT>0xc0000000</TT> (3 GB). Kernel
virtual memory occupies the rest of the virtual address space, from
<CODE>PHYS_BASE</CODE> up to 4 GB.
</P>
<P>
User virtual memory is per-process.
When the kernel switches from one process to another, it
also switches user virtual address spaces by changing the processor's
page directory base register (see <CODE>pagedir_activate()</CODE> in
<Q><TT>userprog/pagedir.c</TT></Q>). <CODE>struct thread</CODE> contains a pointer to a
process's page table.
</P>
<P>
Kernel virtual memory is global. It is always mapped the same way,
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</CODE>. That is, virtual address
<CODE>PHYS_BASE</CODE> accesses physical
address 0, virtual address <CODE>PHYS_BASE</CODE> + <TT>0x1234</TT> accesses
physical address <TT>0x1234</TT>, and so on up to the size of the machine's
physical memory.
</P>
<P>
A user program can only access its own user virtual memory. An attempt to
access kernel virtual memory causes a page fault, handled by
<CODE>page_fault()</CODE> in <Q><TT>userprog/exception.c</TT></Q>, 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
access memory at an unmapped user virtual address
will cause a page fault.
</P>
<P>
<A NAME="Typical Memory Layout"></A>
<HR SIZE="6">
<A NAME="SEC27"></A>
<H4> 2.2.4.1 Typical Memory Layout </H4>
<!--docid::SEC27::-->
<P>
Conceptually, each process is
free to lay out its own user virtual memory however it
chooses. In practice, user virtual memory is laid out like this:
</P>
<P>
<CENTER>
<TABLE><tr><td> </td><td class=example><pre> PHYS_BASE +----------------------------------+
| user stack |
| | |
| | |
| V |
| grows downward |
| |
| |
| |
| |
| grows upward |
| ^ |
| | |
| | |
+----------------------------------+
| uninitialized data segment (BSS) |
+----------------------------------+
| initialized data segment |
+----------------------------------+
| code segment |
0x08048000 +----------------------------------+
| |
| |
| |
| |
| |
0 +----------------------------------+
</pre></td></tr></table></CENTER>
<P>
In this project, the user stack is fixed in size, but in project 2 it
will be allowed to grow. Traditionally, the size of the uninitialized
data segment can be adjusted with a system call, but you will not have
to implement this.
</P>
<P>
The code segment in Pintos starts at user virtual address
<TT>0x08084000</TT>, approximately 128 MB from the bottom of the address
space. This value is specified in [ <A HREF="pintos_10.html#SysV-i386">SysV-i386</A>] and has no deep
significance.
</P>
<P>
The linker sets the layout of a user program in memory, as directed by a
"linker script" that tells it the names and locations of the various
program segments. You can learn more about linker scripts by reading
the "Scripts" chapter in the linker manual, accessible via <Q><SAMP>info
ld</SAMP></Q>.
</P>
<P>
To view the layout of a particular executable, run <CODE>objdump</CODE>
(80<VAR>x</VAR>86) or <CODE>i386-elf-objdump</CODE> (SPARC) with the <Q><SAMP>-p</SAMP></Q>
option.
</P>
<P>
<A NAME="Accessing User Memory"></A>
<HR SIZE="6">
<A NAME="SEC28"></A>
<H3> 2.2.5 Accessing User Memory </H3>
<!--docid::SEC28::-->
<P>
As part of a system
call, the kernel must often access memory through pointers provided by a user
program. The kernel must be very careful about doing so, because
the user can pass a null pointer, a pointer to
unmapped virtual memory, or a pointer to kernel virtual address space
(above <CODE>PHYS_BASE</CODE>). All of these types of invalid pointers must
be rejected without harm to the kernel or other running processes, by
terminating the offending process and freeing its resources.
</P>
<P>
There are at least two reasonable ways to do this correctly. The
first method is to verify
the validity of a user-provided pointer, then dereference it.
The second method is to check only that a user
pointer points below <CODE>PHYS_BASE</CODE>, then dereference it.
An invalid user pointer will cause a "page fault" that you can
handle by modifying the code for <CODE>page_fault()</CODE> in
<Q><TT>userprog/exception.c</TT></Q>. 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). It is also the way
access to user pointers is implemented in the Pintos version provided.
