i. The kernel.
Android (like many other Smartphone operating systems) runs on the Linux kernel. The Linux kernel was created in the early 1990ís by a gentleman named Linus Torvalds in Helsinki, Finland. Itís incredibly stable, incredibly friendly, and incredibly difficult for the layman to understand and modify. Thankfully itís also very popular so it has been ported on to a multitude of hardware, including our Android devices.
Think of the kernel as an interface layer between the hardware and software on your device. The kernel decides when things happen, such as the LED indicator gets lit or when the soft button's LED gets lit. An application sends a request to the operating system to blink the LED. The operating system then sends the request to the kernel, which makes the light flash for the amount of time requested by the OS.
What sounds like a round-about way to get things done is also what makes the system so scalable and robust. Application developers only have to code in a way the operating system understands and the kernel makes it work on the hardware. This also keeps the application running in itís own user-space and separate from the kernel. That means when you run the latest uber-cool app that wasnít designed for your particular OS version, or is still very beta and it crashes, the kernel gives you the option to Force Close the application and the kernel can run untouched.
In a standard Android ROM (we will leave developer images and the like for another discussion) the kernel is bundled along with a set of instructions that tell the device how to load the kernel and the OS during boot. This is the boot.img that you see inside a zipped ROM that your not able to easily open. The device knows to extract this image to internal memory (the ramdisk) and follow a series of scripts (init scripts) to load the kernel and then the other portions of the OS. Thatís whatís happening while youíre watching the boot animation. Interestingly enough this is done the same way for a PC, your smartphone, an Android tablet, or even a smart Linux powered toaster. If youíre feeling exceptionally geeky, plug your Android phone into the USB port on your PC and let the PC boot from the USB device. No, it doesnít actually load, but you can watch the animation while it tries to match up the hardware support with whatís inside your PC. As I said, Linux is amazingly scalable and as a result so is Android.
What is a kernel? If you spend any time reading Android forums, blogs, how-to posts or online discussion you'll soon hear people talking about the kernel. A kernel isn't something unique to Android -- iOS and MacOS have one, Windows has one, BlackBerry's QNX has one, in fact all high level operating systems have one. The one we're interested in is Linux, as it's the one Android uses. Let's try to break down what it is and what it does.
Android devices use the Linux kernel, but it's not the exact same kernel other Linux-based operating systems use. There's a lot of Android specific code built in, and Google's Android kernel maintainers have their work cut out for them. OEMs have to contribute as well, because they need to develop hardware drivers for the parts they're using for the kernel version they're using. This is why it takes a while for independent Android developers and hackers to port new versions to older devices and get everything working. Drivers written to work with the Gingerbread kernel on a phone won't necessarily work with the Ice Cream Sandwich kernel. And that's important, because one of the kernel's main functions is to control the hardware. It's a whole lot of source code, with more options while building it than you can imagine, but in the end it's just the intermediary between the hardware and the software.
When software needs the hardware to do anything, it sends a request to the kernel. And when we say anything, we mean anything. From the brightness of the screen, to the volume level, to initiating a call through the radio, even what's drawn on the display is ultimately controlled by the kernel. For example -- when you tap the search button on your phone, you tell the software to open the search application. What happens is that you touched a certain point on the digitizer, which tells the software that you've touched the screen at those coordinates. The software knows that when that particular spot is touched, the search dialog is supposed to open. The kernel is what tells the digitizer to look (or listen, events are "listened" for) for touches, helps figure out where you touched, and tells the system you touched it. In turn, when the system receives a touch event at a specific point from the kernel (through the driver) it knows what to draw on your screen. Both the hardware and the software communicate both ways with the kernel, and that's how your phone knows when to do something. Input from one side is sent as output to the other, whether it's you playing Angry Birds, or connecting to your car's Bluetooth.
It sounds complicated, and it is. But it's also pretty standard computer logic -- there's an action of some sort generated for every event. Without the kernel to accept and send information, developers would have to write code for every single event for every single piece of hardware in your device. With the kernel, all they have to do is communicate with it through the Android system API's, and hardware developers only have to make the device hardware communicate with the kernel. The good thing is that you don't need to know exactly how or why the kernel does what it does, just understanding that it's the go-between from software to hardware gives you a pretty good grasp of what's happening under the glass. Sort of gives a whole new outlook towards those fellows who stay up all night to work on kernels for your phone, doesn't it?
