xv6 is a re-implementation of Dennis Ritchie's and Ken Thompson's Unix
Version 6 (v6).  xv6 loosely follows the structure and style of v6,
but is implemented for a modern x86-based multiprocessor using ANSI C.
This version is a fork from the original xv6 implementing the VFS (Virtual
Filesystem) layer to enable more than one filesystem operate in the same
time. A simple implementation of ext2 filesystem is also added.

ACKNOWLEDGMENTS

xv6 is inspired by John Lions's Commentary on UNIX 6th Edition (Peer
to Peer Communications; ISBN: 1-57398-013-7; 1st edition (June 14,
2000)). See also http://pdos.csail.mit.edu/6.828/2012/v6.html, which
provides pointers to on-line resources for v6. You can find documentation
about the VFS implementantio on Documentation/ folder soonly.

xv6 borrows code from the following sources:
    JOS (asm.h, elf.h, mmu.h, bootasm.S, ide.c, console.c, and others)
    Plan 9 (entryother.S, mp.h, mp.c, lapic.c)
    FreeBSD (ioapic.c)
    NetBSD (console.c)
    GNU/Linux (ext2.h, ext2.c)

The following people have made contributions:
    Russ Cox (context switching, locking)
    Cliff Frey (MP)
    Xiao Yu (MP)
    Nickolai Zeldovich
    Austin Clements
    Caio Lima (ide driver changes and VFS layer implementation)

In addition, we are grateful for the bug reports and patches contributed by
Silas Boyd-Wickizer, Peter Froehlich, Shivam Handa, Anders Kaseorg, Eddie
Kohler, Yandong Mao, Hitoshi Mitake, Carmi Merimovich, Joel Nider, Greg Price,
Eldar Sehayek, Yongming Shen, Stephen Tu, and Zouchangwei.

The code in the files that constitute xv6 is
Copyright 2006-2014 Frans Kaashoek, Robert Morris, and Russ Cox.

ERROR REPORTS

If you spot errors or have suggestions for improvement of this fork version,
send email to caiolima@dcc.ufba.br. If you found an error on the official
xv6 code, follow the instruction on the official site (point to the site).

BUILDING AND RUNNING XV6

Tehre are 2 ways to build and run the xv6 OS. The first one is using a
pre-configured vagrant machine and we recommend follow this way to avoid
headaches. The second solution we recommend follow this documentation
https://pdos.csail.mit.edu/6.828/2014/tools.html

Required software:
- Vagrant: https://www.vagrantup.com/downloads.html
- git: https://git-scm.com/book/en/v2/Getting-Started-Installing-Git
- QEMU (Run the xv6 OS on a virtual machine): https://en.wikibooks.org/wiki/QEMU/Installing_QEMU

To configure an environment using vagrant, the first thing to do is
download and install the Vagrant software. you can do this following
isntructions from https://www.vagrantup.com/downloads.html. If you
are not use to use vagrant, don't worry. Almost all configurations
are already done on this Vagrantfile (https://gist.github.com/caiolima/fdc6974c1fec0e57caac).
The workflow using the Vagrant is simple: share the folder where the
xv6 source code is beteween Vagrant machine and host OS. This way,
you can decide your own development env (IDE, Editor, etc.) that you
are accustomed.

After install and configure the vagrant env, you need to download
our xv6 code and place it on the shared folder between vagrant machine
and your host OS. It is recommended clone the source code on the same
folder where the Vagrantfile is placed. You can find this repository on
https://github.com/caiolima/xv6-public or download the code using the
following command:

-------------------------------------------------------
git clone https://github.com/caiolima/xv6-public.git
-------------------------------------------------------

Ps.: This clone can be done from the vagrant machine, because it already
have git installed.

After the clone command, your folder tree shall be like this one supposing
that you placed the Vagrant file on ~/xv6-dev:

-------------------------------------------------------
- Vagrantfile;
- xv6-public;
-------------------------------------------------------

Now, we are almost in the end of the build step. We need to run the commands:

-------------------------------------------------------
cd ~/xv6-dev
vagrant up
vagrant ssh
-------------------------------------------------------

Maybe the first time you run ```vagrant up``` you will need to wait for a while
because the vagrant need to download the VM image. After the download, the load
time will be faster.

When you successfully ssh into the vagrant vm, run the following commands to
build the xv6 OS:

-------------------------------------------------------
cd /vagrant/xv6-public/src
make
-------------------------------------------------------

To create an valid ext2 filesystem, you need to run the following commands on
/vagrant/xv6-public/src folder:

-------------------------------------------------------
dd if=/dev/zero of=ext2.img bs=10M count=1
sudo mkfs.ext2 -b 1024 -T "EXT2_TEST" ext2.img
-------------------------------------------------------

To run and test the xv6, you need to open the terminal application of your host
OS and run:

-------------------------------------------------------
make qemu
-------------------------------------------------------

The QEMU will start and boot the xv6. If you have already compiled the xv6 before
you will notice that you have a program to mount a new filesystem from a new device
to mount a new filesystem, you need to create an folder and run the mount command.
To execute these steps, you need to type the commands on the xv6 terminal:

-------------------------------------------------------
mkdir mnt
mount /dev/hdc mnt ext2
-------------------------------------------------------

The mount program requires 3 parameters: the block device that contais your
FS (/dev/hdc), the folder where this device will be mounted on (mnt) and
the type of the FS (ext2). If you try mount a filesystem that is not implemented
on the xv6, this program will return an error.

Now, you have an mounted on ext2 fs on mnt/ folder. You can run the commands over
this folder, but the current ls program does not work with ext2 filesystems because
they are hardcoded to the xv6 default FS (we call it s5).

VFS DOCUMENTATION

We are documenting the VFS implementation and when we finish it, we will post
on the wiki page of this repository and on Documentation/ folder.
We are working on the following documentations right now:
- The VFS architecture and code comment;
- An tutorial to add a new Filesystem on xv6 using our VFS implementation;