</P>
<P>
In either case, one needs to make sure not to "leak" resources. For
example, suppose that your system call has acquired a lock or
allocated memory with <CODE>malloc()</CODE>. If you encounter an invalid user pointer
afterward, you must still be sure to release the lock or free the page
of memory. If you choose to verify user pointers before dereferencing
them, this should be straightforward. It's more difficult to handle
if an invalid pointer causes a page fault,
because there's no way to return an error code from a memory access.
</P>
<P>
<A NAME="Project 0 Requirements"></A>
<HR SIZE="6">
<A NAME="SEC29"></A>
<H2> 2.3 Requirements </H2>
<!--docid::SEC29::-->
<P>
<A NAME="Project 0 Design Document"></A>
<HR SIZE="6">
<A NAME="SEC30"></A>
<H3> 2.3.1 Design Document </H3>
<!--docid::SEC30::-->
<P>
Before you turn in your project, you must copy <A HREF="start.tmpl">the
project 0 design document template</A> into your source tree under the name
<Q><TT>pintos/src/intro/DESIGNDOC</TT></Q> and fill it in. We recommend that
you read the design document template before you start working on the
project. See section <A HREF="pintos_7.html#SEC93">C. Project Documentation</A>, for a sample design document
that goes along with a fictitious project.
</P>
<P>
<A NAME="Alarm Clock"></A>
<HR SIZE="6">
<A NAME="SEC31"></A>
<H3> 2.3.2 Alarm Clock </H3>
<!--docid::SEC31::-->
<P>
Reimplement <CODE>timer_sleep()</CODE>, defined in <Q><TT>devices/timer.c</TT></Q>.
Although a working implementation is provided, it "busy waits," that
is, it spins in a loop checking the current time and calling
<CODE>thread_yield()</CODE> until enough time has gone by. Reimplement it to
avoid busy waiting.
</P>
<P>
<A NAME="IDX1"></A>
</P>
<DL>
<DT><U>Function:</U> void <B>timer_sleep</B> (int64_t <VAR>ticks</VAR>)
<DD>Suspends execution of the calling thread until time has advanced by at
least <VAR>x</VAR> timer ticks. Unless the system is otherwise idle, the
thread need not wake up after exactly <VAR>x</VAR> ticks. Just put it on
the ready queue after they have waited for the right amount of time.
<P>
<CODE>timer_sleep()</CODE> is useful for threads that operate in real-time,
e.g. for blinking the cursor once per second.
</P>
<P>
The argument to <CODE>timer_sleep()</CODE> is expressed in timer ticks, not in
milliseconds or any another unit. There are <CODE>TIMER_FREQ</CODE> timer
ticks per second, where <CODE>TIMER_FREQ</CODE> is a macro defined in
<CODE>devices/timer.h</CODE>. The default value is 100. We don't recommend
changing this value, because any change is likely to cause many of
the tests to fail.
</P>
</DL>
<P>
Separate functions <CODE>timer_msleep()</CODE>, <CODE>timer_usleep()</CODE>, and
<CODE>timer_nsleep()</CODE> do exist for sleeping a specific number of
milliseconds, microseconds, or nanoseconds, respectively, but these will
call <CODE>timer_sleep()</CODE> automatically when necessary. You do not need
to modify them.
</P>
<P>
If your delays seem too short or too long, reread the explanation of the
<Q><SAMP>-r</SAMP></Q> option to <CODE>pintos</CODE> (see section <A HREF="pintos_1.html#SEC6">1.1.4 Debugging versus Testing</A>).
</P>
<P>
The tests for the <A HREF="pintos_2.html#SEC31">2.3.2 Alarm Clock</A> assignment are executed by changing the
working directory <Q><TT>intro</TT></Q>. The run <Q><SAMP>make</SAMP></Q> to build the
Pintos kernel. Finally run <Q><SAMP>make check</SAMP></Q> to run the tests,
followed by <Q><SAMP>make grade</SAMP></Q> to obtain your score.
</P>
<P>
The alarm clock implementation is not needed for later projects.
</P>
<P>
<A NAME="Argument Passing"></A>
<HR SIZE="6">
<A NAME="SEC32"></A>
<H3> 2.3.3 Argument Passing </H3>
<!--docid::SEC32::-->
<P>
Currently, <CODE>process_execute()</CODE> does not support passing arguments to
new processes. Implement this functionality, by extending
<CODE>process_execute()</CODE> so that instead of simply taking a program file
name as its argument, it divides it into words at spaces. The first
word is the program name, the second word is the first argument, and so
on. That is, <CODE>process_execute("grep foo bar")</CODE> should run
<CODE>grep</CODE> passing two arguments <CODE>foo</CODE> and <CODE>bar</CODE>.
