- 30 August 2018: Under “Adding the system call” indicate that
syscall.his where system call dispatching is implemented, and mention the files where the actual individual system calls are implemented.
- 31 August 2018: Add some additional notes about options for testing correctness and debugging under “Testing your system call”.
- 1 September 2018: Correct link to xv6 book to link to the commentary book, not the (much less useful) source code book.
- 1 September 2018: Add note that we don’t care how
writecount()works with multiple processors, which are not used by default in our version of xv6.
- 2 September 2018: More careful phrasing of not caring about multiple processors; add explanation of locks not being required to hints, and note this alongside referencing
- 3 Septmeber 2018: Mention user.h in hints.
- 18 August 2019: add section re: this assignment being based on Arpaci-Dusseau’s
- Obtain a copy of a VM or another Linux environment (most likely not OS X, see below) with:
- a working C and C++ compiler
- a recent copy of qemu installed (configured to support full 32-bit x86 emulation).
Download and get xv6 to boot in an emulated machine. For how to do this, see the instructions below. Run it and make sure you can run some simple commands in a shell in the emulated machine.
Add a new system call
writecount()that takes no arguments and returns the number of times the
writesystem call has been called across all processes. See below for hints about how to do this.
(We do not care how your
writecount()system call counts calls to
writethat fail, such as from having invalid arguments. We also don’t care how it treats simulataneous calls to write() and/or writecount() on multiple processors (not used by default in our version of xv6).)
Write a program that uses and tests this new system call. See below for hints on how to do this.
make submitto create a
.tar.gzarchive and upload it to the 4414-002 submission site. (For 4414-001, submit on Collab.)
Getting xv6 running
Download our version of xv6 using git:
git clone https://github.com/uva-reiss-cs4414/xv6.git
In the newly created
Boot xv6 in an emulator using one of the following methods:
To boot the OS in an emulator with a graphical user interface, run
To boot the OS in an emulator without a graphical user interface, run
make qemu-nox. (
noxprobably standards for “no X”; X is short for the X windowing system, the primary graphics system on Unix-like operating systems.)
Adding the system call
For this task, you might want to read
how system call dispatching is implemented in
syscall.h, and how the handlers for individual system calls are implemented in
Basic steps for adding system calls:
sys_writecountfunction based on an existing simple system call function like
sys_uptime. (For this assignment, you do not need to (but are allowed to) use a spinlock and
uptimedoes since we do not care how your code works with multiple processors.)
Add a system call number for your new system call to
sys_writecountto the table in
sys_write. You can use a command like
grep sys_write *.cto find out where it is.
user.hto create a system call wrapper function that invokes your system call from a normal user program.
Testing your system call
echo.cas a template, create a new program to run your writecount system call and print the results.
Makefileby adding your program to
UPROGS, similar to how
echois included on this list.
make qemuto boot the OS with your new program included.
If your test program crashes after finishing, you may have forgotten to
exit()at the end. (Returning from
main()will not work.)
You could run other programs that call
write()(e.g. by outputting anything to the console) and/or have your test program make writes (using the
write()system call wrapping function directly or by taking advantage of
printf()calling write) to verify that the count make sense.
In kernel code, you can do debug printing using the
cprintf()function. You can use this to help debug any problems with your code, or get a better idea what’s going on.
Locks not required
sys_uptimeuses a spinlock, which it uses to handle the case where xv6 is running on multiple processors. For this assignment, you don’t need to worry about this problem.
An implementation of
writecountthat correctly handled multiple procesors would probably use a spinlock around accesses to the counter (from both
xv6 book: note on terminology
CS 3330 and the xv6 book and source code use slightly different terminology related to exceptions:
|CS 3330 Term||most common xv6 term||description|
|exception||trap||any event in which the processor transfers control from whatever program was running to the OS (to the “kernel”) at a location chosen by the OS|
|fault||exception||a program performs an illegal action, causing control to be transferred to the OS to decide what to do|
|interrupt||interrupt||an event external to the program, such as a timer or an input/output device needs the OS’s attention|
|trap||(specific kind) trap||an exception deliberately triggered by an instruction; on x86 this is via the
a note on OS X
Because xv6 expects ELF format binaries, xv6 will require a cross-compiler on OS X. We recommend (and will support) using a Linux environment instead (such as through a VM) instead, but if you can get it working natively on OS X using a cross-compiler, that’s great (and your fellow students would probably appreciate the information).
This assignment is based loosely on Arpaci-Dusseau’s initial-xv6 assignment.