[SOLVED] Comp8051 – operating system engineering assignment 1
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Install qemu on your Ubuntu vm
sudo apt-get update && sudo apt-get install git nasm build-essential
qemu gdb
Download xv6 using git.
Open a terminal window
git clone git://github.com/mit-pdos/xv6-public.git
cd xv6-public
run make to build xv6
and
make qemu
to run xv6
Execute some shell commands to get familiar with the user part of the OS. The
shell commands are separate programs e.g. ls.c, cat.c.
To get a feel for how programs look in xv6, and how various APIs should be
called, you can look at the source code for other utilities: echo.c, cat.c, wc.c, ls.c.Hints:
In places where something asks for a file descriptor, you can use either an actual
file descriptor (i.e., the return value of the open function), or one of the standard
I/O descriptors: 0 is “standard input”, 1 is “standard output”, and 2 is “standard
error”. Writing to either 1 or 2 will result in something being printed to the
screen.The standard header files used by xv6 programs are “types.h” (to define some
standard data types) and “user.h” (to declare some common functions). You can
look at these files to see what code they contain and what functions they define.Write a program for xv6 that, prints “Hello world” to the xv6 console. This can be
broken up into a few steps:
1. Create the file hello.c in the xv6 directory
2. Edit the file Makefile, find the section UPROGS (which contains a list of
programs to be built), and add a line to tell it to build your hello.c code. When
you’re done that portion of the Makefile should look like:
UPROGS=
_cat
_echo
_forktest
_grep
_init
_kill
_ln
_ls
_mkdir
_rm
_sh
_stressfs
_usertests
_wc
_zombie
_hello
3. Run make to build xv6, including your new program
4. Run make qemu to launch xv6, and then type hello in the QEMU window. You
should see “Hello world” being printed out.
(1 mark)a) Write a program that prints the first 10 lines of its input for the xv6 operating
system. If a filename is provided on the command line (i.e., head FILE) then
head should open it, read and print the first 10 lines, and then close it. If no
filename is provided, head should read from standard input.
See how the program cat.c works e.g
cat README
grep the README | head
should show ten lines each line has the word the in it.Hints:
Many aspects of this are similar to the wc program: both can read from standard
input if no arguments are passed or read from a file if one is given on the
command line. Reading its code will help you if you get stuck.
b) What is a user program for the xv6 operating system? Explain how a user
program links to library functions such as printf and how does it access the
operating system?
c) Explain how the xv6 shell works – refer to the xv6 source code for your
answer.
d) Explain how xv6 implements the ls program – refer to the xv6 source code for
your answer.
(10 marks)Add basic versions of the commands cp, mv, to xv6. The cp and mv commands
are to work on files only (no dirs). System calls to be used are reported in
brackets. Please see user.h (on xv6) for a complete list of syscalls and library
functions available.These commands are available on Linux.
* cp src dst (open, read, write)
* mv oldname newname (link, unlink)
The build procedure can be broken up into a few steps:
1. Create the file cp.c in the xv6 directory
2. Edit the file Makefile, find the section UPROGS (which contains a list of
programs to be built), and add a line to tell it to build your cp.c code.
3. Run make to build xv6, including your new program
4. Run make qemu to launch xv6, and then type execute cp in the QEMU
window.
(4 marks)a) Read chapter 3 of the XV6 book and the xv6 source and describe how
xv6 dispatches interrupts and system calls – refer to the xv6 source code
for your answer.b) Explain how the keyboard driver buffers keystrokes for the xv6
operating system.
(12 marks)The trace syntax is
int trace(int)
When called with a non-zero parameter, e.g., trace(1), system call tracing is
turned on for that process. Each system call from that process will be printed to
the console in a user-friendly format showing:
the process ID
the process name
the system call number
the system call name
Any other processes will not have their system calls printed unless they also call
trace(1).Calling trace(0) turns tracing off for that process. System calls will no longer be
printed to the console
In all cases, the trace system call also returns the total number of system calls
that the process has made since it started. Hence, you can write code such as:
printf(“total system calls so far = %d ”, trace(0));
How to add a new system call to XV6You need to touch several files to add a system call in xv6. Look at the
implementation of existing system calls for guidance on how to add a new one.
The files that you need to edit to add a new system call include:
user.h
This contains the user-side function prototypes of system calls as well as utility
library functions (stat, strcpy, printf, etc.).
syscall.hThis file ontains symbolic definitions of system call numbers. You need to define
a unique number for your system call. Be sure that the numbers are consecutive.
