[U-Boot] Regarding Start.S file

[Albert ARIBAUD]

Hi Asmit,

Le 14/09/2011 19:48, Asmit Patel a écrit :
> Hi All,
> I am new to u-boot and ARM. I am trying to understand uboot thtough start.S
> file.
> I have few questions about it.

As the custodian for the ARM u-boot repository, I feel compelled to try 
and answer your questions. :)

> 1) What is the deifference between ARM Supervisior and User mode. why uboot
> is not running in user mode?

As for the difference between ARM user and supervisor modes, you should 
go to the ARM Ltd. website and look up their documentation, especially 
the ISA Reference Manual for the specific ISA you're interested in. 
However, in a very broad and inexact way, user mode is for application 
code and has restrictions on some instructions, while supervisor mode is 
for OS code and has no restrictions -- which explains why U-Boot is in 
user mode.

> 2) At the start of startup file I found below code.
>
>   .globl _start
>   _start: b start_code
> ldr pc, _undefined_instruction
>   ldr pc, _software_interrupt
>   ldr pc, _prefetch_abort
> ldr pc, _data_abort
> ldr pc, _not_used
> ldr pc, _irq
> ldr pc, _fiq
>
> _undefined_instruction: .word undefined_instruction
> _software_interrupt: .word software_interrupt
>   _prefetch_abort: .word prefetch_abort
> _data_abort: .word data_abort
>   _not_used: .word not_used
> _irq: .word irq
>   _fiq: .word fiq
>
>   .balignl 16,0xdeadbeef
>
>
> What is this code will do? as i believe the instruction "b start_code" will
> branch it to start_code then how does the other code below branch
> instruction will work?

That's where you should really go to the ARM Ltd. documentation, because 
this code is nothing U-Boot specific and purely derived from ARM 
architecture.

> 3) where does exception vector table located during power on? on boot rom or
> at any other place?

Again, this is not an U-Boot related question; this time, it is a core 
or even SoC or board specific thing, as various platforms have various 
boot processes.

> Thanks&  Regards,
> Asmit Patel"

Amicalement,
-- 
Albert.