</P>
<P>
Within a command line, multiple spaces are equivalent to a single
space, so that <CODE>process_execute("grep foo bar")</CODE>
is equivalent to our original example. You can impose a reasonable
limit on the length of the command line arguments. For example, you
could limit the arguments to those that will fit in a single page (4
kB). (There is an unrelated limit of 128 bytes on command-line
arguments that the <CODE>pintos</CODE> utility can pass to the kernel.)
</P>
<P>
You can parse argument strings any way you like. If you're lost,
look at <CODE>strtok_r()</CODE>, prototyped in <Q><TT>lib/string.h</TT></Q> and
implemented with thorough comments in <Q><TT>lib/string.c</TT></Q>. You can
find more about it by looking at the man page (run <CODE>man strtok_r</CODE>
at the prompt).
</P>
<P>
See section <A HREF="pintos_2.html#SEC39">2.5.1 Program Startup Details</A>, for information on exactly how you
need to set up the stack.
</P>
<P>
<A NAME="Project 0 FAQ"></A>
<HR SIZE="6">
<A NAME="SEC33"></A>
<H2> 2.4 FAQ </H2>
<!--docid::SEC33::-->
<P>
</P>
<DL COMPACT>
<DT><B>How much code will I need to write?</B>
<DD><P>
Here's a summary of our reference solution, produced by the
<CODE>diffstat</CODE> program. The final row gives total lines inserted
and deleted; a changed line counts as both an insertion and a deletion.
</P>
<P>
The reference solution represents just one possible solution. Many
other solutions are also possible and many of those differ greatly from
the reference solution. Some excellent solutions may not modify all the
files modified by the reference solution, and some may modify files not
modified by the reference solution.
</P>
<P>
<TABLE><tr><td> </td><td class=example><pre> devices/timer.c | 40 +++++++++++-
threads/thread.h | 3 +
userprog/process.c | 148 ++++++++++++++++++++++++++++++-----------
3 files changed, 150 insertions(+), 41 deletions(-)
</pre></td></tr></table></DL>
<P>
<A NAME="Threads FAQ"></A>
<HR SIZE="6">
<A NAME="SEC34"></A>
<H3> 2.4.1 Threads FAQ </H3>
<!--docid::SEC34::-->
<P>
</P>
<DL COMPACT>
<DT><B>How do I update the <Q><TT>Makefile</TT></Q>s when I add a new source file?</B>
<DD><P>
<A NAME="Adding Source Files"></A>
To add a <Q><TT>.c</TT></Q> file, edit the top-level <Q><TT>Makefile.build</TT></Q>.
Add the new file to variable <Q><SAMP><VAR>dir</VAR>_SRC</SAMP></Q>, where
<VAR>dir</VAR> is the directory where you added the file. For this
project, that means you should add it to <CODE>threads_SRC</CODE> or
<CODE>devices_SRC</CODE>. Then run <CODE>make</CODE>. If your new file
doesn't get
compiled, run <CODE>make clean</CODE> and then try again.
</P>
<P>
When you modify the top-level <Q><TT>Makefile.build</TT></Q> and re-run
<CODE>make</CODE>, the modified
version should be automatically copied to
<Q><TT>threads/build/Makefile</TT></Q>. The converse is
not true, so any changes will be lost the next time you run <CODE>make
clean</CODE> from the <Q><TT>threads</TT></Q> directory. Unless your changes are
truly temporary, you should prefer to edit <Q><TT>Makefile.build</TT></Q>.
</P>
<P>
A new <Q><TT>.h</TT></Q> file does not require editing the <Q><TT>Makefile</TT></Q>s.
</P>
<P>
</P>
<DT><B>What does <CODE>warning: no previous prototype for `<VAR>func</VAR>'</CODE> mean?</B>
<DD><P>
It means that you defined a non-<CODE>static</CODE> function without
preceding it by a prototype. Because non-<CODE>static</CODE> functions are
intended for use by other <Q><TT>.c</TT></Q> files, for safety they should be
prototyped in a header file included before their definition. To fix
the problem, add a prototype in a header file that you include, or, if
the function isn't actually used by other <Q><TT>.c</TT></Q> files, make it
<CODE>static</CODE>.