Once the kernel is loaded, the init scripts tell the Operating System to load. Android is the user interface for a custom built Java virtual machine called Dalvik. Dalvik was written by Dan Bornstein, who named it after the fishing village of Dalvik in Iceland, where his family originated from. The debate of which Java VM is superior is best left for another discussion, so Iíll simply say that DalvikVM is a register-based machine versus true JavaVMs which are stack based.
The Dalvik machine creates executable files (.dex files) which can be interpreted by the OS and run by the end user. These .dex files are OS version dependant. That simply means that applications and core functions built to work with one version of Android may or may not work well with other versions. Google provides the tools through itís Software Development Kit (SDK) for applications to communicate with the OS.
No smartphone would be complete without a set of functions that allow the device to be used as intended. Things like the phone and dialer interface, the calendar, the messaging system are core functions of the Operating System. In Android, these are run on top of the kernel as separate applications. The merits (or lack of) of providing these needed functions as separate applications is once again best left for another discussion, but this is what allows developers like HTC or Motorola to replace the standard functions with alternatives that provide a different look and feel from stock. HTCís onscreen keyboard or Motorolaís MotoBlur contact list are great examples of this. The ďlittle guyĒ isnít left out of the mix either. Handcent SMS or Chomp SMS can integrate into the OS very well, as most of us already know.
An additional set of Core Functions are provided by Google. Popularly called GoogleBits, things like Gmail, sync, Gtalk and the Android Market are applications written by Google that give an extra set of useful functions to the OS. Youíll find these on all smartphones, as well as many other Android devices.
These are applications provided by the manufacturer to give the device even more usability. Things like the Amazon MP3 store, PDF readers, Corporate Calendar etc. allow you to do even more with your device. Remember - Droid Does
B. How is a ROM packaged?
In most cases a ROM will come packaged in a .zip file. The recovery imageís kernel (yes, it has one too!) has the ability to unzip and copy the contents into the correct place. Inside this zip file is a folder (META-INF\com\google\android\) that contains a script prepared by the ROM ďcookerĒ (another of those techie terms - it means the person(s) who developed the ROM) that tells the system what to format, what to copy and where, and any file operations that need to be done. Each device does things a bit differently, but this script is where it all gets done. More on this folder later.
Youíll also see a /system folder. This is the meat of the ROM. It has the necessary OS files, the Core functions, and any optional applications the cooker decided to include. The folder is structured the same way it is on your device - /system/app, /system/framework, etc. The whole tree is usually copied over and the existing /system folder is overwritten. The cooker uses the script to tell the kernel to erase the existing system folder, copy the new folder over, and set the file permissions.
Sometimes you will also see a data folder. This usually is space set up for optional applications, including optional system tools like busybox or SuperUser white list. These applications could be placed in the /system folder, but placing them in the data folder makes it easier for the end user (you and I) to remove or update them as needed.
Youíll also notice a META-INF folder. This contains the update script we talked about earlier, as well as secure keys that need to be provided so the device knows the update can be trusted. A special note needs made here. Trusted means that the update is trusted to be in the correct form to load the device. It in no way means the ROM is safe from malicious code. Anyone is able to use a set of test keys and create a ROM that will flash and run your device - even those people with bad intentions. Flashing and running a custom 3rd party ROM is putting faith in the cooker that he or she not only knows what they are doing, but are honest as well. Also, some Motorola custom ROMs will have a small update.zip stored inside this folder to be run on first boot of the device.
Finally we are left with the boot.img file. This is the kernel and ramdisk image we discussed earlier. Your phone copies this over to be decompressed and run when the device boots.
In this section weíre discussing how to install a custom 3rd party ROM. ROMs from the manufacturer usually have a utility that runs on your PC to flash and load the new image.
A. Got Root???
What is Root?
Android rooting is the process of allowing users of smartphones, tablets, and other devices running the Android mobile operating system to attain privileged control (known as "root access") within Android's subsystem.
Rooting is often performed with the goal of overcoming limitations that carriers and hardware manufacturers put on some devices, resulting in the ability to alter or replace system applications and settings, run specialized apps that require administrator-level permissions, or perform other operations that are otherwise inaccessible to a normal Android user. On Android, rooting can also facilitate the complete removal and replacement of the device's operating system, usually with a more recent release of its current operating system.
