just have a look into this file, it is well documented.
* Kernel startup code for all 32-bit CPUs
And in linux/arch/Doumentation/arm/README it is written:
Kernel entry (head.S)
The initial entry into the kernel is via head.S, which uses machine
independent code. The machine is selected by the value of 'r1' on
entry, which must be kept unique.
Due to the large number of machines which the ARM port of Linux provides
for, we have a method to manage this which ensures that we don't end up
duplicating large amounts of code.
We group machine (or platform) support code into machine classes. A
class typically based around one or more system on a chip devices, and
acts as a natural container around the actual implementations. These
classes are given directories - arch/arm/mach-<class> and
arch/arm/mach-<class> - which contain the source files to/include/mach
support the machine class. This directories also contain any machine
specific supporting code.
For example, the SA1100 class is based upon the SA1100 and SA1110 SoC
devices, and contains the code to support the way the on-board and off-
board devices are used, or the device is setup, and provides that
machine specific "personality."
This fine-grained machine specific selection is controlled by the machine
type ID, which acts both as a run-time and a compile-time code selection
You can register a new machine via the web site at: