Analysis & Opinion

[Android ABC] What's a Bootloader,ROM,Kernel,Firmware,ADB,Root etc

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By iONEx, Senior Member on 10th October 2012, 11:54 PM
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Android ABC

I've gathered some info for newcomers to the Android world.
Copied over from my thread at androidforums...

I've tried to keep it relatively simple. So if you want more info, follow the links!

And please if you want anything added, do post!

I hope this helps someone....


Custom ROMs

What's A Bootloader?
Taken from: Android 101: What is a bootloader? |

In literal terms, the bootloader is code that is executed before any Operating System starts to run. Bootloaders basically package the instructions to boot operating system kernel and most of them also have their own debugging or modification environment. Think of the bootloader as a security checkpoint for all those partitions. Because if you’re able to swap out what’s on those partitions, you’re able to break things if you don’t know what you’re doing.

As the bootloader kicks off before any piece of software on your device, it makes it extremely processor specific and every motherboard has it’s own bootloader. This is one reason that all Android phones have different custom ROMS developed due to high variance of processing hardware present on the device.

Android Bootloader
Every Android phone has a bootloader that instructs the operating system kernel to boot normally. But you need to understand one thing here that as Android OS is an open source OS and is available on a variety of different hardware, every manufacturer has their own version of bootloader specific for the hardware present in it’s environment. At its most basic level, your Android smartphone is like a hard drive, made of up several partitions. One of those partitions holds the Android system files, another holds all the app data you accumulate (which is how you’re usually able to update without losing all your stuff), and others to do more behind-the scenes stuff.

A lot has been said about bootloaders being “locked” and even the developer-friendly Nexus devices shipped with a locked bootloader (Nexus devices and a couple tablets are easily unlocked with a single command).In fact, a lot bootloaders are locked and encrypted, meaning simple commands like “fastboot oem unlock”, won’t do a thing.

Why are Bootloaders Locked?
A bootloader is usually locked on an Android device because although it’s an open source OS, still the manufacturers want you to stick to their Android OS version specifically designed for the device. In order to apply this concept, manufacturers lock the bootloader. With a locked bootloader on Android devices, it is virtually impossible to flash a Custom ROM and forced attempts void warranty as well as usually end up in bricks. Therefore, the first step is to always unlock the bootloader.

Why keep a bootloader out of reach? One of the biggest reasons is that the carriers and manufacturers don’t want to have to support hacked phones. The other is that a lot of time and money is spent developing these things. HTC Sense ain’t cheap. Neither is TouchWiz. But Samsung and HTC both have managed to find a middle ground with the modding community, and pressure is on other companies to do so as well.

Also a very good read about bootloaders:

What's a kernel?
Taken from: Android A to Z: What is a kernel? | Android Central

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?

What's CWM?
Taken from: AddictiveTips » Blog ArchiveWhat Is ClockworkMod Recovery And How To Use It On Android [Complete Guide]

ClockworkMod, abbreviated as CWM, is a popular custom recovery for Android phones and tablets developed by Koushik Dutta (Koush), a well-known name in the Android dev community. ClockworkMod recovery allows you to perform several advanced recovery, restoration, installation and maintenance operations on your Android device that aren’t possible with the stock recovery, and is one of the most common ways used to gain root access, back up device data, install a custom ROMs, kernels, themes, mods and more. However, for anyone new to Android customization and hacking, some of its options might prove to be a tad confusing. In what follows, we will cover all that this recovery is capable of doing, and how to do it.

About Android Recovery
All Android devices ship with a recovery console that is basically a partition on the device’s internal memory and can be booted into. The stock recovery of almost all Android devices provides a few basic yet handy options that allow you to factory reset your device and also to recover its operating system using an official ROM in zip format, but that’s all you can do with it. That’s where a custom recovery comes handy.

A custom Android recovery basically replaces the stock recovery with one that lets you do all you can do with the stock recovery, plus a plethora of more options to give you a lot more control on your device. With a custom recovery, you can install official and unofficial ROMs as well as other updates including apps, themes, kernels etc. using zip files, wipe not just user data but pretty much every partition on your device, mount the storage card for USB mass storage access without leaving recovery, partition your SD card, wipe Dalvik cache and battery stats, fix permissions, perform, manage and restore backups and so on.

