The new Android releases usually come with two reference platforms. Developers can test the new Android releases on Android emulator first. This can be very useful in the preview stages. After the official release, the Google hardware platforms, such as Nexus or Pixel, usually become the devices for developers. The emulator and Nexus/Pixel builds are the earliest builds available in AOSP.

In this book, we will use Android emulator as the virtual hardware reference platform for our topics. Since the Android emulator build is already available in AOSP, you may wonder what we can do with it. Actually, we can customize an existing platform by adding new features to it. This is what OEM/ODM companies usually do using a reference platform from a silicon vendor. With Android emulator, we will demonstrate how to create a new device so that we can customize it. If you know any commercial emulator products, such as Genymotion and AMI DuOS, then you may know what features these products added to the emulator. We will extend Android emulator in a very similar way.

After we explore the topics about the customization of a new device, we will explore more advanced topics about porting. The major work with porting is the changes to the kernel and HAL. To discuss advanced topics about porting and debugging, we will also use VirtualBox as another virtual hardware reference platform. Even though VirtualBox has been used by many commercial emulator products, such as Genymotion, AMI DuOS, Leapdroid, and so on, it is not supported by AOSP directly. Most Android emulators for the PC are based on VirtualBox and they are designed for gamers to run Android games. In this book, we will learn how to create a similar build using various open source resources.

Android emulator has been changed dramatically as well in Android 4, 5, 6, and 7. Before Android 5, Android emulator was built on a virtual hardware reference board called goldfish.

The goldfish virtual hardware platform was built on QEMU 1.x to emulate ARM devices on the x86 environment. The x86 host environments could be a Windows, Linux, or macOS X computer. Since the target device architecture is emulated using QEMU, the performance is poor. The emulator is very slow and difficult to use for application developers. However, QEMU is actively developed on the x86 architecture and widely used together with various virtualization technologies, such as VT-x, AMD-V, and so on.

Since Android 4.x, Intel developed an x86-based Android emulator using KVM on Linux and Intel HAXM for Windows and macOS X. With the introduction of virtualization technology to the emulator, the Intel x86-based emulator is much faster than the emulated one for the ARM or MIPS architecture. For the sake of Android application developers, Google officially integrated the Intel x86-based Android emulator to Android SDK. The Intel x86-based Android emulator has become the recommended choice for developers to test their Android applications.

With the introduction of Android 5 (Lollipop), the 64-bit hardware architecture is available for both ARM and Intel platforms. However, 64-bit hardware devices for Android were still under development at that time. The only choice for developers was to get a hardware reference platform from silicon vendors.

To help developers test their applications on 64-bit architecture, the engineers at Linaro did an excellent job enabling a virtual hardware platform on QEMU to test ARMv8-A 64-bit architecture. They gave this virtual hardware platform a code name, ranchu. You may refer to the blog at Linaro by Alex Bennée at https://www.linaro.org/blog/core-dump/running-64bit-android-l-qemu/.

This change was adopted by Google later and was used as the hardware reference platform for the next generation of Android emulators. If you install the Android SDK images, you can see two kernel images starting from Android 5. The kernel image kernel-qemu is the image to be used with the goldfish virtual hardware platform and the image kernel-ranchu is the image to be used with the ranchu virtual hardware platform.

To respond to this change, both Intel and MIPS worked on their architectures to support their 64-bit hardware emulation in ranchu. You can refer to the group discussions at https://groups.google.com/forum/#!topic/android-emulator-dev/dltBnUW_HzU.

The ranchu hardware platform is based on a newer QEMU version and the architecture is changed to have less dependency on Google modification and goldfish-specific devices. For example, it uses virtio-block devices to emulate the NAND and SD card. This has the potential of providing much better performance and also makes it possible to utilize the features provided by the latest QEMU code base. The ranchu kernel is built on a new version in the android-goldfish-3.10 branch, while the latest goldfish kernel is in the android-goldfish-3.4 branch. You can notice this difference by running your Android virtual device using different kernels from Android SDK.

With the ever evolving nature of virtualization technology, there are many commercial Android emulator products developed on the market as well. You may have heard of some of them such as Genymotion, AMIDuOS, Andy, BlueStacks, and so on. Many of them are built using VirtualBox from Oracle, such as Genymotion, AMIDuOS, and Andy. The reason that VirtualBox is used instead of other solutions such as VMware is because VirtualBox is an open source solution.

To achieve the best performance and user experience, both host and target need to be customized in the commercial emulator products. Besides Android emulator, we will also use VirtualBox as the virtual hardware platform to demonstrate how to port Android to a new platform. The reason that we need another virtual hardware platform in this book is because Android emulator is already supported in AOSP. We will use Android emulator as a platform to teach how to extend and customize an existing platform. While VirtualBox is not supported in AOSP, it can be used as a target platform to teach how to port Android to a new platform. Even though Android has been ported to VirtualBox by Genymotion, AMI, and others, none of them are open source products.