That is, there are no missing number in the sequence. These numbers are
indices into a table of pointers defined in syscall.c (see next item).
syscall.cThis file contains entry code for system call processing. The syscall(void)
function is the entry function for all system calls. Each system call is identified by
a unique integer, which is placed in the processor’s eax register. The syscall
function checks the integer to ensure that it is in the appropriate range and then
calls the corresponding function that implements that call by making an indirect
funciton call to a function in the syscalls[] table. You need to ensure that the
kernel function that implements your system call is in the proper sequence in the
syscalls array.
usys.SThis file contains macros for the assembler code for each system call. This is user
code (it will be part of a user-level program) that is used to make a system call.
The macro simply places the system call number into the eax register and then
invokes the system call. You need to add a macro entry for your system call
here.
sysproc.c
This is a collection of process-related system calls. The functions in this file are
called from syscall. You can add your new function to this file.
Per-process state is stored in a proc structure: struct proc in proc.h. You’ll need
to extend that structure to keep track of the process related metrics. You’ll also
need to find where the proc structure is allocated so that you can ensure that the
elements are initialized appropriately.When you implement your trace call, you’ll need to retrieve the incoming
parameter. The file sysproc.c defines a few helper functions to do this. The
functions argint, argptr, and argstr retrieve the n th system call argument, as
either an integer, pointer, or a string. argint uses the esp register to locate the
argument: esp points at the return address for the system call stub.Implementation steps
1. Write a test program.
Add the test program to the Makefile so that it will be compiled and built
whenever you run make.
In the Makefile, add your program (e.g., try.c) to the list of user commands in
the UPROGS= section. That should be all you need to do to that file.Beware that programs don’t have access to the typical stdio library that you
expect to find on most systems. You’ll have many of the functions you expect
but some of the behavior might be different. For example, printf accepts an
initial parameter that is the output stream: 1 represents the standard output and
2 represents the standard error stream. There is no FILE* type and no fopen,
fclose, fgets, etc. calls. Look through usertests.c for examples on how all of the
system calls provided with xv6 are used.2. Add system call tracing to the kernel.
Print a message identifying everysystem call that is requested by any process as
well as the process ID and process name. You do not need to print the
arguments to the system calls. When you run any program, including the shell,
you will see output similar to this:
…
pid: 2 [sh] syscall(5=read)
pid: 2 [sh] syscall(5=read)
pid: 2 [sh] syscall(1=fork)
pid: 2 [sh] syscall(3=wait)
pid: 3 [sh] syscall(12=sbrk)
pid: 3 [sh] syscall(7=exec)
pid: 3 [try] syscall(20=mkdir)
pid: 3 [try] syscall(15=open)
pid: 3 [try] syscall(16=write)
pid: 3 [try] syscall(21=close)
pid: 3 [try] syscall(2=exit)
pid: 2 [sh] syscall(16=write)
pid: 2 [sh] syscall(16=write)
…
Use the cprintf function in the kernel, which prints using direct access to the vga
controller. It works just like the normal Linux printf function. For example:
cprintf(“hello, I’m number %d ”, num);3. Restrict this output to a single process.
Create a new system call called trace(int). This turns console-based system call
logging on and off for only the calling process.
Extend the proc structure (the process control block) in proc.h to keep track of
whether tracing for the process is on or off. Be sure that the elements are
cleared (initialized) whenever a new process is created.In implementing your system call, you’ll need to access the single parameter
passed by trace. Use the helper functions defined in syscall.c (argint, argptr,
and argstr). Take a look at how other system calls are implemented in xv6. For
example, getpid is a simple system call that takes no arguments and returns the
current process’ process ID; kill and sleep are examples of system calls that take
a single integer parameter.4. Add system call counting.
Be sure to count calls on a per-process basis. You
will need to keep track of this in the process control block, the proc structure.
(7 marks)The process table and struct proc in proc.h are used to maintain information on
the current processes that are running in the XV6 kernel. Since ps is a user space
program, it cannot access the process table in the kernel. So we’ll add a new
system call. The ps command should print:
process id
parent process id
state
size
nameThe system call you need to add to xv6 has the following interface:
int getprocs(int max, struct uproc table[]);
struct uproc is defined as (add it to a new file uproc.h):
struct uproc {
int pid;
int ppid;
int state;
uint sz;
char name[16];
};Your ps program calls getprocs with an array of struct proc objects and sets max
to the size of that array (measured in struct uproc objects). Your kernel code
copies up to max entries into your array, starting at the first slot of the array and
filling it consecutively. The kernel returns the actual number of processes in
existence at that point in time, or -1 if there was an error.
(6 marks)