Introduction To ClockworkMod
ClockworkMod recovery is one of the most widely used custom Android recoveries that is available for most mainstream Android devices. It is our custom recovery of choice here at AddictiveTips and almost every custom ROM that we install on our devices is done using this recovery.

ClockworkMod recovery has been developed by Koushik Dutta (also known as Koush) – the same guy who brought us the Android ROM Manager. He can be found at his blog hacking away at Android devices and at Twitter.

CWM options explained:
[REF] CWM - Clockworkmode menu options & Partitions– GENERAL KNOWLEDGE - xda-developers

What's Firmware?
Taken from: What is Firmware, Rom and Firmware Flashing ? - I Teach Android

What the heck is this firmware? Definition of firmware is permanent software programmed into a read-only memory
In Simple words, you can understand it like windows for pc , in case of android we are going to do same thing – installing firmware (Froyo,Gingerbread, ICS, Jelly Bean etc.) on your phone. All phones have their different firmwares and installing tools regard less to the Andriod version (Froyo,Gingerbred). So never think that we can install any firmware on any android phone like we do in PCs.
Wiki link for even more info: Firmware - Wikipedia, the free encyclopedia

What's Flashing?

Flashing refers to the overwriting of existing data on ROM modules present in an electronic device with new data. This can be done to upgrade a device or to change the provider of a service associated with the function of the device, such as changing from one mobile phone service provider to another or installing a new operating system.
In simple words flashing is called installing firmware on your phone.

What's Rooting?
Taken from: Rooting for Android: What, why and how? | Ubergizmo
WiKi link:

When carriers and manufacturers sell you your device, it is almost certain that the device would come with certain software restrictions in place. There are a variety of different reasons why they might do that – some claim that this is done to protect the user, preserve the device’s warranty (this policy will vary from manufacturer to manufacturer), prevent users from getting rid of carrier bloatware apps or simply because the manufacturer would prefer if your device was distinguishable from the competition based purely on its user interface (i.e. Samsung’s TouchWiz, HTC Sense UI, etc).

Whatever their reasoning may be, chances are if you are looking to customize your device on a deeper level, you’d be out of luck and this is where rooting comes into play.

Rooting is essentially a process that allows users of smartphones, tablets or other devices running on Android to gain “superuser” access to the software. This will allow the user to perform administrative tasks such as writing to locations normally restricted by the system which in turn will allow for deeper customization. For iOS users, rooting on Android devices could be thought of as a close equivalent to jailbreaking your device.

What are custom ROMs?
Taken from: Custom ROMs For Android Explained - Here Is Why You Want Them

A stock ROM is the version of the phone's operating system that comes with your phone when you buy it.

A custom ROM is a fully standalone version of the OS, including the kernel (which makes everything run), apps, services, etc - everything you need to operate the device, except it's customized by someone in some way.
So what does the "customized" part mean? Since Android is open source, developers are free to take stock ROMs, modify them, strip them of garbage, optimize them, add things, and pretty much do whatever their imagination and skills allow.

What is ADB?
Taken from: Android 201: What is adb? | Android Central

According to Google "Android Debug Bridge (adb) is a versatile tool lets you manage the state of an emulator instance or Android-powered device." That certainly sounds like Google, doesn't it? To put it simply, adb is two different applications -- one running on your computer (Windows, Linux or Mac) and one running on your phone. When your phone is connected, and USB debugging is enabled, you can issue commands and communicate with the phone using your computer screen and keyboard.

Your Android phone uses a modified Linux kernel and tools as a base. This means that quite a few Linux commands can be sent via the adb server (the one running on your computer) to the adb client (the one running on your phone) and they will be executed. In our example picture, I've sent the "top" command over the wire to my phone, and my phone sent me back the information and printed it to my terminal.

This can be awfully handy for debugging things that aren't going right, as well sending those weird commands you need when you're hacking away in the middle of the night. Chances are, if you aren't actively debugging something or trying to break hack at your phone, you won't have much use for adb. And that's OK -- there's more than one way to have fun with an Android device.