</P>
<P>
</P>
<DT><B>What is the interval between timer interrupts?</B>
<DD><P>
Timer interrupts occur <CODE>TIMER_FREQ</CODE> times per second. You can
adjust this value by editing <Q><TT>devices/timer.h</TT></Q>. The default is
100 Hz.
</P>
<P>
We don't recommend changing this value, because any changes are likely
to cause many of the tests to fail.
</P>
<P>
</P>
<DT><B>How long is a time slice?</B>
<DD><P>
There are <CODE>TIME_SLICE</CODE> ticks per time slice. This macro is
declared in <Q><TT>threads/thread.c</TT></Q>. The default is 4 ticks.
</P>
<P>
We don't recommend changing this value, because any changes are likely
to cause many of the tests to fail.
</P>
<P>
</P>
<DT><B>How do I run the tests?</B>
<DD><P>
See section <A HREF="pintos_1.html#SEC8">1.2.1 Testing</A>.
</P>
<P>
See section <A HREF="pintos_8.html#SEC100">D.4 Backtraces</A>, for more information.
</DL>
<P>
<A NAME="Alarm Clock FAQ"></A>
<HR SIZE="6">
<A NAME="SEC35"></A>
<H3> 2.4.2 Alarm Clock FAQ </H3>
<!--docid::SEC35::-->
<P>
</P>
<DL COMPACT>
<DT><B>Do I need to account for timer values overflowing?</B>
<DD><P>
Don't worry about the possibility of timer values overflowing. Timer
values are expressed as signed 64-bit numbers, which at 100 ticks per
second should be good for almost 2,924,712,087 years. By then, we
expect Pintos to have been phased out of the curriculum.
</DL>
<P>
<A NAME="Userprog FAQ"></A>
<HR SIZE="6">
<A NAME="SEC36"></A>
<H3> 2.4.3 Userprog FAQ </H3>
<!--docid::SEC36::-->
<P>
</P>
<DL COMPACT>
<DT><B>The kernel always panics when I run <CODE>pintos -p <VAR>file</VAR> -- -q</CODE>.</B>
<DD><P>
Did you format the file system (with <Q><SAMP>pintos -f</SAMP></Q>)?
</P>
<P>
Is your file name too long? The file system limits file names to 14
characters. A command like <Q><SAMP>pintos -p ../../examples/echo -- -q</SAMP></Q>
will exceed the limit. Use <Q><SAMP>pintos -p ../../examples/echo -a echo
-- -q</SAMP></Q> to put the file under the name <Q><TT>echo</TT></Q> instead.
</P>
<P>
Is the file system full?
</P>
<P>
Does the file system already contain 16 files? The base Pintos file
system has a 16-file limit.
</P>
<P>
The file system may be so fragmented that there's not enough contiguous
space for your file.
</P>
<P>
</P>
<DT><B>When I run <CODE>pintos -p ../file --</CODE>, <Q><TT>file</TT></Q> isn't copied.</B>
<DD><P>
Files are written under the name you refer to them, by default, so in
this case the file copied in would be named <Q><TT>../file</TT></Q>. You
probably want to run <CODE>pintos -p ../file -a file --</CODE> instead.
</P>
<P>
You can list the files in your file system with <CODE>pintos -q ls</CODE>.
</P>
<P>
</P>
<DT><B>All my user programs die with page faults.</B>
<DD><P>
This will happen if you haven't implemented argument passing
(or haven't done so correctly). The basic C library for user programs tries
to read <VAR>argc</VAR> and <VAR>argv</VAR> off the stack. If the stack
isn't properly set up, this causes a page fault.
</P>
<P>
</P>
<DT><B>How can I disassemble user programs?</B>
<DD><P>
The <CODE>objdump</CODE> (80<VAR>x</VAR>86) or <CODE>i386-elf-objdump</CODE>
(SPARC) utility can disassemble entire user
programs or object files. Invoke it as <CODE>objdump -d
<VAR>file</VAR></CODE>. You can use GDB's
<CODE>disassemble</CODE> command to disassemble individual functions
(see section <A HREF="pintos_8.html#SEC102">D.5 GDB</A>).
</P>
<P>
</P>
<DT><B>Why do many C include files not work in Pintos programs?</B>
<DD><DT><B>Can I use lib<VAR>foo</VAR> in my Pintos programs?</B>
<DD><P>
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.)
