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is able to mount the root file system from different physical media, for
example from a directory exported via Network File System (NFS) by the
host. This strategy (kernel image and RFS retrieved from the network)
saves time during the development phase, since no flash reprogramming
or removable storage (SD, usb pen drives, external disks) is required to
test new versions or updates of the software components.
7.3.2
Software components
7.3.2.1
Toolchain
With the term "toolchain" we refer to the set of programs that allow the
building of a generic application. For applications built to run on the same
platform as the tool chain, we use a native toolchain. On the contrary, for
applications built to run on a target architecture different from the host
architecture, we use a cross-toolchain. In this case all the tools involved
in this process are lead by the “cross-” prefix. So we talk about cross-
compiler, cross-toolchain and so on. The cross-toolchain used to build U-
Boot and the Linux kernel is the GNU toolchain for the ARM architecture
built for x86 hosts. In other words, the toolchain runs on x86 machines
but generates binaries for ARM processors. As for all the software
compliant to the GPL license, it is released in source code. Thus the first
thing to do to set up the developing environment should be building the
cross-toolchain. This is not a trivial task, it takes a lot of time and hard
disk space. To avoid this tedious task, we suggest use of a pre-built
toolchain as explained in the following sections.
7.3.2.2
Bootloader
U-Boot is a very powerful boot loader and it became the "de facto"
standard on non-x86 embedded platforms. The main tasks performed by
U-Boot are:
●
hardware initialization (external bus, internal PLL,
SDRAM controller etc.)
●
starting a shell on the serial port allowing the user to
interact with the system through the provided
commands
●
automatic execution of the boot script (if any)
After system power-up, U-Boot prints some information about itself and
about the system it is running on. Once the bootstrap sequence is
completed, the prompt is printed and U-Boot is ready to accept user's
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