What's baseband?
Baseband is the Radio or Modem version depending upon the Phone Model, Carrier and Android Software Stack version. The Radio/Modem file is flashed via Recovery tool (other options are ADB/ODIN). The mismatched Radio/Modem and ROM will lead to things not working. You need to find the matching Radio/Modem for the particular ROM you are running.

The radio firmware controls basic low-level functions like network connectivity, Wi-Fi, and GPS. Upgrading Radio firmware will fix connectivity issues, increase range or performance, decrease battery usage, etc. Incorrec tRadio frimeware can disable some functions in your phone such as MMS, 3G Data, VM Notifications, etc. Network operators/carriers select the correct version of the Radio firmware that is suitable for the phone, network and bandwidth.

There is also Modem and Baseband Radio Processor chipsets in Mobile phones. Usually, Google, Phone Manufacturers and carriers develop various types of modem firmware/software that controls the functions of these chipsets.

Firmware is the overall version of the Android system on your phone. Baseband version is the version of the radio embedded in the device. Since Android is based on the Linux operating system, they show you the current version of the Kernel used in the heart of the system. The Build number is just an indicator of which numerical version of the current overall system was built by developers for your device.

You cannot update any of these from the official web site. Updates to the Android system are pushed to the phone over-the-air by the manufacturer or the cell phone carrier. The only other way to update or change an Android phone it to install custom modified ROMs in place of the existing system firmware. That usually requires rooting the phone and a fairly considerable knowledge of how to hack hardware.

What's Dalvik?
Dalvik is named after a fishing village in Iceland where ancestors of Dan Bornstein, the person who wrote the VM’s original code, lived. Dalvik is designed for fast execution speeds and operatation in resource-constrained environments like those in mobile devices (with limited memory, CPU and battery power). A Dalvik VM is designed to run multiple instances of itself with each instance hosted on its own separate process and running one application each. When one instance crashes, other concurrently running applications don’t suffer.

Although Android apps are written in Java, they are first compiled into the Dalvik Executable (DEX) format to make them run on the Dalvik VM. DEX files are generally smaller than compressed .JAR (Java Archive) files, making them suitable for mobile devices.

The main difference between Dalvik and a typical Java VM is that the former is register-based while the latter is stack-based. Register-based VMs require fewer instructions than their stack-based counterparts. Although the register-based VMs also require more code, they are generally considered to exhibit faster startups and have better performance than stack-based VMs.

The Dalvik source code license is based on the Apache license. That means, it is free to modify and hence attractive to mobile phone carriers.

What's init.d?

init.d is a folder located at /system/etc
To keep it simple, it allows the user to run scripts at system startup/ boot.
You can adjust many different things/settings with scripts. You can tweak system settings, prolong battery life etc.
To enable init.d and to get some scripts, go here:
Last edited by iONEx; 16th November 2012 at 09:24 PM. Reason: added init.d
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11th October 2012, 02:45 AM |#2  
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great job brother, do much to newcomers become familiar with android and they need to know
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6th February 2013, 06:27 AM |#3  
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woooow , thats nice and great thread ...... thx ..... but between that , can u continue explain many things like what each android device need to boot up and what the most commen partitions in android devics , and getting deeper in android world ad then give some tut about adb using

thx so much
Last edited by speedman2202; 6th February 2013 at 06:32 PM.
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13th February 2013, 10:51 AM |#4  
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Good stuff, thanks!
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26th April 2014, 10:40 PM |#5  
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Great guide for android noobie who want to learn how to root
17th October 2014, 05:58 PM |#6  
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Thumbs up add CID and MID ... ?
Hey -- a really great resource. great work.

could be nice to include CID, MID, etc.

also, would like to understand why ROM has to be built for specific carrier variant of phone.

Example: HTC ONE M8 has multiple different ROM threads -- ATT, Tmobile, Verizon, etc. While I understand there are some small frequencies support differences between an M8_tmobile and M8_Verison, why doesn't a Rooted with S-off M8 care whether it's a ATT or Verizon model?

