[U-Boot] How does u-boot know where to put its start code?

[Albert ARIBAUD]

Le 20/04/2011 04:23, Hebbar, Gururaja a écrit :
> Hi,
>
> On Wed, Apr 20, 2011 at 02:43:23, Rogan Dawes wrote:
>> Hi folks,
>>
>> I'm trying to understand a bit more about how u-boot creates the image, such that the CPU reset vector is pointing to the right piece of code when it is reset.
>>
>> i.e. my DNS323 (Orion5x) has a reset vector of 0xffff0000. But for the life of me, I can't find anywhere that actually references that value to place the start code at that point.
>>
>
> Placing the final boot image is left to user who flashes/burns it board. But it should be same as _TEXT_BASE (this is being removed now. Orion5x is arm based). Also look at<u-boot-src>\arch\arm\cpu\arm926ejs\start.S&  <u-boot-src>\arch\arm\cpu\arm926ejs\u-boot.lds for more info on how linker is instructed to place the starting code at predefined address.
>
>> I'm basically trying to make sure that my CONFIG_SYS_TEXT_BASE is correct (the address in the flash to which I write the whole u-boot.bin file, right?.
>>
> This is passed to linker as the entry point.

There is another point re: orion5x based boards: often, their designers 
preferred generating a linear image for U-Boot, but the fact that the 
vector address is at FFFF0000 makes it impossible to directly the image 
there because it is always greater than 64K. So the designers put some 
"pseudo-rom boot code" at FFFF0000 that will finally jump to an address 
lower in FLASH; for ED Mini V2 it is FFF90000, and that's where the 
U-Boot image is supposed to be flashed.

Rogan, I bet in the DNS323 case, the same applies modulo your Flash 
size. Try tracing through the FFFF0000 code, it should not last more 
than a few tens of instructions before it jumps to some absolute address.

>> Thanks
>>
>> Rogan

Amicalement,
-- 
Albert.