As Android derives from the Linux kernel, rooting an Android device is similar to accessing administrative permissions on Linux or any other Unix-like operating system such as FreeBSD or OS X.
The process of rooting varies widely by device, but usually includes exploiting a security bug(s) in the firmware (i.e. in Android) of the device, and then copying the su binary to a location in the current process's PATH (e.g. /system/xbin/su) and granting it executable permissions with the chmod command. A supervisor application like SuperUser or SuperSU can regulate and log elevated permission requests from other applications. Many guides, tutorials, and automatic processes exist for popular Android devices facilitating a fast and easy rooting process.
For example, shortly after the HTC Dream was released, it was quickly discovered that anything typed using the keyboard was being interpreted as a command in a privileged (root) shell. Although Google quickly released a patch to fix this, a signed image of the old firmware leaked, which gave users the ability to downgrade and use the original exploit to gain root access. Once an exploit is discovered, a custom recovery image that skips the digital signature check of a firmware update package can be flashed. In turn, using the custom recovery, a modified firmware update can be installed that typically includes the utilities (for example the Superuser app) needed to run apps as root.
The Google-branded Android phones, the Nexus One, Nexus S, Galaxy Nexus and Nexus 4, as well as their tablet counterparts, the Nexus 7 and Nexus 10, can be boot-loader unlocked by simply connecting the device to a computer while in boot-loader mode and running the Fastboot program with the command "fastboot oem unlock". After accepting a warning, the boot-loader is unlocked, so a new system image can be written directly to flash without the need for an exploit.
Recently, Motorola, LG Electronics and HTC added security features to their devices at the hardware level in an attempt to prevent users from rooting retail Android devices. For instance, the Motorola Droid X has a security boot-loader that puts the phone in "recovery mode" if a user loads unsigned firmware onto the device, and the Samsung Galaxy S II displays a yellow triangle indicator if the device firmware has been modified.
Custom ROMís simply will not load on devices that arenít rooted. In theory, it may be possible to sign a 3rd party ROM with the keys that the stock recovery image will flash, but for the most part you need to have flashed a custom recovery image before you can change your deviceís ROM. Instructions and tutorials on how to root your device are all over the internet. Some are good, some are bad. The hacking forum is a great place to go and learn more about rooting and how to successfully get it done on your device.
Most Android devices have had a custom recovery image written for them. This will overwrite the stock recovery image, allowing you to flash 3rd party ROMs as well as giving extra functionality. Help with finding and flashing the custom recovery image for your device can also be found in the hacking forum. The installation of a custom recovery image also allows for a very important function. Backup and restore.
Nandroid is a set of bash scripts and code written by that copies the state of your system and stores it in a folder on your SD card. You can then use the restore function of Nandroid to restore to this point at any time. This is a priceless feature and reason enough to root your phone. Itís included by default in most custom recovery images, and the code is freely available to use if youíre inclined to write your own recovery image.
1. Verify you have a memory card with enough free space (~300MB to backup, ~500MB to restore).
2. Reboot your device into recovery. Itís slightly different for each device, once again hacking forum FTW!
3. Navigate through the menu and select the Nandroid Backup function.
4. Apply your choice and wait for the device to tell you itís finished.
Itís always good practice to copy the entire nandroid folder from your SD card to a safe place. You can then copy it back to the SD card if the card is ever damaged, lost or erased.
D. Copy and Flash
Youíre rooted, have downloaded a custom ROM, have your system backed up and are now ready to flash your device. This is not nearly as scary as it sounds.
1. Mount your SD card to your PC, and copy the .zip file to the root folder of the card. Donít unzip the file, and donít look for a folder called root. The root folder in this case means the base folder, what you will see when you mount your card to a PC or the device.
2. Reboot your phone into recovery.
3. Navigate through the recovery menu and select the flash update option. Depending on your recovery image, the file may need to be named update.zip, or you may be able to select any zip file on your card as long as itís the correct format. The cooker knows this as well and if the ROM needs to be named update.zip it will be.
4. Apply your choice and wait for your device to tell you itís finished.
Dirty flash and Clean flash
A dirty flash is only wiping cache and davlik then flashing your ROM....
a Clean flash is at LEAST factory reset/data wipe + wiping davlik(factory wipe takes care of /cache also)... Maybe doing a format /system also.