27th January 2015, 10:29 PM |#7  
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Thanks iONEx, this post helped me some. I already have 20 years of experience with Linux on PCs and Macs, so I already understood concepts like Bootloader, Kernel, Rooting, Flashing, Firmware, and init.d. I've had to flash a new BIOS on several PC motherboards, so I understand the difference between nonvolatile storage in firmware mounted on an integrated circuit of the motherboard versus nonvolatile storage in a physical spinning magnetic hard drive connected to the motherboard via a SCSI or SATA bus and controller. I rooted my first Android (a Motorola Atrix) a year ago, so I also understand CWM, Custom ROMs, and ADB. But your explanation of Baseband and Dalvik was new and helpful to me.

Right now I'm running Paranoid Android on my Oneplus One and using the Settings app in it, I see that I have Android version 4.4.4, ParanoidAndroid version 4.6-BETA6, Baseband version MPSS.DI.2.0.1..., Kernel version 3.4.0-ParanoidAndroid (Mon Nov 3 21:55:14 UTC 2014), Build number pa_bacon-userdebug...).

I found your post while trying to understand more about my OPO that I rooted a few days ago. I installed TWRP, F-Droid, Busybox, MultiROM, and a few other major customizations on it, but I feel like there's still a whole lot that I don't understand at all. For example, in this thread [] I commented that I was unable to get Beanstalk 5.0.2 to function reliably on my OPO.

From chineel's reply "The Steps To Have Better Experience With OnePlus One With Lollipop ROMs" though, I realized that I must still be missing some important concepts, so I started searching for a comprehensive picture of my OPO and of Android phones in general, and although your post helped some, I'm still looking for a much more comprehensive understanding of this device.

I do understand that the nonvolatile storage in my phone must be partitioned into several mutually exclusive sections and that's how it's possible for me to wipe (using TWRP) all of the partitions (Dalvik Cache, System, Data, and Cache) except Internal Storage and flash a new ROM like Beanstalk and yet I still have the contents of /sdcard/ as they were before I wiped and flashed. Obviously, /sdcard/ as mounted in ParanoidAndroid and Beanstalk must be on the Internal Storage partition that did not get wiped.

But when chineel wrote that I should download latest “Cm Nightly” and “(CM Nightly Is for Modem and firmware Update only ) you can Just Flash Firm ware Update [] Instead of...”, that's when I realized that when I flashed a new ROM, I was apparently still leaving something aside from the Internal Storage partition untouched: the modem/baseband/radio?

And so if I flash the latest CM nightly from [] then I'll end up doing what I have not been doing before which is to also change the modem/baseband/radio. Is that right?

So then if I flash a new ROM (like Beanstalk) AFTER flashing the CM Nightly, then I'll be replacing the ROM (from the CM Nightly to Beanstalk), but I won't be changing again the modem/baseband/radio that was changed when I flashed the CM Nightly. Is that right?

If so, then where in this partition system is the modem/baseband/radio firmware (which is apparently separate from the whole ROM) stored in nonvolatile storage? Is it also on Internal Storage? Or is it stored on a separate integrated circuit (like the BIOS is on a PC) or on some other hidden partition?

And what about flashing the kernel? When I flashed my PA ROM, I got a new kernel with it, without explicitly installing from TWRP a new kernel. So sometimes flashing a ROM gives you a new kernel and sometimes flashing a ROM does not change the existing kernel? Is that right? And so is it also possible to flash a ROM and then subsequently flash a kernel and that second flash replaces the kernel that was part of the ROM of the first flash?

I just need to understand where all of this information is getting stored (in which partitions). I know I flash a ROM, then I flash GAPPS, then I flash a kernel, then I flash a modem/radio/baseband. But I can't tell; is all that software going to the System partition? If so, then why don't all the later flashes completely write over all the earlier flashes?

My real question here is what to read for a comprehensive explanation of all these pieces and how they fit together and why flashing sometimes replaces something that was there before, but it doesn't replace everything (like the modem/radio/baseband)? I think I need a book or something. Can you recommend one?

Thanks, and sorry for the long post.
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