Introduction
This page contains information about the Cr-48 Chrome Notebook that is interesting and/or useful to software developers. For general information about getting started with developing on Chromium OS (the open-source version of the software on the Chrome Notebook), see the Chromium OS Developer Guide. Entering Developer Mode
You might want to enter developer mode if you're following the instructions in the Chromium OS Developer Guide, or if you just want to get access to a shell on your device to poke around. To get your device into Developer Mode, you'll need to flip the developer switch to the "Developer Mode" position.
Caution: Modifications you make to the system are not supported by Google, may cause hardware issues and may void warranty.
The first time a Chrome Notebook boots in Developer Mode after leaving Normal Mode it will:
- Show
a scary warning that its software cannot be trusted, since verified
boot is disabled (press Ctrl-D or wait 30 seconds to dismiss).
- Erase all personal data on the "stateful partition" (i.e., user accounts and settings - no worries, though, since all data is in the cloud!).
- Make you wait between 5 and 10 minutes to while it erases the data.
- Boot from any self-signed image on its SSD, negating the security of verified boot.
The erase and wait steps only happen when you first switch to Developer Mode, to help prevent someone from quickly reimaging your device while you're away from the keyboard. Successive boots in Developer Mode will only:
- Show a scary warning that its software cannot be trusted, since verified boot is disabled (press Ctrl-D or wait 30 seconds to dismiss).
- Boot from any self-signed image on its SSD, negating the security of verified boot.
Entering Developer mode is easy:
- Remove the battery.
- Peel off the sticker that hides the developer switch (see image 1).
- Flip the developer switch towards the battery connector (see image 2).
- Put the battery back in.
- Turn the device back on.
- Press Ctrl-D at the scary warning screen.
- Wait 5-10 minutes and any saved information on your device will be erased.
- Congratulations, enjoy hacking in Developer Mode!
Here are some pictures that might help:
Image 1: Location of Developer Switch
Image 2: Developer Switch in "Developer Mode" position.
Getting the recovery kernel
Leaving Developer Mode
Returning to normal mode is as simple as entering developer mode: shut down, remove the battery, flip the switch, reboot. There are a couple of things to note, however.
First, assuming you haven't modified anything, the first time you boot
in normal mode after leaving developer mode, the stateful partition will
be erased. This is a much faster erase process than when entering
developer mode, usually only 30 or 40 seconds, and only happens with the
first boot.
Second, verified boot will be enabled, meaning that only Google-signed images will be bootable. If you haven't modified the original kernel or rootfs partitions in any way, you should have no problems. If you've made changes to the kernel partitions, the Cr-48 will refuse to boot that kernel and will display a recovery screen. You'll have to create a recovery USB drive to restore your Cr-48 to the factory condition. If you've made changes to the rootfs partition but not the kernel, the Cr-48 may appear to boot normally, but may later suddenly reboot and/or display the recovery screen. This happens because the kernel verifies the rootfs as each block is read from the SSD, so it may not encounter a modified block until sometime later. When it does, it will reboot immediately.
How to use the Recovery Mode button
Recovery mode is a special boot operation in which the BIOS will:
- Refuse to boot from the SSD
- Prompt you to insert a recovery USB drive
- Only boot a Google-signed image from the USB drive
You will encounter recovery mode when the BIOS is unable to find a valid kernel to boot, either because the SSD has become corrupted or (more likely) because you modified all the kernel partitions while in developer mode and have switched back to normal mode. While in developer mode, you will be presented with the scary boot screen at every boot. Pressing SPACE or RETURN will take you to recovery mode.
You can also force your Cr-48 into recovery mode (even in normal mode) by using the recovery mode button.
On the bottom of the Cr-48, directly below the ESC key, there is a tiny pinhole:
If you stick a paperclip into this hole, you'll feel it press a button. To force recovery mode, turn the Cr-48 off, press this button, and while keeping it pressed, turn the Cr-48 on again. This sometimes requires three hands or a bit of contortion, but you'll know it worked when you see the recovery screen instead of booting normally.
There is little use for this button other than booting a recovery USB drive. In developer mode you can run your own scripts from the recovery USB drive, but in developer mode you can trigger recovery mode by just pressing SPACE at boot. Still, there it is.
How to boot your own (non-Chromium OS) image from USB
Sometimes,
you just want to have a shell or maybe you want to install another
operating system. If you're in Developer Mode, then you can totally do
that.
This tip could actually be useful
to someone who wasn't a Chromium OS developer. However, at the moment
the only easy way to get the tools is to follow many of the steps in
the Chromium OS Developer Guide.
If you don't care about developing for Chromium OS and just want to
get to booting your own stuff, you should read the developer guide with
these thoughts in mind:
- The instructions in the Chromium OS Developer Guide assume that you're running the Ubuntu Lucid distribution of Linux. If you're not, you may not be able to follow them.
- You
should only need to follow the instructions up to the point of "Enter
the chroot". You don't actually need to build a Chromium OS image. You
probably also want to choose the "minilayout" when downloading the
source code, since that will be faster.