</P>
<P>
The chances are good that the library you want uses parts of the C library
that Pintos doesn't implement. It will probably take at least some
porting effort to make it work under Pintos. Notably, the Pintos
user program C library does not have a <CODE>malloc()</CODE> implementation.
</P>
<P>
</P>
<DT><B>How do I compile new user programs?</B>
<DD><P>
Modify <Q><TT>src/examples/Makefile</TT></Q>, then run <CODE>make</CODE>.
</P>
<P>
</P>
<DT><B>Can I run user programs under a debugger?</B>
<DD><P>
Yes, with some limitations. See section <A HREF="pintos_8.html#SEC102">D.5 GDB</A>.
</P>
<P>
</P>
<DT><B>How can I run user programs that need more than 4 kB stack space?</B>
<DD><P>
You may modify the stack setup code to allocate more than one page of
stack space for each process. In project 2, you will implement a better
solution.
</P>
<P>
</P>
<DT><B>What happens when an open file is removed?</B>
<DD><A NAME="Removing an Open File"></A>
<P>
You should implement the standard Unix semantics for files. That is, when
a file is removed any process which has a file descriptor for that file
may continue to use 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.
</P>
<P>
</DL>
<P>
<A NAME="Argument Passing FAQ"></A>
<HR SIZE="6">
<A NAME="SEC37"></A>
<H3> 2.4.4 Argument Passing FAQ </H3>
<!--docid::SEC37::-->
<P>
</P>
<DL COMPACT>
<DT><B>Isn't the top of stack in kernel virtual memory?</B>
<DD><P>
The top of stack is at <CODE>PHYS_BASE</CODE>, typically <TT>0xc0000000</TT>, which
is also where kernel virtual memory starts.
But before the processor pushes data on the stack, it decrements the stack
pointer. Thus, the first (4-byte) value pushed on the stack
will be at address <TT>0xbffffffc</TT>.
</P>
<P>
</P>
<DT><B>Is <CODE>PHYS_BASE</CODE> fixed?</B>
<DD><P>
No. You should be able to support <CODE>PHYS_BASE</CODE> values that are
any multiple of <TT>0x10000000</TT> from <TT>0x80000000</TT> to <TT>0xf0000000</TT>,
simply via recompilation.
</DL>
<P>
<A NAME="80x86 Calling Convention"></A>
<HR SIZE="6">
<A NAME="SEC38"></A>
<H2> 2.5 80<VAR>x</VAR>86 Calling Convention </H2>
<!--docid::SEC38::-->
<P>
This section summarizes important points of the convention used for
normal function calls on 32-bit 80<VAR>x</VAR>86 implementations of Unix.
Some details are omitted for brevity. If you do want all the details,
refer to [ <A HREF="pintos_10.html#SysV-i386">SysV-i386</A>].
</P>
<P>
The calling convention works like this:
</P>
<P>
<OL>
<LI>
The caller pushes each of the function's arguments on the stack one by
one, normally using the <CODE>PUSH</CODE> assembly language instruction.
Arguments are pushed in right-to-left order.
<P>
The stack grows downward: each push decrements the stack pointer, then
stores into the location it now points to, like the C expression
<Q><SAMP>*--sp = <VAR>value</VAR></SAMP></Q>.
</P>
<P>
</P>
<LI>
The caller pushes the address of its next instruction (the <EM>return
address</EM>) on the stack and jumps to the first instruction of the callee.
A single 80<VAR>x</VAR>86 instruction, <CODE>CALL</CODE>, does both.
<P>
</P>
<LI>
The callee executes. When it takes control, the stack pointer points to
the return address, the first argument is just above it, the second
argument is just above the first argument, and so on.
<P>
</P>
<LI>
If the callee has a return value, it stores it into register <CODE>EAX</CODE>.
<P>
</P>
<LI>
The callee returns by popping the return address from the stack and
jumping to the location it specifies, using the 80<VAR>x</VAR>86 <CODE>RET</CODE>
instruction.
<P>
</P>
<LI>
The caller pops the arguments off the stack.
</OL>
<P>
Consider a function <CODE>f()</CODE> that takes three <CODE>int</CODE> arguments.
