This is a VM running Linux used for software development -- it contains Xinu sources
and a text editor (e.g., vi) that can used to modify them.  It also contains a C
compiler/assembler/linker (gcc) used to produce a binary image.  It also runs a
DHCP server as well as a TFTP server that are both connected to an internal network.
Once an image has been produced, it is copied to /srv/tftp/xinu.boot and a backend
VM is powered on.  The backend VM is configured to use PXE boot, which means it
broadcasts a DHCP request that is answered by the server.  The response tells the
backend machine to download file "xinu.grub" -- a copy of the grub boot program set
up to boot Xinu.  When it runs, grub sets up the VM in 32-bit "protected" mode,
downloads Xinu, and branches to symbol "start".  Intel chips always boot in 16-bit
"real" mode (as if we were living in the 1980s), and changing to 32-bit mode is painful
because it involves CPU-specific steps.  So, we let grub handle the details.

Interestingly, although both VMs have access to the global Internet, there is no
problem with a TFTP or DHCP server because the servers only run on an "internal"
network (i.e., a virtual network that is only visible to VMs inside of Vbox).
To obtain Internet access, each VM is configured with two network adapters.  The
diagram below shows how various components in the VMs use the two networks.
Xinu only has a driver for an Intel 82545EM Ethernet device.  Therefore, it only
looks for such a device, and only finds Adapter 2 (i.e., it cannot send or
receive packets over the internal network).  Like a commerical NAT box (i.e., a
wireless router), the NAT box runs a DHCP server and provides IP addresses from a
private address space.  If more than one backend VM is powered on, they will all
obtain the same Xinu image as other backend VMs, but each will have a unique IP
address, so they can communicate with one another.


           This VM (runs Linux)                      Backend VM (runs Xinu)

 ----------------------------------------     ---------------------------------------
|                                        |   |                                       |
|  ---------       --------   --------   |   |                       ----------      |
| |  Other  |     |  DHCP  | |  TFTP  |  |   |                      |   Xinu   |     |
| |  Linux  |     | server | | server |  |   |                       ----------      |
| | network |      --------   --------   |   |  ----------               |           |
| |  comm.  |         |          |       |   | | PXE boot |              |           |
|  ---------          |          |       |   |  ----------               |           |
|     |                ----      |       |   |      |                    |           |
|     |                    |     |       |   |      |                    |           |
|  -----------          -----------      |   |  -----------          -----------     |
| | Adapter 2 |        | Adapter 1 |     |   | | Adapter 1 |        | Adapter 2 |    |
|  -----------          -----------      |   |  -----------          -----------     |
|      |                    |            |   |      |                    |           |
|      |                    |            |   |      |                    |           |
 ------|--------------------|------------     ------|--------------------|------------
       |                    |                       |                    |
       |                    |  internal network     |                    |
       |                 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~                 |
       |                                                                 |
       |            Private addresses "natted" to the Internet           |
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
                                          |
                                          |
                                       -------
                                      |  NAT  |
                                       -------
                                          |
                                          V
                                  To the global Internet


To try Xinu, navigate to xinu/compile, and run make, which will link an image and
copy the result to /srv/tftp/xinu.boot.  Then run:

	sudo minicom --color=on

Once minicom starts, power on the backend VM.  It will PXE-boot grub, which will
then boot xinu.boot.  All output from Xinu will appear in the minicom window, and
input typed to the minicom window will be sent to Xinu.

The software development process repeats the following steps:

	1. Change Xinu.  For example,  navigate to the system directory, and add
	   the following to main.c: 

		kprintf("Hello world. I'm trying something new.\n");

	2. Navigate to the compile directory and run make.

	3. Run minicom --color=on

	4. Double click on the backend VM.

	5. Once you are finished, power down the backend VM.