AT91Bootstrap is a second step bootloader which provides a set of algorithms to manage hardware initialization (GPIO, Clock, SDRAM, etc), to download your main application from specified FLASH media to main memory and to start it.
In the Android4SAM pre-built demo it is used to configure the hardware and to load and start the Linux kernel directly.
For more information, please refer to: Linux4SAM AT91Bootstrap
U-Boot is optional for demo package. If you want the recovery mode and fastboot features added in, you need u-boot.
For more information, please refer to: Linux4SAM u-boot
Android Linux Kernel
The Android Linux kernel is based on the kernel on Linux4SAM Kernel
Android4SAM kernel is release through git repository. It can be found at: https://github.com/Android4SAM/linux-at91
. And each release is tag with release name, for example: Android4SAM v4.2
For more information, please refer to: Linux4SAM Kernel
Android is an open-source software stack for a wide range of mobile devices and a corresponding open-source project led by Google. Here you can find the information and source code you need to learn more about the Android platform. From there you can create custom variants of the Android software stack, port devices and accessories to the Android platform, and ensure your devices are compatible with the Android compatibility definition.
For more information, please refer to: Android