This diagram shows a sample stack frame as seen by the callee at the
beginning of step 3 above, supposing that <CODE>f()</CODE> is invoked as
<CODE>f(1, 2, 3)</CODE>. The initial stack address is arbitrary:
</P>
<P>
<CENTER>
<TABLE><tr><td> </td><td class=example><pre> +----------------+
0xbffffe7c | 3 |
0xbffffe78 | 2 |
0xbffffe74 | 1 |
stack pointer --> 0xbffffe70 | return address |
+----------------+
</pre></td></tr></table></CENTER>
<P>
<A NAME="Program Startup Details"></A>
<HR SIZE="6">
<A NAME="SEC39"></A>
<H3> 2.5.1 Program Startup Details </H3>
<!--docid::SEC39::-->
<P>
The Pintos C library for user programs designates <CODE>_start()</CODE>, in
<Q><TT>lib/user/entry.c</TT></Q>, as the entry point for user programs. This
function is a wrapper around <CODE>main()</CODE> that calls <CODE>exit()</CODE> if
<CODE>main()</CODE> returns:
</P>
<P>
<TABLE><tr><td> </td><td class=example><pre>void
_start (int argc, char *argv[])
{
exit (main (argc, argv));
}
</pre></td></tr></table><P>
The kernel must put the arguments for the initial function on the stack
before it allows the user program to begin executing. The arguments are
passed in the same way as the normal calling convention (see section <A HREF="pintos_2.html#SEC38">2.5 80<VAR>x</VAR>86 Calling Convention</A>).
</P>
<P>
Consider how to handle arguments for the following example command:
<Q><SAMP>/bin/ls -l foo bar</SAMP></Q>.
First, break the command into words: <Q><SAMP>/bin/ls</SAMP></Q>,
<Q><SAMP>-l</SAMP></Q>, <Q><SAMP>foo</SAMP></Q>, <Q><SAMP>bar</SAMP></Q>. Place the words at the top of the
stack. Order doesn't matter, because they will be referenced through
pointers.
</P>
<P>
Then, push the address of each string plus a null pointer sentinel, on
the stack, in right-to-left order. These are the elements of
<CODE>argv</CODE>. The null pointer sentinel ensures that <CODE>argv[argc]</CODE>
is a null pointer, as required by the C standard. The order ensures
that <CODE>argv[0]</CODE> is at the lowest virtual address. Word-aligned
accesses are faster than unaligned accesses, so for best performance
round the stack pointer down to a multiple of 4 before the first push.
</P>
<P>
Then, push <CODE>argv</CODE> (the address of <CODE>argv[0]</CODE>) and <CODE>argc</CODE>,
in that order. Finally, push a fake "return address": although the
entry function will never return, its stack frame must have the same
structure as any other.
</P>
<P>
The table below shows the state of the stack and the relevant registers
right before the beginning of the user program, assuming
<CODE>PHYS_BASE</CODE> is <TT>0xc0000000</TT>:
</P>
<P>
<CENTER>
</P>
<TABLE>
<TR><TD>Address </TD><TD> Name </TD><TD> Data </TD><TD> Type</TD>
</TR>
<TR><TD><TT>0xbffffffc</TT> </TD><TD> <CODE>argv[3][<small>...</small>]</CODE> </TD><TD> <Q><SAMP>bar\0</SAMP></Q> </TD><TD> <CODE>char[4]</CODE></TD>
</TR>
<TR><TD><TT>0xbffffff8</TT> </TD><TD> <CODE>argv[2][<small>...</small>]</CODE> </TD><TD> <Q><SAMP>foo\0</SAMP></Q> </TD><TD> <CODE>char[4]</CODE></TD>
</TR>
<TR><TD><TT>0xbffffff5</TT> </TD><TD> <CODE>argv[1][<small>...</small>]</CODE> </TD><TD> <Q><SAMP>-l\0</SAMP></Q> </TD><TD> <CODE>char[3]</CODE></TD>
</TR>
<TR><TD><TT>0xbfffffed</TT> </TD><TD> <CODE>argv[0][<small>...</small>]</CODE> </TD><TD> <Q><SAMP>/bin/ls\0</SAMP></Q> </TD><TD> <CODE>char[8]</CODE></TD>
</TR>
<TR><TD><TT>0xbfffffec</TT> </TD><TD> word-align </TD><TD> 0 </TD><TD> <CODE>uint8_t</CODE></TD>
</TR>
<TR><TD><TT>0xbfffffe8</TT> </TD><TD> <CODE>argv[4]</CODE> </TD><TD> <TT>0</TT> </TD><TD> <CODE>char *</CODE></TD>