The rest of the
instructions will assume that you've followed the instructions enough to
make a chroot and that you're currently in the chroot.
Getting the recovery kernel for your Chrome OS Notebook
You'll
need to get a "recovery kernel" for your Chrome OS Notebook in order to
follow these instructions. The instructions for that are also in the
Developer Guide, though in a different context. You should follow the
instructions in the section "Get the Recovery Kernel for your Chrome OS
Notebook" under the section Getting your image running on a Chrome OS Notebook. Creating a USB disk that will boot a simple script
Using the make_developer_script_runner.sh tool,
you can create a USB disk that will run shell script. In this example,
we'll show how to create a USB disk with a script that will leave you
running the shell.
Create the script
echo -e '#!/bin/sh\nexec /bin/sh\n' > run_a_shell.sh
Build a disk image
Now, use make_developer_script_runner.sh. The script will create a file called dev_runner_image.bin that can be placed on a USB disk. Note that these instructions assume that you've set the RECOVERY_KERNEL variable properly, as would happen if you followed the instructions in the Developer Guide.
./make_developer_script_runner.sh --kernel_image=${RECOVERY_KERNEL} --developer_script=run_a_shell.sh
Find your USB disk
You
should plug a USB disk in to your computer now and wait a few seconds
for it to be detected. You can then run this quick command to use some
Chromium OS tools to detect the disk and find out where it is:
bash -c 'source chromeos-common.sh;
echo
for disk in $(list_usb_disks);
do echo /dev/$disk - $(get_disk_info $disk manufacturer) - $(get_disk_info $disk product);
done'
...assuming
your disk was detected, it should print out as a result of that
command. For instance, on my computer, I see this, which tells me that
my USB disk is in /dev/sdc:
/dev/sdc - Kingston - DataTraveler G3
Format your USB disk with the disk image
WARNING: This step will erase all data on your USB disk. Make sure there's nothing important on it.
ALSO: If you mistype this step (don't put the right /dev/sdX in the command), it can blow away some other disk on your computer.
Really, I'm serious. Be very careful. If you shoot yourself in the foot with these instructions, you only get to blame yourself.
OK, now that you've read all of the disclaimers, the command you'll need to enter is this, replacing ${MY_USB_KEY_LOCATION} with the /dev/sdX value that is for your USB disk (see the "Find your USB disk" instruction above).
TODO:
You may need to unmount your USB disk before running this instruction
if Ubuntu mounted it for you. Put a set of instructions that tell how
to do that.
sudo dd if=dev_runner_image.bin of=${MY_USB_KEY_LOCATION} bs=16M conv=fsync
Boot your Chrome OS Notebook with your USB image
Now,
shut off your Chrome OS Notebook and turn it back on. Since you're in
Developer Mode (right?), you should see the warning. Hit the space bar
to enter recovery mode. Once prompted, insert your USB disk.
Assuming
that you haven't reimaged the built-in SSD with a developer-signed
image, you'll need to wait 5 minutes each time you boot up with this
disk image. This is a security precaution, to prevent someone from quickly rebooting your device from their USB key while you're getting more coffee (see the Developer Mode design document).
Create a USB disk that will boot another operating system
TODO:
This should be possible for someone skilled in the art of Linux. The
above instructions tell you how to run any arbitrary Linux program, and
(with root access) you should be able to do pretty much anything you
want.
It would be nice to get some good instructions here, though.
How to install a different OS on your SSD
There's an example of configuring a Cr-48 to dual-boot Chrome OS and Ubuntu here.
What's inside?
WARNING: Opening the case and fiddling with the stuff inside could easily brick your system and make it unrecoverable. DO NOT ATTEMPT if you are not familiar with this process.
Components
Here is a rundown of the parts that are not soldered down:
- Power supply: 19.5V
 (DC) 2.05A  (positive polarity tip)
- SSD: 16GiB mSATA
- RAM: one slot for 204-Pin DDR3 SO-DIMM laptop memory
Disassembly
Taking apart your laptop is not encouraged. If you have hardware troubles, please seek assistance from a friend knowledgeable in the area. If you just want to see what the inside looks like, gaze upon this (click for a high res version):
 
However, we acknowledge that some people like to tinker. So here's a quick guide to taking it apart:
- Shut the system down and close the lid
- Flip the laptop over so the bottom is facing up
- Pop out the battery using the release switch in the middle of the case
- Remove the visible screws
- There should be 10 in total (a line of 3 black, a line of 3 silver, and a line of 4 silver)
- The black screws are M2x2x0.4 while the silver screws are M2x3x0.4
- Remove the two screws under the rubber feet (M2.5x6x0.45)
- If you pry them up starting at the edge closest to the hinge, you're less likely to strip the glue
- Using a flat instrument (your nails, a wide screw driver, etc...), pry the case apart starting at the side w/the VGA port
- Work your way around the front and to the other side
- By now, it should be pretty easy to wiggle the whole thing apart
You now have access to the upgradable components.
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ď Douglas Anderson, Dec 8, 2010, 2:31 PM ď Douglas Anderson, Dec 8, 2010, 2:30 PM
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