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<title>Homework: xv6</title>
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<h1>Lab: xv6</h1>
This lab makes you familiar with xv6 and its system calls.
<h2>Boot xv6</h2>
<p>Login to Athena (e.g., ssh -X athena.dialup.mit.edu) and attach the course
locker: (You must run this command every time you log in; or add it to your
~/.environment file.)
<pre>
$ add -f 6.828
</pre>
<p>Fetch the xv6 source:
<pre>
$ mkdir 6.828
$ cd 6.828
$ git clone git://github.com/mit-pdos/xv6-riscv.git
Cloning into 'xv6-riscv'...
...
$
</pre>
<p>XXX pointer to an update tools page
<p>Build xv6 on Athena:
<pre>
$ cd xv6-public
$ makeriscv64-linux-gnu-gcc -c -o kernel/entry.o kernel/entry.S
riscv64-linux-gnu-gcc -Wall -Werror -O -fno-omit-frame-pointer -ggdb -MD -mcmodel=medany -ffreestanding -fno-common -nostdlib -mno-relax -I. -fno-stack-protector -fno-pie -no-pie -c -o kernel/start.o kernel/start.c
...
$ make qemu
...
mkfs/mkfs fs.img README user/_cat user/_echo user/_forktest user/_grep user/_init user/_kill user/_ln user/_ls user/_mkdir user/_rm user/_sh user/_stressfs user/_usertests user/_wc user/_zombie user/_cow
nmeta 46 (boot, super, log blocks 30 inode blocks 13, bitmap blocks 1) blocks 954 total 1000
balloc: first 497 blocks have been allocated
balloc: write bitmap block at sector 45
qemu-system-riscv64 -machine virt -kernel kernel/kernel -m 3G -smp 3 -nographic -drive file=fs.img,if=none,format=raw,id=x0 -device virtio-blk-device,drive=x0,bus=virtio-mmio-bus.0
hart 0 starting
hart 2 starting
hart 1 starting
init: starting sh
$
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<p>
If you type <tt>ls</tt> at the prompt, you should output similar to the following:
<pre>
$ ls
. 1 1 1024
.. 1 1 1024
README 2 2 2181
cat 2 3 21024
echo 2 4 19776
forktest 2 5 11456
grep 2 6 24512
init 2 7 20656
kill 2 8 19856
ln 2 9 19832
ls 2 10 23280
mkdir 2 11 19952
rm 2 12 19936
sh 2 13 38632
stressfs 2 14 20912
usertests 2 15 106264
wc 2 16 22160
zombie 2 17 19376
cow 2 18 27152
console 3 19 0
</pre>
These are the programs/files that <tt>mkfs</tt> includes in the
initial file system. You just ran one of them: <tt>ls</tt>.
<h2>sleep</h2>
<p>Write a program that sleeps for a user-specified number of seconds,
compile it, and run it.
<p>Some hints:
<ul>
<li>Look at some of the other programs in <tt>user/</tt> to see
how you can obtain the arguments passed to a program. If the user
forgets to pass an argument, sleep should print an error message.
<li>The argument is passed as a string; you can convert it to an
integer using <tt>atoi</tt> (see user/ulib.c).
<li>Use the system call <tt>sleep</tt> (see user/usys.S).
<li>Make sure <tt>main</tt> calls <tt>exit()</tt> in order to exit
your program.
<li>Add the program to <tt>UPROGS</tt> in Makefile and compile
user programs by typing <tt>make fs.img</tt>.
</ul>
<p>Run the program from the xv6 shell:
<pre>
$ make qemu
...
init: starting sh
$ sleep 5
(waits for a little while)
$
</pre>
<p>Optional: write an uptime program that prints the uptime in terms
of ticks using the <tt>uptime</tt> system call.
<h2>pingpong</h2>
<p>In the previous exercise, if you made an error in sleep, the
program may have exited prematurely, but it didn't affect other
processes because xv6 isolates processes. Sometimes you want
processes to interact with each other. Xv6 provides two ways to do:
either through the file system (one process can create a file and
another process can read that file) or through pipes. In this
exercise you explore interprocess communication through pipes.
<p> Write a program that uses UNIX system calls to ``ping-pong'' a
byte between two processes over a pair of pipes, one for each
direction. The parent sends by writing a byte to <tt>fd[1]</tt> and
the child receives it by reading from <tt>fd[0]</tt>. After
receiving a byte from parent, the child responds with its own byte
by writing to <tt>fd[1]</tt>, which the parent then reads.
<p>Some hints:
<ul>
<li>Use <tt>pipe</tt> to create a pipe.
<li>Use <tt>fork</tt> to create a child.
<li>Use <tt>read</tt> to read from the pipe, and <tt>write</tt> to write to the pipe.
</ul>
<h2>find</h2>
<h2>Optional: modify shell</h2>
<p>Modify the shell to support lists of commands, separated by ";"
<p>Modify the shell to support sub-shells by implementing "(" and ")"
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