</TR>
<TR><TD><TT>0xbfffffe4</TT> </TD><TD> <CODE>argv[3]</CODE> </TD><TD> <TT>0xbffffffc</TT> </TD><TD> <CODE>char *</CODE></TD>
</TR>
<TR><TD><TT>0xbfffffe0</TT> </TD><TD> <CODE>argv[2]</CODE> </TD><TD> <TT>0xbffffff8</TT> </TD><TD> <CODE>char *</CODE></TD>
</TR>
<TR><TD><TT>0xbfffffdc</TT> </TD><TD> <CODE>argv[1]</CODE> </TD><TD> <TT>0xbffffff5</TT> </TD><TD> <CODE>char *</CODE></TD>
</TR>
<TR><TD><TT>0xbfffffd8</TT> </TD><TD> <CODE>argv[0]</CODE> </TD><TD> <TT>0xbfffffed</TT> </TD><TD> <CODE>char *</CODE></TD>
</TR>
<TR><TD><TT>0xbfffffd4</TT> </TD><TD> <CODE>argv</CODE> </TD><TD> <TT>0xbfffffd8</TT> </TD><TD> <CODE>char **</CODE></TD>
</TR>
<TR><TD><TT>0xbfffffd0</TT> </TD><TD> <CODE>argc</CODE> </TD><TD> 4 </TD><TD> <CODE>int</CODE></TD>
</TR>
<TR><TD><TT>0xbfffffcc</TT> </TD><TD> return address </TD><TD> 0 </TD><TD> <CODE>void (*) ()</CODE></TD>
</TR></TABLE>
</CENTER>
<P>
In this example, the stack pointer would be initialized to
<TT>0xbfffffcc</TT>.
</P>
<P>
As shown above, 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</CODE> (defined in <Q><TT>threads/vaddr.h</TT></Q>).
</P>
<P>
You may find the non-standard <CODE>hex_dump()</CODE> function, declared in
<Q><TT><stdio.h></TT></Q>, useful for debugging your argument passing code.
Here's what it would show in the above example:
</P>
<P>
<TABLE><tr><td> </td><td class=example><pre>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 00-66 6f 6f 00 62 61 72 00 |n/ls.-l.foo.bar.|
</pre></td></tr></table><P>
<A NAME="System Call Details"></A>
<HR SIZE="6">
<A NAME="SEC40"></A>
<H3> 2.5.2 System Call Details </H3>
<!--docid::SEC40::-->
<P>
We already know 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 (see section <A HREF="pintos_5.html#SEC68">A.4.3 External Interrupt Handling</A>).
</P>
<P>
The operating system also deals with software exceptions, which are
events that occur in program code (see section <A HREF="pintos_5.html#SEC67">A.4.2 Internal Interrupt Handling</A>). These can be errors such as a page fault or division by
zero. Exceptions are also the means by which a user program
can request services ("system calls") from the operating system.
</P>
<P>
In the 80<VAR>x</VAR>86 architecture, the <Q><SAMP>int</SAMP></Q> instruction is the
most commonly used means for invoking system calls. This instruction
is handled in the same way as other software exceptions. In Pintos,
user programs invoke <Q><SAMP>int $0x30</SAMP></Q> 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 (see section <A HREF="pintos_2.html#SEC38">2.5 80<VAR>x</VAR>86 Calling Convention</A>).
</P>
<P>
Thus, when the system call handler <CODE>syscall_handler()</CODE> 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()</CODE> as the <Q><SAMP>esp</SAMP></Q> member of the
<CODE>struct intr_frame</CODE> passed to it. (<CODE>struct intr_frame</CODE> is on the kernel
stack.)
</P>
<P>
The 80<VAR>x</VAR>86 convention for function return values is to place them
in the <CODE>EAX</CODE> register. System calls that return a value can do
so by modifying the <Q><SAMP>eax</SAMP></Q> member of <CODE>struct intr_frame</CODE>.
</P>
<P>
You should try to avoid writing large amounts of repetitive code for
implementing system calls. Each system call argument, whether an
integer or a pointer, takes up 4 bytes on the stack. You should be able
to take advantage of this to avoid writing much near-identical code for
retrieving each system call's arguments from the stack.
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